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““TThhiirrttyy YYeeaarrss ffrroomm tthhee RRoommaanniiaa EEaarrtthhqquuaakkee ooff MMaarrcchh 44,, 11997777””

BBuucchhaarreesstt,, RRoommaanniiaa

11--33 MMaarrcchh 22000077

A VIEW ON THE EXPERIENCE, IMPACT AND IMPLICATIONS

OF THE 4 MARCH 1977 EARTHQUAKE, AFTER THIRTY YEARS

Horea SANDI1, Emil Sever GEORGESCU2, ioan Sorin BORCIA2

ABSTRACT

An attempt at summarizing the experience, impact and implications of the destructive earthquake of 1977.03.04 is presented. This is related to the knowledge acquired, to the main contributions of foreign experts participating in the making and discussion of post-earthquake surveys, to the implications for the regulatory basis of structural design, to a critical view on the earthquake risk reduction activities developed to date and on the development of a comprehensive risk reduction strategy, to some actions that are complementary to classical engineering earthquake protection activities. The presentation relies essentially on the experience and activities of a group having worked for a long time in the frame of INCERC - Bucharest.

1. INTRODUCTION

Three decades elapsed from the occurrence of the earthquake of 1977, the most destructive seismic event having affected Romania during historical times. The MGR = 7.2, Mw = 7.5 Vrancea earthquake having occurred in Romania on 1977.03.04 represented a major event, generating a severe economic and social impact, but also a strong favourable scientific impact. An attempt of developing a summary view at this moment is, of course, enrightened. Some early scientific and technical publications on this subject, devoted to the aspects relevant to the engineers, namely the papers (Cişmigiu, 1977) and (Sandi & al., 1978), as well as the comprehensive monograph (Bălan & al., 1982) must be mentioned. It must be also mentioned that ICCPDC (the former Central Institute for Research, Design and Guidance in Civil Engineering) organized a comprehensive collection of data, aimed for internal use. The publication by seismologists of numerous papers on the subject of the earthquake must be mentioned too. The data and developments presented in this paper, which is concerned essentially with aspects that are relevant to engineers, are due essentially to the group of INCERC –Bucharest in the frame of which the authors were active for a long time. The activities dealt with may be considered to a high extent as a part of the activities of research in the field of structural safety and earthquake engineering performed in INCERC – Bucharest, presented in (Sandi, 2006). __________________________________________________________________________ 1 M., Acadeny of Technical Sciences of Romania. E-mail: sandi@geodin.ro 2 INCERC (National Building Research Institute), Bucharest.

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E-mail: ssever@incerc2004.ro, isborcia@incerc2004.ro.

Of course, the view presented cannot be complete. Some complementary developments that are relevant for the field dealt with can be seen in other contributions to this symposium, like (Georgescu & Pomonis, 2007) and (Vlad, 2007).

It may be stated that the most significant effects of this dramatic event were related to: - the social and economic impact; - the lessons of the earthquake, from the viewpoint of various socio - professional

groups The bulk of this presentation is related to topics of interest for engineers, with the remark that the most extended part of it is devoted to topics of engineering seismology.

2. A BRIEF SUMMARY VIEW

2.1. Some References to the Economic and Social Impact An official view on the impact of the 1977 earthquake was released by Romanian authorities at the end of 1977. Some main data are as follows (Bălan & al., 1982):

- 1570 lives lost, as identified, and more than 11300 persons injured, out of which about 90% in Bucharest;

- 32900 housing units destroyed or heavily damaged; - 35000 families remained homeless; - heavy damage or destruction for numerous buildings of the educational, medical

and commercial networks recorded; - important damage having affected 763 units of the basic networks of industry,

construction and transportation; - additionally, numerous cases of heavy damage or destruction for agricultural

buildings, mechanical workshops, silos, hothouses etc., involving numerous cases of loss of animals.

As a summary estimate, the losses exceeded US$ 2 × 109. This estimate is subject to some comments in Section 8. The size of the impact, as shown before, was generally confirmed, with some slight modifications, in the World Bank Report (World Bank, 1978). Some further data may be seen in (Georgescu & Pomonis, 2007). Nevertheless, some reservations on these estimates are presented further on. It is widely accepted that the socio – psychological impact was very heavy and had serious consequences, even long term ones, but quantifications in this direction are a shaky task.

2.2. A Short View on the Scientific and Technical Impact of the Earthquake

Two destructive Vrancea earthquakes have affected Romania during the 20th century,.on 1940.11.10 (MGR = 7.4, Mw = 7.7) and on 1977.03.04 (MGR = 7.2, Mw = 7.5) respectively (note that the most severe historical earthquake occurred on 1802.10.26, with an estimated magnitude MGR = 7.5 … 7.7). Such events represent in principle a major source of learning in order to improve the system of earthquake protection measures. It must be noticed nevertheless that, while in 1940 the professional community that should have been involved in earthquake protection was poorly prepared to learn from the earthquake, the situation was totally different in 1977, when a lot of competence had been accumulated in this field. This scientifically favourable situation was due to the joint action of several factors, increasingly at hand during the second half of the 20th century: specific training of engineers in the frame of civil engineering faculties, existence of quite strong research centers,

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existence of a certain regulatory basis of wide use in practice, strong practical activities in the frame of design institutes, that helped to develop a national school of practicioners of good quality. It may be stated that the most significant and explicit scientific and technical lessons provided by this event concerned:

- data concerning the seismic conditions of Romania; - data concerning the earthquake performance of structures of various categories.

One should add, to these ones, data of various natures about the economic and social impact, which are out of the reach of this paper. The data concerning the seismic conditions of Romania referred mainly to:

- a memento on the high frequency of recurrence of strong, perhaps destructive, Vrancea earthquakes, leading to a wide scale revival of the interest for the characteristics of high magnitude recurrence;

- a memento on the long distance radiation of high intensities, as well as a stimulation of the interest for the analysis of the specific features of attenuation;

- gathering of direct instrumental data about the features of ground motion acceleration, which made it possible to put to evidence especially the rather non-usual spectral features of ground motion due to Vrancea earthquakes.

These data were considerably extended subsequently to the strong Vrancea earthquakes of 1986 and 1990, when a rich instrumental information was obtained. The most extended part of this paper, in Section 6, is devoted to a discussion of the seismic conditions related to the [VSZ] (Vrancea seismogenic zone) activity. The data concerning the eartthquake performance of structures referred mainly to:

- a wealth of case studies, developed as a rule in an individual frame and insufficiently summarized to date;

- studies of statistical nature, of two sub-categories: - studies based on visual observation only; - studies based on engineering analyses.

Some references to these studies are presented in Section 7, besides some methodological developments.

2.3. Some References to Actions Undertaken in the Short Run

As in case of any destructive earthquake occurring in a country with a decent level of development, the occurrence of this disastrous event led to the mobilization of consderable resources in view of coping with the new situation created. One should mention here primarilly:

- emergency actions of rescue of persons trapped under debris; - emergency actions to prevent further collapses of buildings or other structures

heavily damaged; - measures to clean up places with debris; - measures for emergency rehabilitation of communication and transport ways; - measures for emergency rehabilitation of the functional capacity of medical units; - measures for providing shelter to people rendered homeless; - organization of groups of engineers asked to summarize the situation created

and to yield a first view on the proportions of damage and risk, in order to provide the necessary premisses for systematic emergency action;

- some emergency actions aimed at correcting obvious shortcomings of the regulatory basis put to evidence by the earthquake experience (zonation of the territory, microzonation, dynamic factor of design code);

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- organization of activities of scientific interest; - cooperative actions with foreign experts having visited Romania at the times.

In case one thinks of engineering activities, one should mention the concern for preventing further collapses, that was conducted on an individual basis. There existed no widespread regulations or recommendations on the ways of selecting rehabilitation and strengthening solutions. So, one could witness a considerable variety of solutions, able to strengthen more or less efficiently the damaged structures tackled. Given the philosophy of drastic limitation of expenditure of the holders of political power of the time, the General Inspection of Construction (IGSC), together with the Central Institute for Research, Design and Guidance (ICCPDC) issued under political pressure the Decision no. 25 / 4 July 1977, according to which the rehabilitation of damaged structures must be limited to local intervention at places where damage is obvious. Of course, such a philosophy was in direct contradiction with the views of responsible structural engineers, who tried, in several cases, to bypass such constraints. It must be also mentioned that researchers in the fields of seismology and engineering were prompt in collecting information required by their activities. Among other, one should mention a project of large proportions, initiated in April 1977, devoted to an in depth survey of damaged buildings, aimed initially to lie at the basis of an improved microzonation of Bucharest, but resulting also in deriving vulnerability functions for several categories of buildings on the basis of statistical damage spectra developed for various areas of Bucharest (see Section7 of the paper).

2.4. Some References to Longer Term Actions

Following references are limited to the actions related quite directly to earthquake risk reduction. They encompassed primarily:

- actions of revision of the regulatory basis related to earthquake protection; - actions of rehabilitation and strengthening of hazardous structures, including a

first attempt of systematic activities; - improvement of organization of disaster prevention activities; - initiation of disaster preparedness activities.

The revision of the regulatory basis included primarily the development of a new zonation of the territory and of a new earthquake resistant design code. A tabular presentation of the evolution of regulations is presented in Section 5. The zonation of 1977 kept the philosophy of considering maximum observed intensities. Rather local corrections were introduced at places where destructive effects of the 1977 earthquake were observed (the most important change: increasing the intensity for Bucharest from VII to VIII MSK, as it had been specified up to 1963). The revisions of 1991 and 1993 benefitted from rich accelerographic information and adopted the philosophy of specifying return periods for the values prescribed. The main earthquake resistant design code was replaced in 1978, when a restructuring took place. Reference accelerations prescribed were in the range of expected PGA or EPA values with explicit (and credible) return periods. The dynamic factor corresponded to the response spectrum determined for the (NS) INCERC record. Quantitative criteria wedre introduced in order to provide ductility to structures. A new revision, in 1981, brought additional improvements and refinements, among other a consistent development of specification of seismic input for 3D analysis. The revision of 1992 brought important new

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modifications. The zonation, relying on rich instrumental data obtained during the strong earthquakes of 1986 and 1990, combined with previous macroseismic information, was revised, eliminating some previous inconsistencies. The zonation became bi-parametric. The conditions for providing ductililty were improved. New sections, concerning the protection of equipment and the evaluation of existing structures, together with recommendations on intervention, were added. The sections dealing with existing structures were revised subsequently in1996. The main way to deal with the reduction of earthquake risk was represented by the intervention for rehabilitation and strengthening hazardous structures. The most significant actions of rehabilitation were related to the educational and medical networks. Interventions on administrative buildings, on residential buildings, on some industrial facilities etc. were initiated as a rule on an individual basis. No strategy of systematic replacement of old structures was initiated. No standardized solutions (even if designed) were put into practice to date to counteract the shortcomings observed especially in the case of more vulnerable post-war high-rise buildings built according to standardized solutions. Quite late, an inventory of hazardous buildings was initiated. Nevertheless, a wide public dissatisfaction due to the slow pace of action rose. Obtaining funding for interventions on existing buildings was always difficult, due to the shape of the national economy. It may be mentioned, nevertheless, that there were cases when these funds could not be used, due to managerial / logistic problems (e.g. the reluctance of occupants to temporary evacuation required in some cases by radical strengthening). In some cases when, in agreement with regulations referred to, funds could be provided for the lower income occupants, the relatively richer ones, whose income exceeded the thresholds of income per occupant established, hardly could find a source of borrowing money as required by the strengthening of apartment houses.

A reasonable adoption of intervention priorities was sometimes hampered by the outcome of engineering evaluations of building vulnerability. The methods of evaluation, which relied basically on the methods prescribed by codes for the design of new buildings, led often to too pessimistic conclusions on the ability of buildings to withstand earthquakes, distorting thus priorities (sometimes those conclusions were in direct contradiction with direct observation, even of instrumental nature, provided by the experience of the strong earthquakes of 1986 and 1990). In a different connection, the bodies responsible for the safety of the NPP now in operation showed no readiness for reanalyzing the seismic conditions specified about two decades ago for design. Instrumental data obtained in 1986 and 1990 close to the site (Sandi, 1996) and probabilistic hazard analyses were convergent in showing that the upper level of seismic loading specified corresponds to a return period in the range of not more than one century. A similar example, related to the 165 m high Argeş-Vidraru arch dam (the highest in Romania), located in the epicentral zone of Făgăraş earthquakes, may be referred to for illustration. A reassessment of the seismic conditions, performed in 1993 - 94 by three independent research groups, showed that reference design accelerations with a return period of some 500 years should be three to five times higher than conventional accelerations considered before 1960, in design.

2.5. Some Obstacles to Earthquake Risk Reduction Encountered

The experience at hand and the examination of the causes of non-satisfactory results to date in relation to seismic risk reduction makes it possible, and also necessary, to emphasize some main causes and obstacles having led to this non-satisfactory situation (Sandi, 2000).

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There are definitely important obstacles of financial nature to earthquake risk reduction. While the total resources required in order to bring the existing elements at risk (first of all those for which the primary vulnerability, i.e. the vulnerability against ground motion, counts) to a state of risk as accepted by the regulatory basis currently in force, might reach the range of thousands of millions of US$, the resources made available yearly for risk reduction activities were during last years in the range of millions or of tens of millions of US$. Yet the lack of appropriate financial resources cannot be by far blamed as a unique obstacle hindering these activities. It is a main point at this place to emphasize some other critical factors in this connection. A crucial category of obstacles, routinely not referred to, is represented by the limits of various kinds to knowledge and by the reluctance to spend more resources in order to improve knowledge. First of all, there do not exist satisfactory databases listing the elements at risk to be tackled, in all fields where such elements at risk exist. Then, there do not exist satisfactory databases on the outcome of evaluation of the relevant elements at risk. Moreover, there are often quite severe methodological limits concerning the ability of experts to evaluate existing works in a way to lead to results compatible with the outcome of observation of actual performance during strong earthquakes, as well as the ability to derive efficient and economical solutions of reducing vulnerability. In some way, on a complementary side, there are limits to the know-how on hazard evaluation, especially for sites of facilities raising special problems (facilities including high-risk sources, strategic facilities etc.), as illustrated in previous subsection. Another category of obstacles is represented by the shortcomings of the current legislation and regulatory basis. There is e.g. no regulation for creating buffer room (which was so important in 1977 and thereafter) as required for people having become homeless during earthquakes or for occupants of buildings requiring temporary evacuation in order to make possible radical strengthening. There is no regulation to compel to evacuation by occupants in cases when radical strengthening solutions impose this. There is no regulation to provide leverage created by insurance activities in order to push owners / occupants to become active partners in risk reduction activities. The regulations related to disaster prevention activities, based on the governmental ordinance no. 47 referred to, were not developed up to a point to create a consistent system of operational administrative bodies to survey, coordinate and foster risk reduction activities in various fields of interest. Finally, one should not neglect some categories of obstacles of less tangible nature, yet of undeniable importance. They refer essentially to the lack of appropriate willingness of various socio-economic groups involved, to surpass the difficulties and, ultimately, get risk reduced. One could state in this connection that the disaster prevention culture is not sufficiently developed. One can state this often in relation to the attitude of the population as a whole, but also at the level of specialists or of persons or groups having special responsibilities. One could state, from a somewhat different viewpoint, that there is a lack of appropriate political will, at the level of various agencies or persons playing a formal role in the frame of various agencies, institutional structures etc., as well as at the level of what should be a vigorous civil society (even honesty questions may be raised in this latter connection too). The allocation of appropriate financial resources for risk reduction depends heavily on this political will. One can summarize previous considerations stating that not only appropriate financial resources are lacking, but also other categories of resources (knowledge of various natures, sufficiently complete and consistent legislation and regulations, managerial capabilities and general willingness) are in rather short supply. The picture of obstacles presented may be not sufficiently comprehensive or detailed, but the obstacles referred to and their critical character can be hardly denied. It provides a view on the actual size of the task of reducing seismic risk to a level not too far from the philosophy and requirements of regulations in force for new developments.

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2.6. Some Remarks on the Necessary Development of a Comprehensive Risk

Reduction Strategy

The UN initiative of developing the IDNDR starting with 1990 represented a true milestone in the activities devoted to protection against disasters. Romania moved quite slowly during the first IDNDR years, but afterwards our country benefitted considerably from the adhesion to this initiative. Some remarks in relation to a more profound implementation of the IDNDR may be formulated in this connection. In agreement with the holistic view that was characteristic to the IDNDR, a holistic view is needed also at the level of Romania. In face of an event having the potential of disaster generation, the persons involved in protection are in a situation quite similar to that of a general fighting durins a war in the defense. The enemy will almost certainly strike wherever he recognizes a soft or weak point in the defensive system.

3. SOME DATA ON THE ASSISTANCE PROVIDED SUBSEQUENTLY TO THE 1977

EARTHQUAKE BY FOREIGN EXPERTS HAVING VISITED ROMANIA AND BY

FOREIGN INSTITUTIONS

3.1. Main consultancy activities

Romania was visited, subsequently to the 1977.03.04 earthquake, by numerous foreign experts of highest qualification, who provided in several cases and from various viewpoints, particularly valuable consultancy (see more details in Chapter IX of (Bălan & al. 1982)). They arrived in the frame of national delegations, or of assistance projects. A token of gratitude is due to them. A list of experts, which is not complete, includes the UNESCO delegation (N. Ambraseys - UK, J. Despeyroux - France, G. Grandori - Italy and J. Petrovski – Yugoslavia), US experts (G. Berg, M. Sozen, J. Penzien, Chr. Rojahn), the Japanese delegations of engineers and seismologists (referred to below), Soviet experts (S. V. Medvedev, S. V. Polyakov, Ya. M. Eisenberg, A. Zharov, Kilimnik), a New Zealand expert (I. Skinner). A few of the recommendations formulated by them are mentioned subsequently. G.Grandori emphasized the complexity of the problem of interpretation of instrumental data in order to determine design parameters. J. Despeyroux emphasized the possible variability of the spectral contents of ground motion. The celebrated American seismologist Ch. F. Richter stated, in a letter to the National Council on Science and Technology, that nowhere in the world exists a concentration of

population so exposed to earthquakes originating systematically in a same source, as in Bucharest. G. Berg emphasized the peculiarities of the INCERC accelerogram of 1977.03.04. M. Sozen emphasized the importance of limitation of deformation / drift, since excessive deformation may lead to excessive reduction of the part of a section subject to compression, followed by a lack of capacity of transmitting shear forces, ergo to brittle failure. J. Penzien emphasized the need of a comprehensive inventory of the existing building stock and of corresponding evaluation. Recommendations were formulated in relation to the needs of preventing progressive collapse, of appropriate proportioning of separations between buildings, of in depth investigation of the satisfactory performance of large panel buildings, of revising the expression of the dynamic factor prescribed by codes. The high cost and the uncertain results of works of rehabilitation and strengthening of damaged buildings was mentioned.

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A particularly strong delegation came from Japan. K. Nakano and M. Hirosawa provided particularly valuable consultancy especially on the ductile design of r.c. structures, putting into value a summary of experience of the Tokachi-oki 1968 earthquakes, which had become ripe in about 1970. The newly developed philosophy of ductile design (limitation of gravity stresses in columns, limitation of longitudinal reinforcement, concrete confining, avoiding of short columns etc.) took by surprise Romanian experts educated in the spirit of the Soviet school of design of r.c. structures, relying at that time basically upon the consideration of static loading. The main recommendations offered by the Japanese colleagues were implemented already in the 1978 version of the earthquake resistant design code P.100. K. Kubo and E. Kuribayashi were involved mainly in the on site inspection of engineering structures, among which the trans-Danube bridge of Giurgeni – Vadul Oii. Besides the group of engineers referred to, a strong group of seismologists, led by H. Asada, attended Romania. Attention was drawn on the features of Vrancea seismicity (frequent occurrence of strong earthquakes, specific features of spectral contents of ground motion). The possibility of occurrence, anywhere in the country, of magnitude 6 crustal earthquakes having a potential of heavy local destruction, was emphasized too. S.V. Medvedev emphasized the spectral peculiarities of ground motion due to deep earthquakes. Soviet seismologists discussed earthquake prediction techniques. They emphasized the need of taking into account of the spectral contents put to eviidence by instrumental data. S. V. Polyakov advocated the llimitation of drift in case of framed structures. He advocated also the use of prefabricated, vibrated masonry panels. The need of confining of the masonry panels by means of r.c. members was emphasized. The studies under way concerning devices of limitation of earthquake induced deformation and stresses were referred to. Rules of proportioning separations between neighbouring buildings were proposed. The need of considering buildings as a whole when designing rehabilitation and strengthening measures was emphasized too.

As a common factor, the experts appreciated the performance of engineered structures of Romania and emphasized the need of further development of research activities.

3.2. Hardware development assistance

The most important assistance for scientific hardware development was provided by the Agency of International Development (AID) of the US Department of State. This was a part of the total aid having reached US$ 60 M. The share of aid reserved for civil engineering research was oriented towards two major goals:

a) development of the strong motion network of Romania; b) development of a strong testing facility.

The strong motion network was strengthened mainly by a system of 75 strong motion accelerographs SMA-1, which produced during the subsequent strong earthquakes of 1996.08.30, 1990.05.30 and 1990.05.31 a considerable number of accelerographic records, which played, at their turn, a paramount role in a better understanding of the seismic conditions of Romania and in the development of a more realistic seismic zonation of the territory. The main outcome of use of these data is presented in section 6. A strong testing facility was planned and designed for development in the frame of INCERC (Building Research Institute), Bucharest. On one hand, a strong reaction wall of a height of 12 m was bult. This was designed to permit testing up to failure of full scale fragments of structures like shear wall buildings or various engineering structures. On the other hand, two

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shaking tables were built, one of them 3m × 3m, with a payload up to 5 t, the other 6m × 6m, with a payload up to 60 t .Both of them were planned to have 6D motions, with amplitudes of accelerations, velocities and displacements widely covering the parameters determined by similitude criteria from what could be produced by strongest Vrancea earthquakes. Unfortunately, the government of that time decided to distort the destination of funds from AID and to order practically exclusive use of domestic sources of equipment. This had a strong negative impact on the project, such that the system of required actuators and the hydraulic system were not obtained. The reaction wall was used under more modest conditions, while only the smaller shaking table was partially put into operation. It may be mentioned in this connection that the Japanese delegation offered a strong motion accelerograph of the type SMAC-E and that the delegation of IZIIS (IEEES) Skopje offered four accelerographs of the type SMA1.

4. A VIEW ON THE RESEARCH STIMULATED BY THE EARTHQUAKE IMPACT AND

EXPERIENCE

4.1. General

This section is devoted essentially to a summary look at research activities stimulated by the incidence and experience of the destructive earthquake of 1977.03.04, but also to some aspects raised by the incidence and experience of the subsequent strong earthquakes of 1986.08.30, 1990.05.30 and 1990.05.31. While the information obtained in 1977 was of high diversity, due mainly to the destructive effects of the earthquake, the information obtained in 1986 and 1990 was essentially of instrumental nature, due to the important progress achieved in the development of the strong motion network. It must be also mentioned that the activities presented at this place are due mostly to the efforts of a strong staff of INCERC (Building Research Institute, Bucharest), in the frame of which the authors were active for a long time.

4.2. Research based on statistical field survey activities

The hard information provided by the earthquake incidence was of quallitative nature (mainly: effects observed on the earthquake performance of structures and also of other elements at risk), as well as of quantitative nature (mainly: instrumental data obtained). An important, large scale research initiative taken under the auspices of CNST (National Council for Science and Technology) was aimed at performing a damage survey to lie at the basis of a new, improved, microzonation of the City of Bucharest (the observed geographical distribution of damage severity was in strong disagreement with the provisions of the microzonation standards in force at the time of earthquake incidence and the standards referred to had to be put promptly out of force). The CNST project relied on a partition of the territory of Bucharest into squares of 1 km × 1 km. According to a random selection algorithm, about 300 buildings were surveyed for each of 62 selected squares, corresponding to the grid referred to. The number of buildings investigated totaled more than 18,000. The buildings investigated pertained to eight categories, covering: adobe type, masonry walls with non-rigid (e.g. wooden) floors of different age categories, masonry walls with rigid (r.c.) floors of different age categories too, taller buildings with r.c. walls (distant or closely spaced), taller buildings with r.c. frames with masonry infill. The damage grades undergone were quantified based on the provisions of the MSK scale, with some additional detailing relying on the developments of (Shebalin 1975), as presented in (Bălan & al., 1982). Based on fast empirical estimates of the fundamental periods of buildings investigated, statistical damage spectra were developed for the 1 km × 1 km squares referred to. The outcome of the project did not lead finally to a new microzonation of the

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City of Bucharest, but it made it possible to derive, on the basis of additional statistical processing, vulnerability functions for the categories of buildings referred to (Sandi, 1986). The availability of factual information about statistical damage spectra and structural vulnerability stimulated also a concern for analytical research in this field, as presented in next subsection.

4.3. Analytical research Following main directions of analytical research were stimulated by the earthquake impact and experience:

a) reconsideration of the concept of seismic intensity; b) analysis of seismic hazard; c) analysis of seismic vulnerability; d) analysis of seismic risk: e) developing ground motion models; f) developing stochastic relations for random vibration; g) philosophy of design accelerograms; h) extension of the capacity design philosophy; i) decision on intervention upon existing structures; j) development of earthquake scenarios. k) studies on earthquake preparedness; l) studies on a strategy of disaster prevention and on disaster prevention activities; m) studies on insurance for natural hazards under the conditions of Romania; n) comparative analysis of disasters; o) forensic engineering studies for historical earthquakes; p) studies on local seismic culture

Seismic intensity. Given the analysis of earthquake effects, which put to evidence the differentiation of earthquake effects severity depending upon the dynamic characteristics of structures affected, it turned out that a revision of the concept of seismic intensity, in order to make it more suitable from the viewpoint of engineering activities is necessary. The alternative approaches developed relied essentially on the use of instrumental data, but attention was paid also to the importance of the concept of seismc vulnerability in relation to the development of macroseismic criteria. Some successive attempts were as follows:

- introduction of destructiveness spectra and tensors (Sandi, 1979) determined on the basis of motion of pendulums with parametric dynamic characteristics, starting from an idea that generalizes Arias' approach based on consideration of ground motion records (Arias, 1970), and definition of intensity on this basis;

- analysis of relationship between seismic vulnerability and seismic intensity (Sandi, 1982), anticipating the developments of the EMS - 98 intensity scale (Grünthal, 1998);

- introduction of spectrum based intensity (Sandi, 1986), using response spectra and introducing also a rule of averaging intensities corresponding to different directions of motion; proposals of revising historical intensities, based on consideration of estimated corner periods of ground motion (Sandi, 1988);

- introduction of a system of alternative instrumental criteria for intensity estimation (Sandi & Floricel, 1998): alternative criteria for global intensities, for intensities related to an oscillation frequency or intensities related to certain frequency bands; determination on this basis of continuous or of discrete intensity spectra (Sandi & Borcia, 2006),

- reconsideration of calibrations adopted in (Sandi & Floricel, 1998), considering the statistical data presented in (Aptikaev, 2005);

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- calculating intensity spectra based on several records obtained in Romania (Sandi & Borcia, 2006).

Seismic hazard. The need to conduct consistent seismic risk analyses raised at its turn the need to use appropriate ways of analysis of seismic hazard. Research was conducted in this connection in several directions:

- development of probabilistic relations (expressed in discrete terms) for determining local hazard, in case of individual structures or of geographically extended networks (Sandi, 1986b);

- determining integral relations for estimate of local hazard (Sandi, 1992) in a way that is equivalent ot the developments of (Mc Guire, 1995);

- develpiing of an attenuation relation generalizing Blake's law (Blake, 1941), introducing the influence of source depth (Sandi, 1992);

- conducting parametric probabilistic hazard analyses for various locations and emphasizing the importance of the attenuation scatter (Borcia & al., 2000), (Sandi & al., 2002a).

Seismic vulnerability. Research in this field was conducted mainly in following directions:

- adaptation of expressions of vulnerabilty developed by the Italian school (Dolce,1984), introducing a generalization able to consider a lower conditional scatter of damage grade (Sandi & Floricel, 1994) and proposing an expression for the average damage grade (Sandi & al., 1990);

- developments related to the evolutionary nature of vulnerability in case of incidence of repeated strong earthquakes (Sandi, 1998);

- development of methodology aimed at considering the conditioning intensities related to a relevant spectral band when calibrating vulnerability (Sandi, 1999).

Seismic risk. Research in this field was conducted mainly ini following directions: - development of probabilistic relations (expressed in discrete terms) for analysis

of seismic risk for individual structures or for geographically extended networks (Sandi, 1985b), (Sandi, 1986b);

- conducting parametric probabilistic risk analyses aimed at putting to evidence the influence of various factors (Sandi & Floricel, 1994).

Ground motion models. In view of providing models required for spatial analyses used in earthquake resistant design, stochastic models were taken into consideration:

- attempts to account for the spatial characteristics of ground motion, developing a model for the 6D motion (3D translational and 3D rotational) of ground, represented as a continuum (Sandi, 1982), (Sandi, 2005);

- developing expressions for the consideration of the simultaneous, non-synchronous, ground motion along the various degrees of freedom of the ground – structure interface (Sandi, 1970), (Sandi, 1975), (Bălan & al. ,1977).

Stochastic relations for random vibration. In view of easing the use of stochastic relations for the analysis of dynamic phenomena, a new („diagonal”) representation for the 2nd order moments of non – stationary random functions was introduced (Sandi, 1989). This is equivalent to the usual representation, but also permits a smooth passage to the limit case of stationary functions. Philosophy of design accelerograms. The use of artificial acccelerograms in engineering activities was analyzed in (Sandi, 1989). A more in depth view on this problem, considering the alternative anticipative representations of ground motions, the spaces in which hazard is

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to be represented, the alternative possible objectives of engineering calculations etc., was dealt with in (Sandi, 2006), (Sandi, 2007). Capacity design philosophy. The experience of the 1977 earthquake put to evidence the fact that, in case of relatively tall and rigid buildings, there should have been quite numerous cases of transient partial uplift of foundations.This uplift appeared not to have unfavourable effects, The institute Proiect Bucureşti, that was monitoring the settlements of tall buildings remarked that post - earthquake uniform settlements of a few centimeters could be observed for these buildings. On the other hand, partial uplift can lead to a limitation of overturning moments corresponding to the fundamental mode and, as a favourable consequence, a limitation of some categories of internal forces / stresses in structures. This leads to the need of revising the appplication of the capacity design phillosophy, including the consideration of performance of the ground - structure interface. Some considerations of chapter V of (Bălan & al. ,1982) and of (Sandi, 1985) are in agreement with the further analyses on this subject, presented in (Pecker, 1998), (Gazetas, 2006). Further considerations on the capacity design philosophy (Sandi 2007) concern the relative levels of protection of nominally structural and nominally non - structural components (including the concern for hazardous equipment)

Decision on intervention. Analysis of risk, conditional upon the time and nature of eventual intervention, upon duration of further exposure and expected frequency of earthquake occurrence was dealt with in (Sandi, 1982c), (Sandi,1983).

Development of earthquake scenarios. The need to develop earthquake scenarios was recognized and preparatory work on this subject was initiated. The cases of Bucharest (Georgescu & Sandi, 1998) and of Romania (Georgescu & Sandi, 2000) were tackled in this connection.

Earthquake preparedness. A concern for earthquake preparedness and for the education of various socio – professional groups and categories was developed. Brochures and posters were developed and distributed. A view on activities of this category is provided in (Georgescu & al., 2006). Strategy of disaster prevention. The need of developing a comprehensive and consistent earthquake protection strategy was recognized and emphasized. An attempt in this relation was presented in (Georgescu & al., 1999), (Georgescu, 2000a,b). Insurance for natural hazards. The features of seismic hazard and risk in Romania were examined from the viewpoint of developing efficient and sustainable insurance activities (Georgescu, 2006).

Comparatiive analysis of disasters. A concern for disaster scaling and comparison for various situations was present in (Georgescu & Kuribayashi, 1992, 1994, 1996, 1998), (Georgescu, 2002c), (Georgescu, 2006). A summary view of the 1997 earthquake impact was presented in (Georgescu & Pomonis, 2007).

Forensic engineering studies for historical earthquakes. The importance of gathering knowledge and estimates concernng the historical earthquakes was recognized. A view on historical earthquakes of Romania was presented in this connection in (Georgescu, 2004).

Studies on local seismic culture. A concern for the development of local culture of earthquake protection on the countryside was developed. This was completed by a

13

summary view on the recent development of earthquake engineering activities (Georgescu, 2002d).

4.4. Research based on strong motion instrumental data

Following main directions of research based on accelerographic data were performed: a) analysis of the features of ground motion: investigation on attenuation and on

the influence of local conditions; b) analysis of the spectral content of motion of structures and of the evolution of

their dynamic characteristics.

A first set of strong motion data of paramount importance was obtained by a few accelerographs having already been installed during the 1977.03.04 earthquake. They referred to the (practically free field) motion at the site of INCERC Bucharest and to the motion at the level of the upper floor of a ten storey building located in Bucharest, abouot 2 km South from INCERC. The INCERC record was of paramount importance: the peak ground acceleration was in the range of 0.2 g, while the dominant period for the strongest (NS) component was in the range of 1.5 s. These data had a direct major impact on the earthquake resistant design code of Romania. The basic design acceleration was modified, from conventional, low and non-realistic values to values in the range of expected peak ground accelerations and a corresponding new seismic zonation was introduced. On the other hand, the expression of the dynamic factor prescribed by the code was strongly modified, in a way to represent a kind of envelope for the INCERC record of 1977.03.04. The wealth of accelerograms obtained during the 1986.08.30, 1990.05.30 and 1990.05.31 earthquakes raised the need of revising the code dynamic factor and to introduce a two parameter zonation, with respect to the reference acceleration and to the velocity / acceleration corner period respectively (MLPAT 1992). The analysis of the set of accelerograms at ground level due to the successive Vrancea earthquakes made it possible to conduct statistical analyses on attenuation and to also investigate the weight of contributions of source mechanisms and of local geological conditions to the spectral features of ground motion. The attenuation was analized in relation to several parameters: PGA, PGV, PGD, EPA, EPV and spectrum based intensities. A first step (Sandi, Floricel 1995) was related to the attenuation irrespective of direction, as well as to the directionality, irrespective of the spectral band. A second step (Sandi & al. 2004) repeated the first step and added an analysis of attenuation in terms of directionality for various spectral bands. It turned out that the rate of attenuation was strongly variable, that the directionality was variable too and that there were important differences between the features of attenuation for various spectral bands. The contributions of source mechanisms and of local conditions to the spectral features of ground motion were analyzed by considering the set of records as a whole. Locations with a trend to stability of dominant periods, as well as locations with a trend to variability were identified (Sandi & al. 2004). The importance of existence of a strong contrast of S-wave propagation velocity at a relatively small depth in order to provide spectral stability was put to evidence. The analysis of the set of accelerograms obtained on some buildings (at upper floor level) made it possible to put to evidence the variation of the dominant frequencies during one strong earthquake and from one earthquake to the other (Sandi & al. 2002).The results obtained were also compared with the results of analysis of ambient vibration (see next subsection).

4.5. Experimental field research

14

Some data on the monitoring of ambient vibration are referred to at this place. Activities in this field started shortly after 1960 (Sandi, Şerbănescu 1969), (Sandi & al. 1969). The direction of interest at this place is that, of monitoring the evolution of stiffness of some structures in relation to the incidence of strong ground motion. The information provided by the monitoring of ambient vibration, performed at various moments in some cases pre – and post – earthquake, prre – and post - intervention, combined with the information provided by strong motion floor accelerograms, made it possible to obtain relevant information on the stiffness evolution determined by the incidence of earthquakes and by the intervention of man. A summary view on these activities, given in (Sandi & al. 2002), puts to evidence some main features of this evolution. 4.6. Research on the seismicity of Romania

Research on the seismicity of Romania, from the viewpoint of engineers, was strongly stimulated by the incidence of the earthquake, as well as by the improvement of instrumentation. The main directions of research in this field were:

- analysis of seismic activity at source level; - analysis of attenuation; - analysis of recurrence of seismic action at site level, for various locations; - analysis of the spectral contents of ground motion, including the analysis of the

contribution of source mechanism and of site conditions upon the spectral contents of ground motion.

The main results obtained are referred to in Section 6.

5. A BRIEF VIEW ON THE DEVELOPMENT OF THE REGULATORY BASIS OF

EARTHQUAKE RESISTANT DESIGN

The 1940.11.10 earthquake (MGR = 7.4, Mw = 7.7) represented a first destructive seismic event having affected Romania during its period of modern development, corresponding to introduction of engineered structures. This event represented a first incentive to develop regulations for the earthquake protection of buildings. The activities in this field developed gradually, in parallel with the accumulation of knowşedge and know how in this field. A tabular view on the development of regulations is given in Table 5.1. A more detailed, critical, view, on the development of the regulatory basis in Romania, is presented in (Vlad, 2007). Table 5.1. A schenatic view on the development of earthquake resistant design regulations

No. Year Document Comments, main features of documents 1 1945 Circular of Ministry

of Public Works Short text, equivalent static model. Compulsory for public works

2 1950 … 1960

Drafts of earthquake resistant design code.

Attempts of introducing up to date knowledge and know how. Not officially endorsed, but used in design practice in parallel with Soviet code PSP.101.

3 1952 First seismic zonation standard, STAS 2923 - 52

Corresponding mainly to intensities observed during the 1940.11.10 earthquake, implicitly considered maximum intensities.

4 1963 New seismic zonation standard, STAS 2923 - 63

Same philosophy. Intensity in Bucharest dropped from VIII to VII, due to administrative pressure.

15

5 1963 First earthquake resistant design code relying on dynamic theory, P.13 - 63.

Low basic coefficients (implicitly including reduction factors), as in Soviet code of the time. Dynamic factor taken also from Soviet code. Working conditions factors (usually > 1.0) prescribed. Little concern for ductility.

6 1970 Revision of previous code, intended to reduce material consumption, P.13 - 70.

Limited modification of basic coefficient (renouncing at geometric ratio 2.0). Modification of expresssion of dynamic factor (reduction of maximum value from 3.0 to 2.0). More emphasis on ductility.

7 1977 New zonation standard, STAS 11100 - 77. Existing microzonation standards out of force.

Revision of zonation, based on observed intensities. Intensity for Bucharest increased from VII to VIII, as before 1963.

8 1978 First post -earthquake code. Intended to provide reasonable earthquake safety, P.100 - 78

Radical modifications of previous code. Basic accelerations close to estimated PGA. Dynamic factor based on response spectrum derived for INCERC record. Emphasis on ductility, introduction of quantitative criteria.

9 1981 Modification of previous code, as P.100 - 81.

Revision of ductiity criteria. Introduction of 3D design methodology, based on stochastic ground motion model.

10 1991 Revision of zonation standard, as STAS 11100 -91

New philosophy, accepting explicitly prescribing intensities for explicit return periods (for Vrancea earthquakes: 50 years; for crustal earthquakes: ≥ 100 years). Zonation relying also on instrumental data obtained during the 1986.08.30, 1990.05.30 and 1990.05.31 earthquakes. Zonation in intensities made compatible with zonation in design parameters prescribed in next code.

11 1992 Modification of previous code, as P.100 - 92.

Revision, using mainly instrumental data obtained during the 1986,08,30, 1990.05.30 and 1990.05.31 Vrancea earthquakes, as well as during the 1991 Banat earthquakes. Two - parameter zonation. Sections on design of equipment, on evaluation and strengthening of existing structures introduced.

12 1996 Revision of sections of previous code concerning existing structures

13 1995 … 2000

Drafts of codes or parts of codes for other structures.

Drafts intended basically to cover topics dealt with by parts of Eurocode EC-8.

14 2006 New code, P.100 - Intended to represent a transition to EC - 8,

16

06. Part I. The activities of earthquake resistant design regulations encompassed also other activities, of less wide interest. One could mention here documents like a standard for industrial equipment or a code for hydrotechnical structures.

6. A VIEW ON THE SEISMIC CONDITIONS RELATED TO THE ACTIVITY OF THE

VRANCEA SEISMOGENIC ZONE

6.1. General

A brief view on some main features of the seismic conditions of Romania is dealt with at this place. This encompasses some data and considerations concerning the activity at source level, the spectral content of ground motion and the features of attenuation. Given the general orientation of the paper, the developments are focused on the seismic conditions corresponding to the Vrancea Seismogenic Zone. The approach is related, according to the understanding of the authors, to the needs of engineering activities.

6.2. Seismic Activity at Source Level

As it well known (Bălan & al. 1982), the seismicity of Romania is determined on one hand by the intermediate depth source zone Vrancea (referred to as [VSZ]) and on the other hand by several crustal zones, out of which the most important are those of the Făgăraş mountains and of the Banat region. A look at earthquake catalogues starting with the year 984, like those of (Constantinescu & Mârza, 1980), updated or of Radu (Bălan & al., 1982), puts to evidence the sustained, practically stationary, [VSZ] activity. It turns out that, for engineering purposes, it is appropriate to adopt a Poissonian [VSZ] magnitude recurrrence model. On the other hand, the activity of crustal zones, which is less sustained, raises difficulties for the development of stochastic recurrence models. Examples: the clusters of relatively strong earthquakes having occurred in the Banat zone during the second half of 1991 (magnitudes close to 6.0), or the cluster of earthquakes having occurred around 1900 in the Şabla source zone located close to Romania, South – East (maximum magnitude: 7.2, in 1901). According to (Bălan & al., 1982), the [VSZ] releases in the average, per century, more than 95% of the total energy released inside the territory of Romania. Its most relevant activity occurs at intermediate depth (about 60 to 180 km deep). Given the stationarity of the [VSZ] activity, put to evidence by the catalogues referred to, the activity of the 20th century, during which magnitudes were determined on an instrumental basis, appears to be relevant in the longer run (note that, according to Radu’s catalogue of (Bălan & al., 1982), there are quite strong random deviations between magnitudes determined on an instrumental basis and those determined on a macroseismic basis; since magnitudes determined on an instrumental basis are more credible, they were adopted as a startpoint for the analysis of magnitude recurrence. The most important recent intermediate depth earthquakes were those of 1940.11.10 (MGR = 7.4), 1977.03.04 (MGR = 7.2), 1986.08.30 (MGR = 7.0), 1990.05.30 (MGR = 6.7), 1990.05.31 (MGR = 6.1). The strongest earthquake recorded historically is that of 1802 (various magnitude estimates: 7.5 to 7.7). A limiting magnitude lies, according to various authors (Constantinescu & Mârza, 1980), (Mârza & al., 1991), Radu (Bălan & al., 1982), somewhere at the level MGR = 7.8 ... 8.0. The [VSZ] activity recorded during the 20th century is represented in graphic terms (for MGR

≥ 6.), according to data of the catalogue (Constantinescu & Mârza, 1980), updated, in Fig. 6.1. The recurrence of earthquakes of various magnitudes MGR ≥ 6. is represented (in graphic terms too) in Fig. 6.2. The analytical expressions determined for the recurrence of magnitudes were

17

lg NM (m, 1 yr.) = 3.4 - 0.7 m (6. ≤ m < 7.) (6.1) and, alternatively, for the two asymptotic branches of the figure, lg NM (m, 1 yr.) = 0.3 - 0.2 m - 0.32 / (7.8 - m) (7. ≤ m < 7.8) (6.2a) lg NM (m, 1 yr.) = 0.4 - 0.2 m - 0.5 / (8. - m) (7. ≤ m < 8.) (6.2b) Several authors put to evidence the tendency to cyclicity of occurrence of strong [VSZ] earthquakes. This tendency made possible forecasts of a next major earthquake. According to (Constantinescu & Enescu, 1985), a next strong event should occur in 2004 ± 4 years (the year 2004 was postponed subsequently by the second author by a few years). A statistical analysis presented in (Sandi & Mârza, 1996) led to a forecast in probabilistic terms. The windows with 20% exceedance probabilities in any sense are: [> 2001, < 2013] for the time (expected year: 2007), and [≤6.8, <7.6] for the magnitudes (expected magnitude: 7.2). The average cyclicity period determined is of 34 to 35 years (see Fig. 6.1).

Fig. 6.1. History of [VSZ] magnitudes Fig. 6.2. [VSZ] magnitude recurrence

after 1900 characteristics, derived on the basis of data of Fig. 6.1 6.3. Activity at Site Level The analysis of seismic activity ay site level is of direct importance for engineers. In order to control risk and safety, the most desirable approach (when feasible) is of probabilistic nature. On the other hand it must be recalled that data of observation at site are never sufficiently rich and certain in practice, in order to make it possible to derive directly on this basis recurrence laws for engineering design parameters. Therefore, the way to derive recurrence laws for the reference parameters is indirect, in the sense that one has to combine the magnitude recurrence laws for the system of sources potentially affecting a definite site with corresponding attenuation laws. The equivalent ways developed in (Sandi, 1992) and (Mc Guire, 1995) are to be used to this purpose. In order to determine recurrence laws at site level, following approach was used:

- the parameter quantifying the earthquake severity at source level was MGR; - the parameter quantifying the severity of ground motion at site level was the

intensity (that could be, in an equivalent way, IMSK, IEMS or the spectrum based intensity IS, as developed in (Sandi, 1986a), (Sandi & Floricel, 1998);

- the attenuation law used was developed in (Sandi, 1992), while it generalizes Blake’s classical law (Blake, 1941);

- in order to account for the expected directionality of radiation in case of Vrancea earthquakes, an artificial correction to the epicentral distance was introduced, replacing circles of equal expected intensity by ellipses having the same area;

18

- the passage from intensity to ground motion parameters like EPA or EPV, was based on developments of (Sandi, 1986a), which consider the influence of velocity / acceleration corner periods;

- the computation of intensity recurrence characteristics was performed in a parametric way, considering alternatively different values of the attenuation scatter characteristic: r.m.s. deviations of the intensity ranging from 0 to 2.0 intensity units.

Note: The approach based on the use of a unique scalar parameter in order to characterize the ground motion severity cannot lead to a full characterization of hazard at site, since various ground motions occurring at a definite site may differ not only by parameters like intensity or amplitude of motion, but also by parameters characterizing the spectral contents, the duration of motion etc.. A discussion on this subject is presented in (Sandi, 2006) and (Sandi, 2007). To illustrate the results of local hazard analysis, plots concerning the results on local hazard obtained for two locations (Bucharest and Focşani, the latter one in the epicentral zone) are presented in Fig’s 6.3 and 6.4. The importance of the assumption on the r.m.s. deviation from the average attenuation law is obvious. Neglecting this factor would lead to coarse underestimate of local hazard.

Fig. 6.3. Intensity recurrence for Bucharest Fig. 6.4. Intensity recurrence for Focşani The outcome of parametric calculations for Bucharest is summarized in Table 6.1. Table 6.1. Return periods of various intensities, depending upon assumptions on limiting

magnitude Mlim, directionality of radiation, ca, and assumption on attenuation scatter, σI

Mlim = 7.8 Mlim = 8.0 I

ca = 1.0 ca = 1.2 ca = 1.0 ca = 1.2 I

σI = 0.5

σI = 1.0

σI = 0.5

σI = 1.0

σI = 0.5

σI = 1.0

σI = 0.5

σI = 1.0

7.0 20.2 17.5 19.0 16.6 20.2 17.4 19.0 16.6 7.0 7.5 33.2 26.3 30.9 24.8 32.7 26.0 30.6 24.6 7.5 8.0 62.5 43.2 56.9 40.3 52.9 42.1 54.1 39.4 8.0 8.5 152.5 78.0 135.0 72.0 131.3 74.4 117.4 68.9 8.5 9.0 554.8 156.0 475.6 144.1 408.3 145.8 354.4 133.4 9.0 9.5 3,421.0 366.5 2,832.0 330.0 2,003.0 323.9 1,881.0 292.8 9.5 10.0 41,030. 993.5 32,640. 882.5 17,610. 833.3 14,050. 743.8 10.0

19

The data of this table put to evidence the fact that, out of the parameters that varied in the frame of this parametric approach, the influence of the assumption on the attenuation scatter is of paramount importance. It turns out that highest attention should be paid to this factor. A summary view on intensities with various return periods expected at several relevant locations is given in Table 6.2. Table 6.2. Intensities with various return periods, assuming Mlim = 7.8 and directionality

factor 1.2, at various locations affected by Vrancea earthquakes

Locations Return periods (years) Bucharest, Vaslui ConstanŃa Buzău, Câmpina,

Focşani, Oneşti, Ploieşti, Văleni

Return periods (years)

50 7.9 ….8.0 6.4 8.4.8.6 50 100 8.3 ….8.4 6.8.6.9 8.8.9.1 100 200 8.6 ....8.7 7.1.7.2 9.1.9.4 200 300 8.8….8.9 7.3.7.4 9.3.9.5 300 500 9.0….9.1 7.6 9.5.9.7 500

6.4. Spectral Contents of Ground Motion

The availability of rich strong motion instrumental information due to the incidence of the strong Vrancea earthquakes of 1977.03.04, 1986.08.30, 1990.05.30 and 1990.05.31 made it possible to obtain a comprehensive picture of the spectral contents of ground motion due to Vrancea earthquakes. To illustrate the data obtained, the sequences of response spectra of absolute accelerations, determined along twelve azimuthally equidistant horizontal directions are presented in Fig’s 6.5 and 6.6. (note: the technique of determining spectra along more than 2 horizontal directions, which offers a more comprehensive picture of the content of ground motion, was introduced in (Stancu & Borcia, 1999)).

Looking at the spectral information at hand, it turned out that it is possible to roughly divide the recording stations into two categories (Sandi & al., 2004b):

a) stations for which one can observe a trend to stability of the spectral contents from one event to the other;

b) stations for which one can observe a trend to significant variability of the spectral contents from one event to the other.

Sequences of response spectra determined for stations of category (a) are reproduced in Fig. 6.6, while homologous sequences for stations of category (b) are reproduced in Fig. 6.5. To provide a further support for following comments, the sets of response spectra for the records obtained inside Bucharest are presented, in a same order, in Fig. 6.7 for the event of 1986.08.30, and in Fig. 6.8 for the event of 1990.05.30. Some main remarks on the system of response spectra are as follows:

- there is sometimes important non-proportionality between amplitudes of spectra corresponding to different stations, from one event to the other (illustrative examples given in Fig’s 6.5 and 6.6: even in Bucharest, according to Fig. 6.5, there is non-proportionality of spectral amplitudes, from one event to the other, between the stations INC and CRL, to which the first two columns pertain);

- there were important differences of the spectral contents (dominant frequencies, corner frequencies etc.) from one recording station to the other during a same event, as well as (for some stations) from one event to the other (illustration: especially the data of Fig. 6.5);

20

- in spite of the fact that the ground conditions are the same for all directions of oscillation, there are important differences between spectral ordinates corresponding to different directions for same event and place (the extreme ratios of ordinates reach, or even exceed, for some oscillation periods, the threshold 3.0, as illustrated in Fig.’s 6.5 and 6.6);

- when considering sequences of strong motion records due to the sequence of strong earthquakes referred to, a quite systematic positive correlation between magnitude and velocity / acceleration corner periods was observed for the recording stations located in Bucharest (Sandi & al., 2004b), but this correlation failed for some recording stations located in several other towns of the most severe seismic zones of Romania (Borcia & al., 2000);

- in some other cases, a positive correlation between local ground motion intensity and dominant periods could be observed (example: the Cernavodă – Town Hall station, Fig. 6.6).

The examination of the sequences of spectra of Fig’s. 6.5 and 6.6 makes it possible to emphasize following additional facts:

- for the Bucharest – INCERC station: while the dominant periods were unusually long (around 1.5 s) in 1977, they became relatively short in 1986 (when the long period spectral peaks of more than 1.0 s got a secondary importance) and totally disappeared in 1990;

- for the Focşani and Râmnicul Sărat stations: there was a strong variation of the shape of response spectra, while the longer period components (almost absent at Bucharest – INCERC in 1990) appeared to have been radiated rather towards those latter sites;

- for the Cernavodă - Town Hall station: there was a quite strong tendency to stability of the spectral contents of ground motion, while the dominant periods were rather short (around 0.4 s); on the other hand, in case one considers the sequence of data at hand, there was a trend to positive correlation between local intensity and dominant motion periods.

A basic remark, relying on data on the geological columns at various recording sites, may be formulated as follows:

- for sites with a strong trend to variability of the spectral contents, there is no strong contrast for S wave propagation velocities at small depth (there is a tendency of gradual increase of this velocity up to important depths);

- for sites with a trend to stability of the spectral contents, a strong contrast for S wave propagation velocities appears at small depths (in the sense of sudden downward increase of velocity at a depth of a few tens of meters).

Microtremors were recorded at some relevant sites too (Sandi & al., 2004b). Their examination permits to derive some conclusions as follows:

- in case of the Bucharest – INCERC site the RFS shape corresponds continually to chaotic motion, with little evidence of dominant frequencies; there are intervals of time for which there appear quite important spectral peaks for very low frequencies (in the range of 0.15 … 0.2 Hz), which correspond to magnified spectral components observed for remote earthquakes (of Asia or Mexico) and which were present, to some extent, also for Vrancea earthquakes (especially for displacements); there are no relevant spectral peaks to correspond to the main spectral peaks for the strong motions recorded (Fig. 6.5, first column);

- in case of the Cernavodă – Town Hall site the RFS shape puts to evidence time segments for which the motion is chaotic, but also segments for which there appear strong spectral peaks (with a frequency of about 2.4 Hz, corresponding

21

very well to the strong spectral peaks of Fig. 6.6, last column); there are thus intervals of time for which there appear strong and sharp spectral peaks for a frequency coinciding with the (stable) predominant frequency observed for the strong motion records; there are also intervals of time for which there appear quite important spectral peaks for very low frequencies (in the range of 0.2 … 0.25 Hz), which would correspond to the fundamental frequencies of geological packages of a thickness in the range of thousands of meters);

- it may be thus stressed that in both cases (more obviously in case of the Bucharest – INCERC site) one may remark, especially for the displacements, very low frequency peaks, which are in accordance with the spectral features observed for remote (especially extra-European) earthquakes.

In order to contribute to the explanation of the facts referred to, transfer functions were determined for dynamic systems corresponding to simple models of the geological columns characterizing some sites dealt with. Since it was not clear at what depth to postulate the presence of base rock, computations were conducted in a parametric way, considering alternative total geological package thicknesses. Some results and remarks corresponding to the sites of Bucharest – INCERC (first column of Fig. 6.5) and of Cernavodă – Town Hall (last column of Fig. 6.6), referred to as reference cases, are as follows (Sandi & al., 2004b):

a) in case of the Bucharest – INCERC site: - the shape of the transfer function changes considerably when the number of

layers considered changes; a certain trend to stability of the transfer function shape appears in case one considers more than four layers (when the interface with stronger contrast at a depth of 600 m, after the fourth layer, is exceeded);

- in case one considers a deep geological package, one remarks that the transfer function has several main peaks of comparable importance, which means that, in this case, the spectral characteristics of ground motion will be determined primarily by the features of the input disturbance;

- the peaks of the transfer function (numbered leftwards) in case of considering all eight layers, up to a depth of 2800 m, present an interesting correspondence with instrumental data at hand: the first peak (period: ∼ 5.7 s) corresponds fairly to the dominant periods of some 6 s, observed for records of remote earthquakes and to the frequencies of 0.15 … 0.2 Hz of the peaks of Fig. 10 (left column); the fourth peak (period: ∼ 1.5 s) corresponds fairly to the main spectral peak of Fig. 3 for the 1977 event; the last important peak to the left (period: ∼ 0.7 s) corresponds fairly to the main spectral peaks of Fig. 3 for the 1986 and 1990 events respectively.

b) in case of the Cernavodă – Town Hall site: looking at the plots derived for this site, where a strong contrast of S – wave velocity exists at a small depth in the range of 20 … 30 m, for alternative assumptions on the thickness and S – wave velocity of the lower layer, one may remark:

- the stability of the oscillation periods corresponding to the transfer function peaks; this represents an explanation of the trend to stability put to evidence by the sequence of response spectra at hand;

- a quite stable fundamental period that is very close to the dominant periods of the last column of Fig. 6.6.

A complementary look at the features of ground motion is provided by the consideration of the response spectra corresponding to various recording stations of Bucharest. The spectra corresponding to the earthquake of 1986.08.30 (arranged in a sequence that is close to a geographic map) are presented in Fig. 6.7. The spectra corresponding to the event of 1990.05.30, arranged in the same sequence, are presented in Fig. 6.8. Looking at these latter figures, it turns out that the changes in the spectral content of records of Bucharest –

22

INCERC occurred, practically synchronously, for all recording stations. One can conclude that the changes were due to remote factors (source mechanism, long distance wave travel path) and that the local ground conditions played a secondary role in determining the spectral features of ground motion. To provide some additional information, the intensity spectra defined as in (Sandi & Floricel, 1998), determined for the same stations and events (Sandi & Borcia, 2006) are given in Fig’s 6.9 and 6.10 respectively. In case one agrees with the sense of intensity spectra, these figures provide information on the severity of earthquake action, differentiated for various categories of structures. The sequence of data of Fig's. 6.7 to 6.10 is as in Table 6.3. Table 6.3. Sequence of stations of Figures 6.7 to 6.10 TIT – Titulescu EXP – ROMEXPO INC – INCERC MLT – Militari CRL – Carlton BLA – Balta Albă PND - Panduri MET - Metalurgiei MTR – Metro Berceni

6.5. Features of Attenuation

The availability of rich instrumental information obtained during the earthquakes of 1986.08.30, 1990.05.30 and 1990.05.31 made it possible to conduct an in depth investigation of the features of attenuation. This analysis was performed at two levels:

- analysis of attenuation in global and directional terms (Sandi & Floricel, 1995); - extension of analysis to consideration of the spectral aspects (Sandi & al.,

2004a). The parameters to which attenuation analysis was related were: PGA, PGV, PGD, EPAS, EPVS, IS (global spectrum based intensity) and is.(ϕ', ϕ") (spectrum based intensities related to various frequency bands (ϕ', ϕ")), as defined in (Sandi & Floricel, 1998). A first summary view on attenuation, irrespective of directionality, is given in Fig. 6.11. The rows correspond to the successive events referred to, while the columns correspond respectively to the global intensity IS and to the frequency band related intensities is

~62 (0.5

Hz, 1.0 Hz), is~

63 (1.0 Hz, 2.0 Hz), is~

64 (2.0 Hz, 4.0 Hz) and is~

65 (4.0 Hz, 8.0 Hz). A more detailed view on attenuation is given in Fig. 6.12, where azimuthal directionality of attenuation is put to evidence in spectral terms. The columns correspond here to the three successive events, while the rows correspond to the global intensity IS and to the frequency band related intensities is

~62 (0.5 Hz, 1.0 Hz), is

~63 (1.0 Hz, 2.0 Hz), is

~64 (2.0 Hz, 4.0 Hz) and

is~

65 (4.0 Hz, 8.0 Hz). The closed curves presented refer to the distances (depending on azimuthal angle) up to which ground motions of the intensities 7.0, 6.0 and 5.0 respectively, were radiated. The examination of the figures presented, besides the earlier results of (Sandi & Floricel, 1995), makes it possible to formulate following remarks:

- unexpectedly (since the source was deeper), the attenuation was faster for the event of 1986.08.30 (h = 133 km) than for the events of 1990.05.30 (h = 89 km) and 1990.05.31 (h = 79 km);

23

- the scatter of various parameters, expressed in units that are comparable with intensity, was high, ranging for the various parameters (in equivalent terms) from about 0.6 to 1.0 intensity units (for events taken individually);

- the general scatter trend (expressed in terms of the comparable units referred to) tended to be highest for peak ground motion parameters PGA, PGV and PGD¸ lower for peak spectrum related values EPAS and EPVS and lowest for the intensity IS;

- the directionality of radiation was strong in all cases; - while the radiation tended to be rather symmetrical for the first two events, it was

strongly non-symmetrical for the last one; - the radiation directionality was different for the three events considered:

approximately NE-SW on 1986.08.30 (as observed on macroseismic basis for the destructive events of 1940.11.10 <MGR = 7.4> and of 1977.03.04 <MGR = 7.2> too), N-S on 1990.05.30 and E on 1990.05.31);

- there were significant differences from the viewpoints of rate of attenuation and directionality for the various frequency bands considered.

24

1977.03.04

MGR = 7.2

771INC Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

?

?

?

1986.08.30

MGR = 7.0

861INC Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861CRL Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861FOC Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861RMS2 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

1990.05.30

MGR = 6.7

901INC Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901CRL Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901FOC1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901RMS2 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

1990.05.31

MGR = 6.1

?

902CRL Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

902FOC1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

902RMS2 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

Stations

→ a) Bucharest – INCERC

(INC) b) Bucharest – Carlton

(CRL) c) Focşani

(FOC) d) Rm. Sărat 2

(RMS)

Fig. 6.5. Response spectra (absolute accelerations, 12 directions) for stations of category (b)

25

1977.03.04

MGR = 7.2

? ? ? ?

1986.08.30

MGR = 7.0

861ONS1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861VLS1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861BAA1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861CVD1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

1990.05.30

MGR = 6.7

901ONS1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901VLS1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901BAA1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901CVD1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

1990.05.31

MGR = 6.1

902ONS1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

902VLS1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

902BAA1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

902CVD1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

Stations

→ a) Oneşti

(ONS) b) Vaslui

(VLS) c) Baia – Dobrogea

(BAA) d) Cernavodă

(CVD)

Fig. 6.6. Response spectra (absolute accelerations, 12 directions) for stations of category (a)

26

861TIT1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861EXP1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861INC1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861MLT1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861CRL1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861BLA1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861PND1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861MET1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

861MTR1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

Figure 6.7. Response spectra along twelve azimuthally equidistant horizontal directions, determined for records obtained inside Bucharest, on 1986.08.30

901TIT1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901INC1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901MLT1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901CRL1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901BLA1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901PND1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901MET1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

901MTR1 Absolute Acceleration Response Spectra n=5%

0

1

2

3

4

5

6

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 T (s)

m/s

2

0°N

15°

30°

45°

60°

75°

90°E

105°

120°

135°

150°

165°

Figure 6.8. Response spectra along twelve azimuthally equidistant horizontal directions, determined for records obtained inside Bucharest, on 1990.05.30

27

Fig. 6.9. Averaged intensity spectra is~(ϕ', ϕ") (red) and id

~(ϕ', ϕ") (blue) for 6 dB intervals, for the records obtained in Bucharest, on 1986.08.30

Fig. 6.10. Averaged intensity spectra is~(ϕ', ϕ") (red) and id

~(ϕ', ϕ") (blue) for 6 dB intervals, for the records obtained in Bucharest, on 1990.05.30

28

IS is

~62 (0.5 Hz, 1.0 Hz)

is~

63 (1.0 Hz, 2.0 Hz)

is~

64 (2.0 Hz, 4.0 Hz)

is~

65 (4.0 Hz, 8.0 Hz)

1986.08.30, MGR = 7.0

1990.05.30, MGR = 6.7

1990.05.31, MGR = 6.1

Fig. 6.11. Regression lines for various events and frequency bands

6.6. Addenda

The developments presented provide a quite comprehensive view on the features of seismicity due to the [VSZ], that are relevant for engineering activities. They were possible, among other, due to the methodological features of research conducted in this field, as well as to the progress of accelerographic instrumentation provided by the foreign aid.

29

1986.08.30, MGR = 7.0 1990.05.30, MGR = 6.7 1990.05.31, MGR = 6.1

IS

is

~62

is

~63

is

~64

is

~65

Fig.6.12. Directionality of attenuation, for various events and frequency bands (common scale, up to epicentral distance of 1000 km)

30

7. A VIEW ON THE VULNERABILITY OF STRUCTURES. SOME ANALYTICAL

CONSIDERATIONS ABOUT STRUCTURAL VULNERABILITY

7.1. General

A first incentive to deal with vulnerability of structures was due to the setting up for the period 1989 – 1983 of the UNDP/UNESCO Balkan Project RER/79/014, “Earthquake risk reduction in the Balkan region”, subsequently to the destructive earthquakes of Romania (1977) and Yugoslavia (1979). Among the five working groups of the project (that were shared by five countries participating in the project), one was devoted to seismic vulnerability and was coordinated by H. Sandi (Sandi, 1984a,b). The work performed was of analytical nature and, also, of statistical nature. Data on vulnerability were gathered from Bulgaria, Greece, Romania, Turkey and Yugoslavia. This work was continued and extended in the frame of a working group organized at European scale by the European Association for Earthquake Engineering (Sandi, 1986b), (Sandi & al, 1990). Further on, a research project at European scale, referred to as ENSeRVES (European Network for Seismic Risk, Vulnerability and Scenarios), was organized for the period 1997 – 2001. The latter project was coordinated by M. Dolce (Italy). The INCERC group was active in this project too. Another direction of vulnerability analysis presented in (Colban & al, 1999). This direction was based on the determination of the ratio R, of actual resistance of a building, to resistance required by codes. Some references to methodologies adopted in this latter frame are given in (Sandi & al., 2007). An aspect of major practical importance is represented by the ability of engineers to predict the real performance of structures subjected to strong seismic action or, in a complementary way, to explain the observed performance of structures during strong earthquakes. Unfortunately, most of the engineers could use for this purpose just the methods of analysis accepted in the code for the conventional design of new buildings, which relied on the consideration of reduced seismic forces, for which structures had to be kept in the elastic stage. A quantitative approach used in this connection was to determine a ratio R, of actual resistance to resistance of new buildings required by the code in force. Buildings of various categories were to be considered in an acceptable shape in case the values obtained for R were equal to 0.5, 0.6 or 0.7. One obtained unfortunately in evaluations for practice quite often much lower values of R even for buildings that obviously withstood strong earthquakes with quite limited damage. The consequences were sometimes quite grave, since they distorted a necessary order of intervention upon existing structures. Among the attempts of correcting this major shortcoming, one could consider the approach proposed in (Mironescu & Bortnowschi, 1983). It is obvious that sustained efforts towards the development of methods of practical use able to predict the real performance of structures subjected to strong seismic action are necessary. Following developments of this section are related to some analytical developments concerning the vulnerability of structures. The authors believe that this look is of interest for providing the necessary basis of use of vulnerability concepts and data.

7.2. Some Analytical Aspects on Seismic Vulnerability

Seismic vulnerability of structures means, in qualitative terms, their proneness to earthquake inflicted damage. This vague definition / characterization must be, of course, detailed, in order to set up a framework of use of vulnerability concepts and data in engineering activities. The observation of earthquake effects puts to evidence the randomness of their severity, and this leads to the idea of using probabilistic tools for

31

vulnerability analysis and use. The framework of vulnerability definition / specification relies (in the simplest possible case) on following main coordinates:

- specification of the category of elements at risk (more specifically, structures), S

(I), considered for vulnerability analysis; - quantification of ground motion severity (usually in terms of a scalar parameter Q

considered as a random variable, that can take continuous values q. or discrete values qj); the sense of the parameters qj is subject to further discussion (see also (Sandi & al., 2007));

- quantification of damage severity (usually in terms of a scalar parameter D considered as a random variable that can take continuous values d. or discrete values dk); a frequently used way of quantifying the damage grades is dk that, of referring to the damage grades defined and described by macroseismic scales like MSK or EMS;

- characterization of distribution of situations, in a probabilistic sense (assuming now discrete formulation) with respect to damage grades dk, conditional upon the nominal severity of seismic action qj, by means of the system of conditional probabilities p(vi)

k/j (referred to often in literature as damage probability matrices).

A standard expression for the probability matrices p(vi)k/j, used by the Italian school (Dolce,

1984) relies on the classical binomial distribution,

b (k, n, dj~) = { n! / [k! / (n – k)!]} (dj

~/ n)k(1 – dj~/ n)n-k (7.1)

( - k: discrete index of current damage grade: integer, where 0 ≤ k ≤ n;

- n: maximum value of k, which is equal to 5, in agreement with the EMS scale; - dj

~ = d~(qj): expected damage grade for an intensity q = qj, where 0 ≤ dj~

≤ n), Note that the mean value of the distribution b (k, n, dj

~) is dj~, while the corresponding

variance is dj~ × (n - dj

~ ) / n. The comparison of empirical conditional damage distributions obtained in Italy subsequently to the destructive Irpinia earthquake of 1980.11.24 with this analytical expression put to evidence a fair fitting (Sandi, 1986b). The expression of the system p(vi)

k/ becomes, on this basis,

p(v)

k/j = b (k, n, dj~) (7.2)

An analytical expresssion proposed for the expected damage grade d~(q), based on

developments of (Sandi & al. 1990) is d

~(q, qd, qs) = (n/2) × {1 + tanh [(q – qd) / qs]} (7.3) where n and q are the same as before, qd is a parameter close to the design intensity (eventually slightly higher) and qs is a measure of the scatter, varying from about 1.5 for relatively ductile structures to about 2.5 for relatively brittle structures (see in Fig. 7.1 a plot for the case qd = 8.0, qs = 2.0). The statistical damage survey conducted after the earthquake of 1977.03.04 made it possible to derive, for eight classes of buildings, empirical distributions p(v)

k/j, represented in graphic terms in Fig. 7.2 (Sandi, 1986b). Note that the roles of axes are changed from one

figure to the other. The examination of the plots of Fig. 7.2 shows that the variance is lower than that, predicted by the analytical expression (7.3). This was due, most likely, to the fact

32

that the samples surveyed in Bucharest were more homogeneous than those investigated in Italy after the 1980 earthquake.

qb=8., qs=2.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

6 6.5 7 7.5 8 8.5 9 9.5 10q

d~(q)

Fig. 7.1. Average damage grade, as a function of intensity q, for qd = 8-0 and qs = 2.0

In order to cope with the discrepancy between the analytical expression (7.3) and the outcome of surveys, the use of the binomial expression (7.1) was adapted while carrying out parametric risk analyses (Sandi & Floricel, 1994), in the following way:

- the parameter n was kept further on as n = 5; - an auxiliary value n' = 3 was introduced; instead of using a reference binomial

distribution b (k, 5, dj~), a binomial distribution b (k, 3, dj

~) was used, adopting following expressions for the conditional distributions p(v)

k/j: a) for dj

~ ≤ 1: for k ≤ 3, p(v)

k/j = b (k, 3, dj~)

for k > 3, p

(v)k/j = 0

b) for 1 < dj~ < 2: for k ≤ 3, p

(v)k/j = (2 - dj

~) b (k, 3, dj~) + (dj

~ - 1) b (k - 1, 3, dj~ - 1)

p

(v)4/j = (dj

~ - 1) b (3, 3, dj~ - 1)

p

(v)5/j = 0

c) for 2 ≤ dj

~ ≤ 3: p

(v)1/j = 0

for 1 ≤ k ≤ 4, p

(v)k/j = b (k - 1, 3, dj

~ - 1) (7.4) p

(v)5/j = 0

d) for 3 < dj

~ < 4: p(v)

0/j = 0

p(v)

1/j = (4 - dj~) b (0, 3, dj

~ - 1) for k ≥ 2, p

(v)k/j = (4 - dj

~) b (k - 1, 3, dj~ - 1) +

+ (dj

~ - 3) b (k - 2, 3, dj~ - 2)

e) for 4 ≤ dj

~: for k < 2, p(v)

k/j = 0

33

for k ≥ 2, p(v)

k/j = b (k - 2, 3, dj~ - 2)

The scatter corresponding to these expressions appears to be closer to that correponding to empirical distributions.

Previous developments concerning seismic vulnerability correspond implicitly to what could be referred to as a classical approach, which is usual in literature and can be characterized as follows:

- it refers to a single, practically instantaneous, event; - the implications of the cumulative nature of effects of successive earthquakes

are not considered. The reality is obviously more complex and some extensions from the classical approach should be considered, at least theoretically. An attempt to deal with such challenges, presented in (Sandi, 1998), can be mentioned in this connection, in relation to the consideration of the evolutionary vulnerability, which corresponds to the consideration of the fact that the vulnerabilty affected by some damage is higher than the initial vulnerability (in the "no damage" state) of a same kind of structure. The introduction of the concept of evolutionary vulnerability leads to the need of considering, in relation to a definite seismiic event, the pre-event state of damage d’, and, also, the post-event state of damage, d”. The distributions characterizing the evolutionary vulnerability will be conditional not only upon the ground motion severity parameter, but also upon the pre-event level of damage and can be represented generically by an expression p(v”)

k”/j,k’. Some logical conditions concerning the vulnerability of damaged structures (more precisely, the distributions p

(v”)k”/j,k) are discussed at this place (Sandi, 1998). They are expressed

primarily in terms of conditions concerning the discrete representation p(v)k”j,k’, but they can

be extended, of course, to the case of continuous representation. 1. A first condition is

p j kk j k

v

k " , '

( ) for any , '=∑ 1"

(7.5)

since p(v)k”j,k’ are probabilities with respect to the possible damage states to which the index

k” is assigned. 2. A second condition is

p k kk j k

v

" , '

( )= 0 for "< ' (7.6)

since the post-event damage cannot be less severe than the pre-event one. 3. In case one considers the expected damage severity d*, given by the expression

d k pj k k j k

v

k, '

*

" , '

( )

""=∑ (7.7)

and two alternative intensities of indices j1 and j2 respectively, or one has two alternative pre-event damage states k1’ and k2”, one has the conditions

d d j jk j k k j k"

*

" , '

*

,'

2 1≥ > for 2 1

(7.8a)

d d k kk j k k j k" ,

*

" ,

'' '2 1

1≥ > for 2

' (7.8b)

The determination of these generalized distributions involves considerably increased requirements and difficulties as compared to the classical case of distributions p(v)

k/j. As an example, in case one wants to use the approach (b) referred to previously, post-earthquake

34

surveys are to be conducted upon samples of buildings for which a pre-event damage survey

Fig. 7.2. Graphic representation of damage vulnerability functions, for eight classes of buildings, based on post-earthquake survey performed in Bucharest

had been performed. It is hardly believable that such in-depth surveys will be performed in practice, given the inevitable evolution of the building stock determined by the general

35

evolution of the economic life. So, rather simple ways of estimating vulnerability, relying to a high extent on the use of expert judgment, are bound to be used in this field.

8. CONCLUDING REMARKS

The impact of the earthquake of 1977.03.04 was particularly severe. Besides the damage and losses on which official sources informed in 1977, it is likely that some "secondary", or hidden aspects of the impact were not less important. Looking at the data published by the National Institutes of Statistics during the years folllowing 1977, it turned out that the balance of foreign trade, that had been quite neutral up to 1977, was seriously upset during the following years. This may have been due to the decrease of the exporting capacity of the country, and this may have been to a high extent the effect of the earthquake impact. It is well known that the eighties of the past century were years of heavy austerity for Romania and the impact of the earthquake may have had a considerable contribution to this fact. The earthquake experience put to evidence the efficiency of earthquake protection measures, even in case when the code provisions of the pre-earthquake period are generally assessed at present to be obsolete and unsufficient. The general record of performance of nominally protected structures may be considered as satisfactory in case one counts the very few cases of collapse recorded. This is, of course, in no case a reason for not improving the situation. Various estiimates of loading and stresses put to evidence that, in many cases, structures had to mobilize resistance reserves that may have been not far from the ultimate ones. There are quite strong and alarming reasons of scare about the expectancy of a new strong, possibly destructive, earthquake. The convergent forecasts referred to in Section 6.2 should be seriously considered in this connection, especially with regard to the time and magnitude windows. This makes of the activities devoted to earthquake risk control and reduction a hasty race against time affected by the uncertainty on the possibility of reaching various desired targets. The experience of the destructive 1977.03.04, as well as of the subsequent earthquakes of 1986.08.30, 1990.05.30 and 1990.05.31, represented a source of first importance of improved / updated knowledge about:the various aspects related to seismic risk. As stated before, the education and experience provided during the second half of the past century to engineers made it possible to learn a lot from the dramatic earthquake effects. It may be stated that much has been achieved in the sense of improving the activities devoted to earthquake risk control and reduction, but much more is to be done still in the future in order to reach the goal of a tolerable level of seismic risk. Besides numerous specific activities that are to be tackled, the important task of developing a sastisfactory, comprehensive strategy of eathquake risk control and reduction, to be widely implemented, should be given a high priority. Romania entered at the beginning of 2007 a new phase of history and development, due to its admission into the European Union. This fact of paramount importance will bring considerable new opportunities and chances. Among other, Romania can benefit to a better extent from the valuable knowledge and know-how accumulated in Europe. There is nevertheless the interest of Romania, as of the Union too, that Romania be not just an absorber of foreign developments, but that, instead, be also a producer of knowledge and of philosophies proper to the specific conditions at hand.

36

A hard problem is that of providing the necessary resources for the wide variety of tasks raised by the central goal of controlling and reducing seismic risk. Major difficulties are raised in case one thinks of the development of a comprehensive system of regulations, of providing differentiated education for various socio – professional groups, of developing and implementing a comprehensive protection strategy, of developing a satisfactory system of earthquake scenarios etc. etc.. In case one takes into account the variety and difficulty of tasks raised by the earthquake risk control and reduction and the nature and size of obstacles encountered in this connection, it turns out also that the goal of improved earthquake risk control and reduction is conditioned by the general development of the country. The accumulation of wealth and of power of scientific and technological nature will lead to increased will of protection and to improved weapons to reach the desired goals.

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41