IC/93/127

INTERNATIONAL CENTRE FOR

THEORETICAL PHYSICS

INTERNATIONALATOMIC ENERGY

AGENCY

UNITED NATIONSEDUCATIONAL,

SCIENTIFICAND CULTURALORGANIZATION

PREDICTION OF THE OCCURRENCEOF RELATED STRONG EARTHQUAKES IN ITALY

Inessa A. Vorobieva

and

Giuliano F. Panza

MIRAMARE-TRIESTE

IC/93/127

International Atomic Energy Agency

and

United Nations Educational Scientific and Cultural Organization

INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS

PREDICTION OF THE OCCURRENCEOF RELATED STRONG EARTHQUAKES IN ITALY

Inessa A. Vorobieva

International Institute of Earthquake Prediction Theory

and Mathematical Geophysics,

Varshavskoye sh. 77 korp. 2, Moscow, Russian Federation

and

Giuliano F. Panza

International Centre for Theoretical Physics, Trieste, Italy

and

Istituto di Geodesia e Geofisica, Via dell'Universita 7, Trieste, Italy.

MIRAMARE - TRIESTE

June 1993

Abstract.

In the seismic flow it is often observed lhat a Strong Earthquake (SE), is 1'ullowed

by Related Strong Earthquakes (RSEs), which occur near ihc epicentre of (he SE wilh

origin time rather close to the origin time of the SE. The algorithm for the prediction of

the occurrence of a RSE has been developed and applied for the first lime to the

seismicity data of the California-Nevada region and has been successfully tested in

several regions of the World, the statistical significance of the result being 97%. So far

it has been possible to make five successful forward predictions, with no false alarms or

failures to predict.

The algorithm is applied here to the Italian territory, where the occurrence of

RSEs is a particularly rare phenomenon. Our results show that the standard algorithm is

successfully directly applicable without any adjustment of the parameters. Eleven SEs

are considered. Of them, three are followed by a RSE, as predicted by the algorithm,

eight SEs are not followed by a RSE, and the algorithm predicts this behaviour for seven

of them, giving rise to only one false alarm. Since, in Italy, quite often the series of

strong earthquakes are relatively short, the algorithm has been extended to handle

such a situation. The result of this experiment indicates thai il is possible to attempt to

test a SE, for the occurrence of a RSE, soon after the occurrence of the SE itself,

performing timely "preliminary" recognition on reduced data sets. This fact, the high

confidence level of the retrospective analysis, and ihe first successful forward

predictions, made in different parts of the World, indicates thai, even if additional tests

are desirable, the algorithm can already be considered for routine application to Civil

Defence.

1. Introduction

In the seismic flow, described by earthquake catalogues, it is often observed ihat

a Strong Earthquake (SE) is followed by slrong shocks, here called Related Strong

Earthquakes (RSEs), which occur near the epicentre of the SE, wilh origin time rather

close lo the origin time of the SE. The SE is usually a Main Shock (MS), bul can also be a

Strong Aftershock (SA) or a Slrong Forcshock (SF). The RSEs may be either SAs, if their

magnitude is less than that of the SE, or other SEs, if their magnilude is larger than that

of the earlier SE. The prediction of the occurrence of a RSE is very important both from

a scientific and a practical point of view. In fact, the study of the phenomena

preceding the occurrence of a RSE may help to understand the laws controlling the

development of the scismogenic process, and, at the same time such prediction is very

important for the reduction of the large hazard caused by the destabilization of

manufacts (e.g. buildings, lifelines) and natural features (e.g. slopes, river beds) due to

the SE.

The algorithm for the prediction of the occurrence of a RSE, based on the

analysis of the aftershocks sequence of the SE, has been developed and applied for the

first lime to the scismicity data of the California-Nevada region (Vorobieva and

Lcvshina, 1992), and has been successfully tested in several regions of the World

(Vorobieva and Levshina, 1992). A total of fifty-two sequences of aftershocks of SEs

have been analysed, and twelve of the SEs arc followed by RSE(s). The score of the

retrospcclive analysis is: ten cases of RSE occurrence arc successfully identified, and

two cases are failures; there are three false alarms in she forty cases of SE which are not

followed by a RSE; the statistical significance of this result is 97% (Vorobieva and

Lcvshina, 1992). So far il has been possible to make five successful forward predictions,

wilh no false alarms or failures to predict. The most important predictions are related to

the 1991, Racinsk earthquake in Georgia, and the Californian earthquakes in Loma-

Prieta (1989), and Joshua Tree (1992). Al present three cases are under monitoring.

The purpose of this paper is lo verify the applicability, also for practical

purposes of Civil Defence, of the algorithm to the different seismoactive regions in

Italy. The results of the retrospective analysis seem to encourage this attempt. In the

period of time considered (1680-1990) eleven SEs are considered. Of them, ihree are

followed by a RSE, as predicted by the algorithm, eight SEs arc nol followed by a RSE, and

the algorithm predicts this behaviour for seven of them, giving rise to only one false

alarm. The high confidence level of the retrospective analysis, and Ihe first successful

forward predictions, made in different parts of the World, indicates that, even if

additional tests are desirable, the algorithm can already be considered for routine

application to Civil Defence.

2. Description of the algorithm

SEs usually are followed by a sequence of aftershocks. Sometimes rather strong

events, comparable wilh the main shock, or even stronger, occur in the aftershocks

sequence. We do not consider here strong shocks which occur in the next few hours or

days after the SE. We are interested in the series of strong earthquakes which occur in

the same place - focal zone - of the SE, in a period of time which ranges from a month lo

several years after the origin time, and we indicate them wilh RSEs. The lime intervals

between the SE and the RSE(s) may be rather long, however, these earthquakes

undoubtedly are not independent. Since, in a few months, the tectonic movements

cannot build up enough new stress to cause an independent SE, ihe RSE(s) can be

considered due to the release of the remnant tectonic, stress which has not been

completely released by the occurrence of the SE, . In other words, al the occurrence of

Ihe SE either practically all tectonic stress is released, or the stress release is only

partial, and the remnant part of stress is redistributed in and around the hypocenlrai

area. In the first case we should nol expect a RSE, while, in the second case, it is

reasonable to expect the occurrence of one or more RSEs. The purpose of the algorithm

is to distinguish these two cases by analysing the local seismicity which follows the SE.

The problem can be formulated as follows (sec also Fig. 1). At the occurrence ofthe SE, with magnitude M, we define the beginning (t0) of ihe aftershock sequence, and

we consider this sequence for the first s days with the purpose of predicting whether

the next strong earthquake will occur within a shon period of time and not far from the

epicentre of the SE. The strong earthquake lo be predicted may be either a SA or a RSE.

More precisely we want to predict the occurrence of a SA and/or RSE - with magnitude

greater or equal to a threshold M4 in the lime interval [s,S], after the occurrence of ihc

SE, and within epicentral distance R, from the epicentre of the SE.

The algorithm for prediction is based on the same ideas used in the intermediate-

term earthquake prediction of a SE, using the whole sequence of main shocks -

algorithms M8 and CN (Keilis-Borok and Kossobokov, 1990; Keilis-Borok and Rotwain,

1990; Keilis-Borok el al., 1992). The main difference here is that the area where the

earthquake is expected to occur is much smaller than that considered in the prediction

of SEs, since RSEs are assumed to be due lo the release of ihe remnant stress in the

hypocentral area of the SE. According to the CN and MS algorithms, a SE is preceded by

(I) ihe increase of the seismic activity, and (2) high irregularity in the space and time

distribution of seismic events. These phenomena are akin to general symptoms of

instability of many non-linear systems. When considering RSEs, the non-linear system

is formed by the seismically active faults, and we make the basic hypothesis Ihat

instability phenomena similar to (1) and (2) occur in the flow of the aftershocks of the

SE. before the occurrence of a RSE.

2.1. Similarity.

In order to make comparable the aftershock sequences of earthquakes with

different magnitudes the flow of aftershocks is normalised with respect to the

magnitude, M, of the SE, in the following way:

- the lower magnitude threshold, in, for ihe analysed aftershocks is m = M-3;

- the area of investigation is the circle with radius R = 0.03-100.5M (km);

- the magnitude, Ma, of the RSEs to be predicted is greater or equal to M - 1.

- She period of time for which the prediction is made ranges from 40 days to 1.5 year

after the SE.

The main assumption here is that, after normalisation, the premonitory

phenomena for earthquakes of different magnitudes will be quantitatively similar, in

other words, after normalisation, if the premonitory phenomena are self-similar, the

average number of aftershocks will be the same for earthquakes with magnitude 6.0

and 8.0. therefore if, for example, fifty aftershocks with M>5 is a number abnormally

large for a SE with magnitude 8, then fifty aftershocks with M>3 is a number

abnormally large for a SE with magnitude 6.

2.2. Functions representing the premonitory phenomena.

All the values of the numerical parameters of the functions, used in this section,

are given in Table 1. The basic assumptions made arc:

(a) large values of the following functions are premonitory phenomena for the

occurrence of a RSE:

N - the number of aftershocks, with magnitude Ma > M - oij occurred in the lime interval

(t0 + Sj , t0 + s2 ), where t0 is the origin time of the SE;

S n - the total area occupied by the sources of the aftershocks, with magnitude Ma > M -

rrij, occurred in the time interval (l0 + Sj , i0 + s2 ), normalised with respect to the source

area of the SE:

Sn = JT I O K M ) (1)

Si

where mj is the magnitude of the i-th aftershock;

V m - the variation of magnitude, from one event to the next, for the aftershocks with

magnitude Ma > M - m t , occurred in the time interval (t0 + Sj , t0 + s2 ):52

(2 )

where n^ is the magnitude of the i-th aftershock;

V m e l j - the variation of the average magnitude from one day to the next for the

aftershocks with magnitude M^ > M - m.j, occurred in the time interval (t0 + S[ , tQ + s2

sivmed=5jM.i+ i -Hi I. (3)

si

where Hj is the average magnitude of the aftershocks occurred in the i-th day.

R z - the deviation from Omory's (1894) law for the aftershocks, with magnitude Mh>M-ni|

occurred in the time interval (t0 + Sj , IQ + s2 ):

Ewhere n( is the number of aftershocks occurred in the lime interval (t0 + Sj , t0 + s2 );

only positive differences are taken into account in the sum.

(b) small values of the following functions are premonitory for the occurrence

of a RSE:

V n - the variation of the number of aftershocks, with magnitude Ma > M - m, , from one

day to the next, occurred in the time interval (t0 + s, , l0 + s2 ):

-V. (5)

where n ; is the number of aftershocks for the i-th day.R m a s " l h e maximum distance, normalised to R, between the main shock and the

aftershocks, with magnitude Ms > M - n^ occurred in the time interval (iQ , ifl + s2).

Nfor - the local seismic activity before the main shock, i.e. the number of earthquakes

with magnitude Ma > M - mj occurred in the lime interval (t0 - S. , In - Si ) before the SE

within a circle of radius 1.5R, centred in the epicentre of the SE.

3. Pattern recognition

In terms of pattern recognition the problem can be formulated as follows. There

are two types of SE: type A - the SEs which are followed by a RSE, and type B - the SEs

which are not followed by a RSE. The available data are the SEs and the first pan of their

aftershock sequence; the problem is lo identify if the SE is of type A or type B.

The first step in pattern recognition is the discretization of the functions defined

in section 2. The values of each function are divided into two intervals "large" and

"small", so that the numbers of objects in each interval are equal. The thresholds for the

discretization are given in Table 1.

The second step is the determination of "typical" values of the functions. We

count, for each function, how often it is "large" (or "small") in correspondence of a SE

of Type A, and how often in correspondence of a SE of Type B. If the function is "large"

(or "small") for at least 2/3 of the objects of Type A (SE of Type A) and for less than 1/2

of the objects of Type B (SE of Type B) then this value is considered typical for the

events of Type A, similarly if the function is "large" (or "small") for at least 2/3 of the

objects of Type B (SE of Type B) and for less than 1/2 of the objects of Type A (SE of Type

A) then this value is considered typical for the events of Type B.

The last step in pattern recognition is voting. For this purpose we count, for each

aftershocks sequence, two numbers nA and nB : nA indicates how many functions are

typical for a SE of Type A, ng indicates how many functions are typical for a SE of Type

B. Therefore the decision rule is:

if nA - ns £ 3 then the SE is of type A (a RSE will occur);

if nA - riB < 3 then the SE is of type B (a RSE will not occur).

To define the value of ihe threshold of voting (3), and the values of all the paramelers of

the algorithm, as "learning material" Californian SEs and iheir aftershock sequences

have been considered . The results of learning are shown in ihe first row of Table 2.

3.1. Test on independent data

The algorithm has been applied, without re adaptation of the values of the

parameters determined for California, to thirty-five SE which have occurred in seven

different regions of the World: Cemral Asia, Caucasus, Lake Baikal area, Ibero-Magreb

area, Dead Sea rift. Turkmenia, and Balkans. The results of the analysis, shown in Table

2, have a statistical significance wilh a 97% confidence level (Vorobieva and Levshina,

1992).

3.2. Forward prediction of RSE{s)

The main difficulty in the forward prediction is represented by the quality of the

available input data, since we must be able to use all the dala which can be quickly

available (e.g. preliminary bulletins). Usually, the quality of these data is obviously

lower than thai of Ihe final versions of earthquake catalogues, which have been used

for ihe retrospective analysis. In the preliminary bulletins, the epicentral parameters

(origin time, co-ordinates, and magnitudes) are determined with large uncertainties,

and sometime part of the information is missing. In some cases it is necessary to compile

dala produced by more than one agency; these data are, in general, not homogeneous

since different agencies may use different procedures for earthquake location (e.g.

differcnl structural models) and quantification (different coefficients in the magnitude

formula). These limits of the preliminary bulletins require to lest if the algorithm is

slablc enough, with respect to the quality of the inpul dala. The firsi results of the

forward prcdiciion, which refers to the lime interval 1989-1993, are given in Table 3.

These results indicate that the algorithm is robust enough, and thai it is reasonable to

attempt practical applications to Civil Defence.

In Table 3 are listed all strong earthquakes occurred in the regions considered.

Fifteen SE occurred in ihe time interval 1989-1993; eight of them have been analysed

and of ihe remaining seven SEs, one has less than len aftershocks with magnitude

greater or equal to M-3, where M is the magnitude of Ihe SE, one is a foreshock which

occurred very close in time (within one day) to the SE. three are aftershocks which

occurred very close in time (same day) to the SE, and for the remaining two events the

data are not available. For the analysed events, in five cases the prediction is successful,

and for three events there is current monitoring; in one case the occurrence of a RSE is

not expected, while in the other two cases there are current alarms. More specifically,

IS * ••:* *H

after Landers earthquake in California (June 28 1992) a RSE, with magnitude above 6.5.

is expected to occur before the end of December 1993 ; after Erzincan earthquake in

Turkey (March 13 1992) a RSE. with magnitude above 5.8, is expected to occur before the

end of September 1993.

4. Analysis of the Italian seismoactive regions

4.1. Tectonics and seismicity of Italy

As it is well known, it is possible to subdivide Italy into three main tectonic zones

with different types of receni motions ( e. g. Dal Piaz and Polino, 1989; Patacca and

Scandone, 1989).

The first one is represented by the northern part of Italy (North of the latitute

44° N, where Ihe Alps are generally uplifting (Mueller, 1982). The relative motions

there, cause compression in the zones of contact between the Southern Alps and the

Northern Apennines, in the western part of Italy, and in Friuli (eastern part of Italy)

(Dal Piaz and Polino, 1989), where one of the branches of the Southern Alps turns to the

South along the Adriatic sea (Dinaric Alps). The zone of Friuli is also characterised by

some strike-slip motion in the western direction (Pavoni et al,, 1992). Ten SEs. wilh

magnitude greater or equal to 6.0, occurred in Northern Italy from 1000 to 1990

(catalogues PFG and ING). Two SEs. boih locaied in the Central Alps, had magnitude 6.8.

Five SEs, with magnitude in the range 6.0 - 6.1, occurred in Friuli.

The second tectonic zone is represented by most of the Italian peninsula, where

two arcs (north-central Apennines and Calabrian Arc) of tectonic shortening meet.

Starting from their presenl-day structure and analysing the time-space evolution of the

thrust belt-foredeep-foreland system, Patacca and Scandone (1989) reached the

conclusion thai the deformation has been strictly controlled by the dipping of the

foreland lilhosphere sinking beneath the mountain chain peninsula and not directly

by the collision between Europe and Africa. This hypothesis is strongly supported by

surface waves dispersion measurements (Calcagnile and Panza, 1981; Panza et al.. 1982;

Suhadolc and Panza. 1988; Delia Vedova, et al. 1991), and more receni investigations,

which combine different geophysical data sets about aeromagnetic and gravity

anomalies with the available structural information about the Hthosphere-

asthenosphere system (Marson et al., 1993). From 1000 lo 1990, twenty-six earthquakes

with magnitude greater or equal to 6.0 occurred in Central Italy, five of them had

magnitude greater or equal to 6.5 (catalogues PFG and ING).

The main tectonic feature of the the third zone, ihe Calabrian Arc (Eastern Sicily

- Calabria), is the old subduction zone, where the deep-focus earthquakes are related to

the presence of a lithospheric slab which may represent, in its deepest parts, the

remnant of the Adriatic lithosphere subducting Corsica-Sardinia before the opening of

ihe Tyrrhenian sea (Palacca and Scandone, 1989). The passive subduction of the Po-

Adriatic-Ionian lithosphere by gravitational sinking appears as a reasonable

mechanism to explain contemporaneous geodynamic events such as mountain building

in the Apennines and extension in the Tyrrhenian area, and the partition of the

Apennines into two major arcs may be related to the differential sinking of the foreland

lithosphere in the Northern Apennines and in the Calabrian Arc. This last region is

characterised by the highest level of seismicity in Italy. All five SE with magnitude 7.0

or more occurred here, and the strongest had magnitude 7.5 (catalogues PFG and ING).

4.2. Input data

We concentrate our attention on the events occurred within the polygon defined

by the vertices (48°N, 12°E); (47°N, 14.5°E); (44°N, 14.5°E); (40°N. 20°E); (36°N, 16°E); (38UN,

11°E); (40°N, 13°E); (43°N, 9°E); (43°N, 6°E); (46°N, 6°E) and with focal depth less or equal

to 60 km, since deep earthquakes, usually, are not followed by aftershocks sequences.

We use the PFGING catalogue, which is composed by the PFG catalogue from 1000 to 1980,

and the ING catalogue , from 1981 to 1990. In the catalogue PFG two types of magnitude

are given: the magnitude, Mt, recalculated from the macroseismic intensity, for the

whole period of time, excluding the last three years; and, since 1900, the local

magniiude, M^. The ING catalogue contains two types of magnitude, too: M]_ and the coda

duration magnitude, Mj. We consider M[_ a more appropriate quantity to be used in

connection with M^, therefore we use M^, whenever available, and, only if ML is

unknown, we use Mj.

The magniiude threshold for which the catalogues are complete changes with

lime, as clearly proven by the frequency of occurrence graphs, for different periods of

time, shown in Fig. 2, For the period which goes from 1000 to 1680 the magnitude

completeness threshold is about 4.5; from 1680 to 1870 it is about 3.5; from 1870 to 1980 it

is about 3.0; and for the last period of time from 1981 to 1990 it is about 2.5. Therefore for

the whole period of time the common magnitude level of completeness is about 4.5.

4.3. Statistics of ihe occurrence of RSEs in Italy.

On the basis of the experience made in the study of the other regions of the

World, listed in Table 2, it is possible to slate that the occurrence of a RSE is, in general, a

rare phenomenon. Including in the statistics the SEs which are followed by few

aftershocks with magnitude greater or equal to M-3, where m is the magnitude of the

considered SE, only 15% of the SEs are followed by a RSE.

To determine such statistics for the Italian seismoactive regions, we have

considered all the fifty-three earthquakes with magnitude greater or equal to 6.0, thai

occurred since 1000. Only five of these events were followed by a RSE. Therefore, the

occurrence of a RSE is a particularly rare phenomenon in Italy. Nevertheless the

frequency of occurrence of RSEs is sufficiently large to warrant further investigations,

with the final goal of making available to the Civil Defence authorities a quite useful

practical tool to be used immediately after the occurrence of a SE.

Three out of the five earthquakes followed by a RSE occurred in Friuli, one in the

Calabrian arc, and one in the zone of dominant strike-slip movement in Central Italy.

There are also several places where RSEs occurred, but wiihin a time interval less lhan

40 days, typically in the range 10-40 days. We call these events "short series" RSEs, to

distinguish them from the others RSEs ("long series").

If the occurrence of a RSE is caused by the incomplete stress release at the

occurrence of a SE, it is reasonable that RSEs occur in places where the tectonic motions

take place in more than one direction. After the SE, which may be caused by one of the

tectonic motions, the redistribution of the residual stress in the focal zone takes place.

This stress redistribution may be sufficient to cause tectonic motion in another

direction, in this case the RSE may occur, sometimes even stronger, than the SE.

In Italy, all places where RSEs have been observed practically satisfy to the

previous hypothesis. With this statement we do not imply that RSEs can not occur in

other places, but, in the case of occurrence of a SE in regions were different tectonic

motions are acting simultaneously, more attention must be paid, since the probability of

occurence of a RSE seems to be higher than elsewhere.

4.4. Application of the algorithm to the Italian seismoactive regions

In order to apply the normalisation procedure described in section 3.1, we need to

consider aftershocks in the magnitude window [M - 3, M], where M is the magnitude of

the SE. Accordingly to the magnitude threshold for which the catalogues are complete

we can consider SE with magnitude greater or equal to 6.5 for the period of time from

1680 to 1870. and 6.0 for the period of time from 1870 to 1990. As we have done in (he

other regions listed in Table 2, we test only the earthquakes which are followed by ten

or more aftershocks, with magnitude in the range [M - 3, M], in the first forty days after

VIn the period of time considered, twenty-two SEs occurred. Two of them are SAs,

which occurred within less than forty days from t0. Nine of the SEs are followed by less

than ten aftershocks with magnitude greater or equal to M-3, and all these nine

earthquakes are not followed by RSEs. This fact is in favour of the hypothesis that a low

level of aftershock activity is against the possibility of occurrence of a RSE. Of the

remaining eleven SE, three are followed by RSEs (SE of type A), and eight are not

followed by a RSE <SE of type B) (see Table 4). The results of the application of the

algorithm are shown in Table 5.

Therefore, without any adjustment of the parameters, the algorithm developed

for California, allows us to correctly recognise ten out of the eleven aftershocks

sequences occurred in Italy. There is only one false alarm after the earthquake

occurred on January 15, 1968.

IP'

5. Modification of the algorithm.

As it was mentioned in section 4.4. several RSEs, which occurred within about one

month from the SE have been observed in Italy (Table 6). In order to analyse these

aftershocks sequences, it is necessary LO modify the algorithm. The goal of the

modification is to allow the classification into type A or B also for the SEs characterised

by short series of aftershocks using the information on the first ten days (and not forty,

as it is done with the standard algorithm used in section 4).

The changes introduced with respect to the standard algorithm are the following:

1. We wail for a RSE during the period of time which goes from 10 days to 1.5

years after [he occurrence of the SE.

2. The functions V m , Vn and V m e d are calculated for 10 days;

3. The function Rz is excluded, because it is not determined for 10 days, due to its

formal definition; the remaining four functions do not change, because they are

already defined for 10 days or less.

4. The threshold for voting is decreased from 3 to 2, because the number of

functions is decreased from 8 to 7. We have not changed the discretization thresholds

for the functions, because the number of objects is too small to allow a new independent

determinat ion,

We have considered the same eleven earthquakes already tested with the standard

algorithm, but, now, ihe earthquake occurred in 1968 is an object of type A. The result is

presented in Table 7. With the modified version of the algorithm it is possible to predict

the four SEs followed by a RSE, and the seven SEs not followed by a RSE, without false

alarms or failures to predict.

6. Conclusions.

The algorithm of prediction of RSEs has been applied to the Italian territory. The

occurrence of RSEs is a particularly rare phenomenon here, but nevertheless it

generates sufficient additional hazard to warrant consideration, Our results show that

ihe standard algorithm is directly applicable with success to the Italian territory,

without any "ad hoc" adjustment of the parameters.

The algorithm has been extended to the prediction of RSEs occurring when the

series of strong earthquakes are relatively short, a situation quite often observed in

Italy. Regretfully, it has been possible to study only one earthquake, shown in Table 6,

because the catalogue is not complete for M=3 before 1870, when they occurred all the

other events which were followed, within a few days, by a RSE. Nevertheless, the result

of this experiment is useful, since it indicates that it is possible to attempt to test a SE, for

the occurrence of a RSE, soon after the occurrence of the SE itself, even if the revised

data usually are available with considerable delay. The modified algorithm allows us to

10

do "preliminary" recognition on reduced data sets, and then to verify the results on the

full data set.

In Italy the series of strong earthquakes has been correctly recognised

retrospectively, with no false alarms nor failures to predict, using only one parameter -

ihe total source area of aftershocks (Fig. 3). However we do not consider this result very

reliable and significant, because, in other regions, this parameter is not sufficient for

the recognition.

The algorithm needs additional tests. Nevertheless, Lhe high confidence level of

the results, the first successful forward predictions and the good retrospective result for

Italy allows us to propose its use for practical purposes of Civil Defence. In decision

making one should remember that, on the basis of the result of the retrospective tests

(including Italy), the probability of a failure to predict is low, (2 cases out of 34, or 6%),

while i! is higher for a false alarm (3 cases out of 10, or 33%).

Acknowledgements

This work was done at the Islituto di Geodesia e Geofisica (University of Trieste).

Author thanks Dr. I. Rotwain and Dr. A. Sadovsky for help and useful discussions.

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now: algorithm M8. Phys. Earth Planet. Inter., 61, 73-83.

Marson, I., Panza, G. F. and Suhadolc, P. (1993), Crust and upper manlle models along the

active Tyrrhenian rim. Submitted to Terra Nova.

11

Mueller S. (1982), Deep structure and recent dynamics in the Alps, in: K. J. Hsu editor

Mountain building processes. Academic Press, 181-199.

Omory, F. (1894), On the aftershocks of earthquake. J. Coll. Sci. Imp, Univ. Tokyo, 7, 111-

200.

Panza, G. F., Mueller, S., Calcagnilc, G., and Knopoff, L. (1982), Delineation of the north

central Italian upper mantle anomaly. Nature, 296, 238-239.

Patacca, E., and Scandone, P. (1989), Post-Tortonian mountain building in the

Apennines, the role of ihe passive sinking of a relic lithospheric slab, ln The

lithosphere in Italy (ed. Boriani, A., Bonafede, M., Piccardo, G.B. and Vai G. B.)

(Accademia Nazionale dei Lincei, Aui dei Convegni Lincei, 80, Roma 1989) pp. 157-176.

Pavoni, N., Ahjos, T., Freeman, R., Grcgersen, S., Langer, H., Leydecker, G., Rolh, Ph.,

Suhadolc, P., and Uski, M, Seismicity and focal mechanisms, In A continent revealed -

The European geolraversc: Atlas of compiled data ( ed. Freeman R. and Mueller S.)

(Cambridge University Press, Cambridge, 1992) pp. 14-19.

PFG, Catalogo dei terrcmoti Italian! dall'anno 1000 al 1980 (ed. Postpischl, D., Bologna)

(CNR-P. F, Geodinamica, 1985).

Suhadolc. P., and Panza, G. F. (1988), The European-African collision and its effects on

Ihe lithosphcre-aslhenosphere system. Tectonophysics, 146, 59-66.

Vorobieva, 1. A., and Levshina., T. A. (1992), Prediction of a reoccurence of strong shock

using the aftershock sequence. Computational Seismology, M. Nauka, 25, 28-46.

12

Table 1. Numerical values of the parameters of the functions. For the definition of the

different functions and parameters see text, section 2.

Function

N

vmv m f l dR*Vn

Nfcr

mi

3233332

si, hr

1111

10 days1-

1 5

s2, days

1010404040402

years 3 :

At, days

--1-11-

nonths

I, days

----10--

Thresholds

of

discretisation240.120.41

0.7, 2.600.980.23

2

Table 2. Results of learning and test of the algorithm.

California

Central AsiaCaucasusLake BaikalareaIbero-MagrebDead Sea riftTurkmeniaBalkanesTotal

Mo Number

LEARNING6.4 17

TEST6.4 86.4 5

5.5 26.0 25.0 15.5 57.0 12

52

Double

Number

6

1_

_1

1312

(type A)

Errors

1

_

__

1

2

Single (type B)

Number

11

7 1

9 140 3

13

Table 3. Results of the forward prediction for the period 1989-1993. AM is the difference

in magnitude between the SE and the RSE; N 4 Q is the number of aftershocks, with

magnitude greater than M, during the first 40 days after the occurrence of the SE. Type

indicates the result of the pattern recognition and Note gives the result of the analysis.

Date

1989199119911992199219921992199219921990199119911992

19921992

10884444666463

810

181 6172325262 628282 02 91513

1924

Epicenter

37.03°N41.70°N41 .82°N33.94°N4 0.37°N40.42°N40.38°N34.18°N34.20°N36.95°N4 2 . 3 9°N42 .37°N39.82°N

41.55CN4 2 . 60°N

121.88°W125.39°W125.40°W116.33°W124.32°W124.60°W124.57°W116.51CW116.83°W

49.40°E43.67°E43.96°E39.94°E

73.46°E44.94°E

M

7 .6.7 .6.7 .6.6 ,7

6 .77

66

76

I13131675671

68

36

R, km

1 0 542

1 0 542

1 0 55466

1 6 854

2 1 21 0 5

597 5

1 3 559

[AM|

1

- 1

20

. 7

-

.2-----. 1. 7-

-

_

-

2482

67

1319152 5852577

2040

Type

B--

AB--

A-

BABA

•

Note

SuccessClose foreshockFew aftershocksSuccessMonitoringClose aftershockClose aftershockCurrent alarmClose aftershockSuccess,regnosisSuccessSuccessCurrent alarm

Data are notavailable

Table A. Italian SE that have been analysed. N40 is the number of aftershocks, with

magnitude S M-3, during the first 40 days after the occurrence of the SE. AM is the

difference in magnitude between the SE and the RSE. AR/R is the relative distance of the

SE from the RSE. AT is the time difference between RSE and SE.

Date

178319761976

18571870187319081915196219681980

259

1210

612

1

81

1 1

Close17831976

29

With168816931783180519051920192819281930

61

3799337

56

15

164

2 928132 11523

l a t

384 6 .4 6

4 09

4 63841413740

33°N25°N27CN

32°N30cN18°N17°N97°N23°N70°N86°N

Earthquakesaftershocks

71 5

lack5

1 12826

8777

23

384 6of4137

384 13 8443837

41

60°N30°N

161313

1 516121513141315

Ion M

Type00cE

.25°E

. 15°E

7 .6.6 .

Type. 93°E.30°E.38°E.58°E,60°E.93°E.10°E.33°E

7 .

66 .7

6666

ft.110

B0

6818005

excluded from

1613

.25°E

.18°E66

af te r shocks ( a l l32°N

,42°N.S3°N.53°N,80°N,25°N.60°N.27°N,07°N

141516141 610161415

,57°E.17°E.50°E,52°E.10°E-28DE.78°E,68°E.35°E

676

676666

70

R, km

1053330

95597 5

1 0 575303053

1

12122

211

the test

663 0

type B)651

503

. 0

. 6

.5

591 6 8

66539 542305953

22121

31

AM|

4010

. 8 0

. 9 0

. 0 0

. 5 0

. 0 0

.00. 0 0. 4 0.80

. 0 0. 8 0

-

. 9 0

. 1 0

. 4 0

. 9 0. 6 0

. 0 0

. 3 0

AR/R |

0 .660.240 .45

0.340 . 0 50 .210 .000 .360 .300 .830.04

0 .500.52

-

0.060.640.240 .240.82

-

1 .000.54

AT,days

501 3 13 6 6

8 0

444 0

1 8 54 5

1747 55 3

4 83 6 6

-7 987

1342 7 52 4 2

-65

1 0 6

5 598

104

1317185 117

121 1 1

5 1

96

005305

003

14 15

Table 5. Summary of the results of the analysis for ihe Italian region.

EarthquakeDate

178319761976

18571870187319081915196219681980

259

1210

612

181

1 1

56

15

164

292813211523

M

7 .6.6 .

7 .6 .6.7 .6 .6.6 .6.

110

06818005

N

AAA

BBBABBAA

vn

BAA

BBBABAAA

Values

vm

BAA

ABBBBBAB

Sn

TypeAAA

TypeBBBBBBAB

of functionsVmed

AAA-

B-BABBBA-

RZj P

AAA

AAAAAAAA

max

A

BB

BAAAABBB

Nfor

AAB

AAAAAAAA

Votincrnft : nB

6:27 : 15:2

3:43:54 :45:33 : 53 : 5

7 :14 : 3

Table 6. List of the SE followed by a RSE wiihin a short period of time ("short series"

RSEs), in Ihc Italian region. AR/R is the relative distance of the SE from the RSE. AT is

the time difference between RSE and SE.

Date

14561456

162716271627

163916391639

17 0217021702

17031703170317 03

19681968

1212

789

101010

344

3222

11

530

3076

81417

1426

1423

25

1525

4 14 1

4 14 14 1

424242

414141

42424242

3737

l a t

. 52°N.52°N

. 7 8 ° N

.78°N. 6 8 ° N

.63°N, 6 3 ° N. 6 3 ° N

. 1 2 ° N,12°N.12°N

.67°N

.42°N

. 50°N

.50°N

,70°N. 70°N

Ion

14.52°E14.52°E

15.30°E15.30°E15.38°E

13.30 ( )E13.30°E13.30°E

14 .95°E14.95CE14 .95°E

13.17°E13.25°E13.25°E13.00°E

13.10°E13.10°E

M |

6 . 15 . 9

6 . 46 .15 .6

6 .15 . 15 . 1

6 .15 .35 . 3

6 .15 . 65 . 15 . 1

6 .05 .7

R, km

33

47

33

33

33

30

AT,days

2 5 . 2

8.338.2

6.09.0

19.02 3 . 5

1 8 . 72 0 . 14 2 . 0

10.3

AR/R

0 .00

0 .000 .27

0 . 0 00 . 0 0

0 .000 . 0 0

0 . 8 90 . 5 90 .67

0.00

16

Table 7. Summary of the results of the analysis of the SE followed by a RSE within a short

period of time ("short series" RSEs), in the Italian region.

Object 1

Date

1783 2 51968 1 151976 5 61976 9 15

1857 12 161870 10 41873 6 291908 12 281915 1 131962 8 211980 11 23

M

7 . 16 .06.16.0

7 . 06 . 66 . 87 . 16 . 86 . 06 .5

N

AAAA

BBBABBA

Vn

BAAA

BBBABAA

Values

vmType

BAAA

TypeBBABBBB

of functions

Sn Vmed Rmax

AA A AA - BA A BA - B

BB B BB B AB - AB B AB B AB B BB - B

Nfor

AAAB

AAAAAAA

Voting

nA : nB

5 : 25 : 16:14 :2

1 : 62 : 53 : 34 : 32 : 52 : 53 : 3

17

§

•normulat:

o"a

o

|

mVEN

T

CO~H

mm

1ED

I

o

i

s

| S

TR

ON

CI

AF

TE

RS

HO

1 J

VI

iS

TR

ON

GE

AR

TH

QU

AK

E

Number of earthquakes Number of earthquakes Number at earthquakes

»

1•a

Number of earthquakes Number ot earthquakes Number of earthquakes

3

D-

GJ -

tn -

Oi -

-^ -

1 J

/

1 ,

;

1.50

1.25

CO

1.00

ffl 0.75 Aoco

0.50-3O

co0.25-

0.00 |20 40 60 80

Mumber of aftershocks

Fig. 3. Comparison of earthquakes of [ypes A and B in Italy. The number of aftershocks

with magnitude > M-3 and their total source area, S, are normalized with the source area

of [he SAs for the first ten days.

20