E L S E V I E R Engineering Geology 46 (1997) 63-70

ENGINEERING 6EOLOQY

Use of seismic microzoning for risk management in Quito, Ecuador

Carlos Villacis a,,, Brian Tucker a, Hugo Yepes b, Fumio Kaneko c, J.L. Chatelain d a GeoHazards International, Stanford, USA

b Escuela Politkcnica Nacional, Quito, Ecuador c 0 YO Corporation, Urawa, Japan

d ORSTOM, Quito, Ecuador

Abstract

A pilot project has been implemented for Quito, Ecuador, to demonstrate the effectiveness of seismic scenarios in reducing earthquake risk in developing countries. The participants included local scientists, government officials, and foreign advisers. Seismic hazard and damage assessments were performed and the results were then used to delineate a seismic risk management program. The uniqueness of the project is the adaptation of techniques developed in Japan and the United States to the local conditions. The results showed that seismic scenarios in developing countries are not only possible but also efficient in raising awareness of the earthquake risk within all the sectors of the society and in managing this earthquake risk.

Keywords: Risk management; Seismic microzoning; Seismic scenarios; Developing countries; Quito; Ecuador

1. Introduction

The worldwide tendency of the people to concen- trate more and more in urban areas increases the seismic risk, especially in the developing countries. Most of the technology and the methods used for risk reduction, however, have been developed in industrialized countries and, therefore, respond to the needs and conditions found in these countries.

A seismic risk management project was imple- mented for Quito, the capital of Ecuador, South America, from July 1992 to December 1993. The purpose of the project was to investigate the effec-

* Corresponding author. Tel.: +1 415 7250057; fax: +1 415 7233624; e-mail: villacis@pangea.stanford.edu.

tiveness of existing microzoning methods in reduc- ing the seismic risk, and their applicability to the conditions found in developing nations. The distri- butions of the ground shaking intensity produced by three plausible earthquakes, as well as the extent of the induced structural and functional damage, were estimated and the results were then used to propose a feasible seismic risk management program for Quito.

The project participants included international experts in the various fields considered in the study (seismology, earthquake engineering, public administration, urban planning, etc.) who provided invaluable advice and revised the results for accu- racy. Due to insufficient data, existing techniques were adapted to meet the local conditions. This

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64 C. Villacis et al. Engineering Geology 46 (1997) 63- 70

paper explains the main steps of the process to estimate the distributions of the ground shaking intensity and of the corresponding damage to structures and lifeline systems.

2. The city of Quito

As can be seen in Figs. 1 and 2, Quito is located on the north-west of South America, on the Andes Mountains, at an altitude of 2800 m. Having an elongated shape in the north-south direction, the city has an area of about 290 km 2 and a popula- tion, as of 1990, of approximately 1 1000000 inhabitants. The city is limited on the west by the slopes of the Pichincha Volcano (5000 m), and on the east by the slopes of a series of small hills. Farther to the east, behind the small hills, steep slopes (10-30 °) lead to a valley 400 m lower than Quito.

The old part of the city, located in its central part, includes many old adobe structures. The modern Quito has extended northwards with, mainly, reinforced concrete (RC) structures that include high-rise buildings and residential and

ATLANTIC OCEAN

CARIBIIBAN PLATIB pZo"/~

Nk,~A2A PLKrB SOlJ~ AMBRICAN , PLA'I~

PACIFIC OCF.AN

Fig. 1. Location and tectonics of Ecuador.

Fig. 2. Epicenters of destructive earthquakes in Ecuador since 1541.

commercial areas. In the southern part of the city there are many non-engineered RC structures of one to three stories.

Similar to other cities in Central and South America, Quito has experienced an explosive growth in recent years and, consequently, hazard- ous areas such as the eastern and western slopes have been used to accommodate the increasing population.

Regarding the geology, the soils consist mainly of cangahua, a mixture of sand, silt, and amor- phous clay. The soil surface layers are generally stiff, with an average shear wave velocity of 250-300 m s -1. Except at the low lands, which were covered by water in ancient times, the ground water table is found generally lower than 8-10 m.

3. Seismology and hypothetical earthquakes

The historical seismology (Fig. 2) indicates that, since 1541, Quito has experienced intensities >6 on at least 39 occasions. In 1906, a great earth- quake of magnitude 8.4 took place off the Pacific

C. Villacls et al. / Engineering Geology 46 (1997) 63-70 65

9.0

2

8.0

7,0

6.0

5.0

4.0

0 Q U l m m 0 o o ~

• • . oeo ~ o e a , m • o .

. . . . . . . . . . ~ . _ ~ . ~ o

° ° 0 , 0 a _ _ _ _ l e e ,

. . . . . . . . I

m

• ° ,

~ I I 0 ~ 1 0

• lntraplate o Subduction

. . . . . . . I a a a mamma I

Distance A (km)

Fig. 3. Earthquake data for the derivation of the attenuation relations.

1ooo

1oo

10 o

< l

0.1

% % %

~ ~ ",,7 **%% %%%

. . . . . Subduction

l l • • • , , . I • , • . . . . .

Distance A (kin)

Fig. 4. Attenuation relations for Quito.

4. Assessment of the seismic hazard

Coast, 200 km from Quito. Big events occurred also in 1755, 1797, 1859, and 1868 causing thou- sands of deaths. More recently, the 1987 earth- quake (M=6.9, A=80km) produced serious damage to several structures in the city.

Two main types of seismic activity were iden- tified from the seismo-tectonic studies: one origi- nated inside the South American plate and the other one originated in the subduction zone where the Nazca plate plunges under the South American plate at a convergence rate of about 78 mm year-1. It was also noticed that local earth- quakes take place regularly in Quito due to the presence of active faults north of the city. Representing these sources of seismic activity, three plausible earthquakes were adopted for the project: (1) an intraplate earthquake in the South American plate (M=7.3, A=80km); (2) a big earthquake in the subduction zone (M= 8.4, A = 200km); (3) a local earthquake (M=6.5, A= 25 kin).

The locations of the adopted hypothetical earth- quakes are shown by the black dots in Fig. 2.

The distributions of the seismic intensity result- ing from the adopted hypothetical earthquakes were estimated in a process that included the derivation of attenuation relations, the zoning of the city, and the evaluation of the non-linear response of the soils to the adopted input motions.

4.1. Attenuation relations for Quito

Countries like Japan and the United States have long histories of instrumental seismicity and sev- eral models have been proposed there to correlate the variation of the ground shaking strength with the magnitude of the seismic event and the distance to its epicenter. However, that is not the case in Ecuador and attenuation relations had to be derived. Twenty-three events were selected from the Ecuador's earthquakes catalog (Egred, 1992) comprising 1104 seismic intensity observations (Fig. 3). Two-step stratified regression analysis was applied to remove the ill-effect of the magni- tude-distance correlation from the database and attenuation relations were derived for the two

66 C Villacis et al. /' Engineering Geology 46 (1997) 63-70

Fig. 5. Variation of slope and adopted soil zones.

main types of earthquakes: intraplate and subduc- tion earthquakes. Damage and seismic intensities observed in Quito in past earthquakes were used to make the attenuation relations applicable to the local soil and construction characteristics. The resulting relations are presented in Eq. (1) and Fig. 4.

For intraplate earthquakes

I = 1 . 5 5 M - 3.72log(A) + 2.39

For subduction earthquakes: (1)

I = 1 . 7 0 M - 4.821og(A ) + 2.97

where, I = M M intensity, M = magnitude, and A = epicentral distance (km).

The expression proposed by Neumann (1954) was used to relate the peak ground acceleration (a) to the seismic intensity (I):

log(a) =0.308 x I - 0 . 0 4 1 (2)

4.2. Soil zoning o f Quito

At first, topographical characteristics were con- sidered to divide the city into three big zones: the western slopes (f) , the central lowlands (1), and the eastern slopes (q). Then, data from more than 2000 borings carried out all over the city were employed to define 20 soil zones. The borders of these zones were determined applying image pro-

Altitude meters

3100

2900

2700

2500

2300

2100

Lake Sediments

Canzahua

~ 1 Young Slidings

Old Slidin~s

Old Volcanic Sediments

I-'-7 Volcanic Sediments

Alluvial Cones

Volcanic Rock

Fig. 6. An example of a geological cross-section for Quito (line A-A' in Fig. 5).

C. Villacls et al. / Engineering Geology 46 (1997) 63-70

Table 1 Estimated soil response to the hypothetical earthquakes

67

Zones Eq. (1) (M=7.3; zl =80 kin) Eq. (2) (114=8.4; A =200 km) Eq. (3) (M=6.5; A =25 km)

Base Amplif. Surface Base Amplif. Surface Base Amplif. Surface accel, ratio accel, accel, ratio accel, accel, ratio accel.

fl 25.4 3.17 78.9 24.0 2.2 52.1 26.7 3.8 102.1 f2 26.1 4.4 114.1 23.8 2.9 68.8 29.2 4.9 141.5 f3 27.6 4.4 121.4 23.3 3.5 81.9 31.7 5.6 173.6 f4 28.4 3.2 89.3 23.2 2.4 54.8 38.6 3.8 147.1 f5 28.2 3.3 91.9 23.4 2.6 59.8 48.0 3.6 173.2 f6 28.1 3.4 96.2 23.6 3.6 84.4 59.6 3.6 211.5 f7 27.0 2.6 69.4 23.6 2.1 49.3 32.3 3.2 101.0 11 26.4 4.0 104.1 23.6 3.8 88.8 27.7 5.3 147.5 12 25.9 3.5 90.7 23.8 2.9 68.9 27.2 4.3 117.1 13s 28.4 4.2 120.5 23.1 3.9 90.0 34.8 4.7 162.0 13n 29.2 4.0 117.5 22.9 3.2 72.6 41.4 4.2 174.8 14 29.0 3.8 110.1 22.9 3.4 78.4 36.8 4.0 149.0 15 28.6 4.1 118.0 23.3 3.8 89.8 47.2 4.1 194.1 pn 27.1 3.1 84.3 23.5 2.2 51.0 31.2 3.8 119.2 ql 27.1 4.3 116.1 23.2 3.0 69.7 27.5 3.8 104.5 q2s 29.3 3.7 109.9 22.8 2.8 62.6 37.2 4.3 159.2 q2n 29.9 3.2 94.0 22.9 2.4 53.4 56.1 3.8 214.0 q3 29.6 4.1 122.9 22.9 4.0 92.3 48.9 4.1 198.7 q4 28.9 3.5 101.9 23.3 3.0 93.9 71.1 3.7 262.3 q5 24.7 3.7 90.9 23.8 2.8 67.5 21.9 3.7 82.1

cessing techniques on GIS data to consider the variations of elevation and ground slope (Fig. 5)

Two problems, however, were found when mod- eling the soil's deep structure: (a) The boring data reached depths of only 10-15 m; (b) the available geological maps of the region did not provide enough information to define the model of the soil's deep structure.

The location of the basement layer and the characteristics of the deep structure were deter- mined using results of deep drillings carried out by the water supply organization, geological cross-sections prepared by local experts, and exist- ing results of seismic refraction tests and electric resistivity prospecting. Studies on the non-linear dynamic response of the soils showed that a layer having a shear wave velocity of 1000ms -1 - - approximately three times the average shear wave velocity of the surface layers - - could be taken as the basement layer. Fig. 6 gives an example of a geological cross-section in the E-W direction.

4. 3. Dynamic soil response and ground shaking distribution

One-dimensional, non-linear response analysis was applied to estimate the seismic acceleration of the ground surface. The input motion at the base- ment for the intraplate earthquake was generated from records obtained in Quito during the 1987 earthquake. Records obtained during a M=4.4, 1990 earthquake that took place north of Quito were used to generate the input motion for the local earthquake. Finally, records at rock sites obtained for the 1985 Michoacan, Mexico, earth- quake were used for the subduction earthquake. The input motions were generated by calculating the response spectra of the available records, modi- fying them to fit target response spectra at the interface between the bedrock and the soil, and then combining the modified amplitudes of the response spectra with the statistically determined phase part of the spectra.

The responses of typical soil columns were calcu-

68 C~ Villaeis et al. / Engineering Geology 46 (1997) 63 70

ci ty o f

|k ing

|- (Sever,I

r+

r- (Strong) $+

|- (ModiK0te)

7 : ,6 3 4 6(km)

Fig. 7. Seismic intensity distribution for the hypothetical local earthquake.

lated for the 20 soil zones. Table 1 presents the estimated dynamic response to the postulated earthquakes and Fig. 7 shows the estimated distri- bution of the seismic intensity for the hypothetical local earthquake. The very strong ground shaking caused by this plausible event could register inten- sities of 8 in the MSK scale in the northern part of the city.

Fig. 8 shows a comparison of the observed and evaluated distribution of the seismic intensity for the actual 1987 earthquake. The good agreement demonstrates the soundness of the process.

5. Damage estimation

An inventory of the buildings in Quito was carried out and 15 structural types were identified,

ASK scale

(Sl)

8- (7.6-S.0)

7+ (7.0-7.6)

7- (e.5-z.o)

6+ (S.0-6.5)

[55] e- (S.S-S.O)

5 snd under

Fig. 8. Observed and evaluated intensity distributions for the actual 1987 earthquake.

ranging from adobe and unreinforced masonry to high-rise RC buildings and steel industrial con- structions. However, there were no damage-inten- sity relations available in Ecuador to estimate the building damage that could be caused by the hypothetical earthquakes. The problem was tack- led dividing the existing structures in Quito into two groups: engineered and non-engineered struc- tures. The first group includes mostly RC and steel structures, and the second adobe, unreinforced masonry, and owner-built structures. Considering that the current Ecuadorian construction code is based on the Californian code, damage matrices for the engineered structures were prepared

c Villacis et al. / Engineering Geology 46 (1997) 63-70 69

adopting the damage matrices provided by the Applied Technology Council (1985; ATC-13) and modifying them to meet the local conditions. The damage matrices for the non-engineered structures were constructed by local experts with the advice of international experts from Peru, Mexico, Guatemala, and other people familiar with Latin American construction.

Building damage was estimated for each urban block (approximately 100x 100m) of the city. There are almost 10 000 blocks in Quito. Fig. 9 shows the estimated distribution of structural damage resulting from the local earthquake, which is the most damaging among the adopted hypo- thetical events.

A similar process was employed to estimate the damage to the lifeline facilities and the city's service systems. The results of the damage estimation were

| i

Fig. 9. Distribution of structural damage resulting from the local earthquake.

presented to and discussed with the local authori- ties, local experts, and the people in charge of the lifeline systems and services of the city. The damage estimates served as the basis for the design of an Earthquake Risk Management Program for Quito. In this way, the project succeeded in raising awareness about the seismic risk in Quito, and in producing a series of recommendations that, if implemented, will reduce that risk.

6. Conclusions

The Quito project demonstrated the feasibility of adapting existing microzoning methods to meet the conditions found in developing countries and the effectiveness of their use in reducing the urban seismic risk. It also showed that the adaptation of existing microzoning methods to different condi- tions can lead to their review and improvement through the incorporation of the latest develop- ments in seismic instrumentation, data manage- ment, and analysis.

It is necessary, however, to develop standard microzoning methodologies that regulate the qual- ity and management of the data, the accuracy of the estimates, and the presentation, use, and dis- semination of the results, regardless of the place were the studies are performed. Standard sources of information, such as remote sensing data, should be incorporated for this purpose.

The use of the GIS allows a fast and convenient handling of large data files. During the Quito Project, the GIS made it possible for the partici- pants to work coordinately in three different conti- nents. The incorporation of GIS techniques is strongly recommended for future microzoning studies.

7. Final remarks

The Earthquake Risk Management Project for Quito improved the management of the city's seismic risk, in several ways. First, it increased the understanding of Quito's earthquake risk. It estimated more comprehensively than ever before the consequences of potential earthquakes on the

70 C Villacis et al. / Engineering Geology 46 (1997) 63-70

city. it produced the most complete survey of Quito's urban infrastructure, with emphasis on its vulnerability to earthquakes.

The awareness of Quito's earthquake risk was raised. Because of the project's nine workshops in Quito and the California tour for Ecuadorian officials, more than 100 Ecuadorian and interna- tional specialists discussed for the first time Quito's earthquake history, hazard and risk. The project brought together for the first time the people in Quito's public and private sectors who are concerned about earthquake hazard.

Finally, the project made significant contribu- tions to the goal of designing programs to manage Quito's earthquake risk. A framework of a com- prehensive, multi-year program to manage Quito's

earthquake risk was developed and submitted to the Mayor. Its significance derives as much from its content as from its being developed by leaders of Quito, who are capable of implementing it and who probably would not have produced it without this project.

References

Applied Technology Council (ATC-13), 1985, Earthquake Damage Evaluation Data for California, Report ATC-13, Redwood City, California.

Egred, J., 1992, Actualizaci6n de Cat~dogos Sismicos, Ecuador, Escuela Polit6cnica Nacional, Quito, Ecuador (in Spanish).

Neumann, F., 1954, Earthquake Intensity and Related Ground Motion, Univ. Press, Seattle, Washington.