Use of seismic microzoning for risk management in Quito, Ecuador

Use of seismic microzoning for risk management in Quito, Ecuador

ENGINEERING 6EOLOQY ELSEVIER Engineering Geology46 (1997) 63-70 Use of seismic microzoning for risk management in Quito, Ecuador Carlos Villacis a,...

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ENGINEERING

6EOLOQY ELSEVIER

Engineering Geology46 (1997) 63-70

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 concentrate 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 implemented for Quito, the capital o f 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: [email protected]

tiveness of existing microzoning methods in reducing the seismic risk, and their applicability to the conditions found in developing nations. The distributions 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 accuracy. Due to insufficient data, existing techniques were adapted to meet the local conditions. This

0013-7952/97/$17.00Copyright© 1997ElsevierScienceB.V. All rights reserved PII S0013-7952 (96) 00086-5

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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 population, 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

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S O l J ~ AMBRICAN , PLA'I~

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, hazardous 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 amorphous 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 PACIFIC OCF.AN

Fig. 1. Location and tectonics of Ecuador.

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

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

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Distance A (km) Fig. 3. Earthquake data for the derivation of the attenuation relations.

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 thousands of deaths. More recently, the 1987 earthquake (M=6.9, A = 8 0 k m ) produced serious damage to several structures in the city. Two main types of seismic activity were identified from the seismo-tectonic studies: one originated 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 earthquakes 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 earthquakes are shown by the black dots in Fig. 2.

The distributions of the seismic intensity resulting 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 relationsfor Quito Countries like Japan and the United States have long histories of instrumental seismicity and several 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 magnitude-distance correlation from the database and attenuation relations were derived for the two

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main types of earthquakes: intraplate and subduction 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 (1) For subduction earthquakes: 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 N e u m a n n (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

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

Lake Sediments

~ 1 Young Slidings

At first, topographical characteristics were considered 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-

Old Volcanic Sediments

Alluvial Cones

Altitude

meters 3100

Canzahua

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I-'-7 Volcanic Sediments

2900 2700 2500 2300 2100 Fig. 6. An example of a geologicalcross-section for Quito (line A-A' in Fig. 5).

Volcanic Rock

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Table 1 Estimated soil response to the hypotheticalearthquakes Zones

fl f2 f3 f4 f5 f6 f7 11 12 13s 13n 14 15 pn ql q2s q2n q3 q4 q5

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 accel,

Amplif. ratio

Surface accel,

Base accel,

Amplif. ratio

Surface accel,

Base accel,

Amplif. ratio

Surface accel.

25.4 26.1 27.6 28.4 28.2 28.1 27.0 26.4 25.9 28.4 29.2 29.0 28.6 27.1 27.1 29.3 29.9 29.6 28.9 24.7

3.17 4.4 4.4 3.2 3.3 3.4 2.6 4.0 3.5 4.2 4.0 3.8 4.1 3.1 4.3 3.7 3.2 4.1 3.5 3.7

78.9 114.1 121.4 89.3 91.9 96.2 69.4 104.1 90.7 120.5 117.5 110.1 118.0 84.3 116.1 109.9 94.0 122.9 101.9 90.9

24.0 23.8 23.3 23.2 23.4 23.6 23.6 23.6 23.8 23.1 22.9 22.9 23.3 23.5 23.2 22.8 22.9 22.9 23.3 23.8

2.2 2.9 3.5 2.4 2.6 3.6 2.1 3.8 2.9 3.9 3.2 3.4 3.8 2.2 3.0 2.8 2.4 4.0 3.0 2.8

52.1 68.8 81.9 54.8 59.8 84.4 49.3 88.8 68.9 90.0 72.6 78.4 89.8 51.0 69.7 62.6 53.4 92.3 93.9 67.5

26.7 29.2 31.7 38.6 48.0 59.6 32.3 27.7 27.2 34.8 41.4 36.8 47.2 31.2 27.5 37.2 56.1 48.9 71.1 21.9

3.8 4.9 5.6 3.8 3.6 3.6 3.2 5.3 4.3 4.7 4.2 4.0 4.1 3.8 3.8 4.3 3.8 4.1 3.7 3.7

102.1 141.5 173.6 147.1 173.2 211.5 101.0 147.5 117.1 162.0 174.8 149.0 194.1 119.2 104.5 159.2 214.0 198.7 262.3 82.1

cessing techniques on GIS data to consider the variations of elevation and ground slope (Fig. 5) Two problems, however, were found when modeling 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 determined using results of deep drillings carried out by the water supply organization, geological cross-sections prepared by local experts, and existing 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 basement 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, earthquake were used for the subduction earthquake. The input motions were generated by calculating the response spectra of the available records, modifying 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-

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city o f |king |- (Sever,I r+ r- (Strong) $+

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(Sl) 8- (7.6-S.0)

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

7+ (7.0-7.6) 7- (e.5-z.o) [55]

6+ (S.0-6.5) e- (S.S-S.O) 5 snd under

lated for the 20 soil zones. Table 1 presents the estimated dynamic response to the postulated earthquakes and Fig. 7 shows the estimated distribution of the seismic intensity for the hypothetical local earthquake. The very strong ground shaking caused by this plausible event could register intensities 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,

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 constructions. However, there were no damage-intensity relations available in Ecuador to estimate the building damage that could be caused by the hypothetical earthquakes. The problem was tackled dividing the existing structures in Quito into two groups: engineered and non-engineered structures. 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

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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 hypothetical 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

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presented to and discussed with the local authorities, 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 conditions can lead to their review and improvement through the incorporation of the latest developments in seismic instrumentation, data management, and analysis. It is necessary, however, to develop standard microzoning methodologies that regulate the quality and management of the data, the accuracy of the estimates, and the presentation, use, and dissemination 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 participants to work coordinately in three different continents. The incorporation of GIS techniques is strongly recommended for future microzoning studies.

7. Final remarks

|

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

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

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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 international 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 contributions to the goal of designing programs to manage Quito's earthquake risk. A framework of a comprehensive, 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,Actualizaci6nde Cat~dogosSismicos,Ecuador, Escuela Polit6cnicaNacional, Quito, Ecuador (in Spanish). Neumann, F., 1954,Earthquake Intensityand Related Ground Motion, Univ. Press, Seattle, Washington.