SITE EFFECT CHARACTERISTICS OF DAMAGE CONCENTRATED AREA DUE TO THE 2003 COLIMA EARTHQUAKE (M7.6), MEXICO

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1 th World Conference on Earthquake Engineering Vancouver, B.C., Canada August -6, Paper No. 5 SITE EFFECT CHARACTERISTICS OF DAMAGE CONCENTRATED AREA DUE TO THE COLIMA EARTHQUAKE (M7.6), MEXICO T. Enomoto, J. Lermo, M. Navarro, N. Abeki, K. Masaki 5 SUMMARY In st of January,, Colima City, the capital city of Colima Province, was attacked the Colima Earthquake (M7.6) occurred in subduction zone along to the Pacific Coast of Mexico and suffered many damages for buildings which were one or two storied masonry or masonry wall with RC framed buildings. The epicentre was located about km southeast far from Colima City. The estimated seismic zone was just northern area of the 985 Michoacan, Mexico Earthquake (M8.). Also there were many big earthquakes occurred in this subduction zone and, actually in 995, Colima Earthquake (M7.9) was occurred at just northern area of the Colima Earthquake and serious damages of buildings were distributed in Manzanillo but the damages were very slight in Colima City. After the 995 Colima Earthquake, the seismic microzoning was performed in Colima City using earthquake and microtremor measurements. So, we investigated the relationship between the damaged building distribution and the result of microzoning. And also, in order to investigate the ground shaking characteristics at damage concentrated area in Colima City, we observed microtremors densely, about 6 points, in this area. The damage concentrated area was divided into three regions, Region was Villa de Alvarez region, Region was historic center region and Region was business region, were located and caught by small rivers. In averaged predominant period of surface shallow soil structures of each region, it s about. and.56 sec in Region, about. and. sec in Region and also about.7 and. sec in Region. And it s not so clear but there appeared long period in H/V spectral characteristics, about.. sec in all regions due to the deep soil structure. Especially, in Region, we performed more detailed analysis about the site effect. INTRODUCTION The Colima, Mexico Earthquake (Ms7.6) occurred at :6: in st January, (Local time), the epicenter had located at near the Pacific Coast in Mexico, and the heavy Kanagawa Univ., Japan, UNAM, Mexico, Almeria Univ., Spain, Kanto Gakuin Univ., Japan, 5 Aichi Institute of Technology, Japan

2 damages for residents and building structures were distributed widely in Colima State and many damages were concentrated in Colima City, the capital city of Colima State. The epicentral distance was about km far from Colima City in south-west direction and the source region was located in subduction zone along to the Pacific Coast at Mexico. This subduction zone is very high seismic activity and then many big earthquakes which have large magnitude, it s about 8. in Mw scale, for examples, the 985 Michoacan, Mexico Earthquake (Mw8.) had occurred in the just south region of the Colima Earthquake along to the subduction zone and recently the 995 Colima, Mexico Earthquake (Mw7.9) had also occurred in the just north region of the Colima Earthquake. In the 995 Colima, Mexico Earthquake, the damage concentrated area was very different from the area where were damaged due to the Colima Earthquake. The heavy damaged area due to the 995 Colima Earthquake was Manzanillo City where is faced to the Pacific Coast but the heavy damaged area due to the Colima Earthquake was Colima City where is located at about km inside from the Pacific Coast and there were not so heavy damages in Manzanillo City. Of course, the location of epicenter, source region, scale of magnitude and damaged building characteristics were different between both earthquakes but it s very strange that even if the epicentral distance of Manzanillo City was shorter than that of Colima City in the Colima Earthquake, but the damages in Colima City were heavier than those in Manzanillo City. One of the reasons that realized such difference in both earthquake, we thought that the underground conditions in deep structure and in shallow structure in this region, mainly concerned to the shallow soil conditions. After the 995 Colima Earthquake, the microzoning study was performed in the damage investigation in Colima City by using microtremor observations and the investigated results were already reported. We have made the densely microtremor observations at damage concentrated areas in Colima City and evaluated the predominant frequency due to the shallow soil structures and compared with the building structures. In this paper, we would like to show the results of the distribution of the predominant period obtained from microtremor observation and the damages of building structures. OUTLINE OF THE COLIMA EARTHQUAKE The epicenter was located at N8., W.6 and the focal depth was about km deep. The magnitude was evaluated Mw7.6 [SSN] and also the several institute have estimated and reported and reported as M w 7. [Harvard Univ.] and M w 7.8[USGS]. The source parameters were estimated as L=6km, W=km, M =.6xNm and D mean =.5m and the source mechanism was also estimated as a low angle thrust type slip occurred in plate boundary between the Pacific plate and the North American plate. The main damaged areas were Colima State and Jalisco State indicated in Fig.. As for the human damages, the death was 6, the injured was about [Reported at rd of January, ] and as for the damages of building structures were about in the damaged area, mainly in Colima State. ) - ) Fig. Location of Epicenter and evaluated seismic intensity due to the Colima Earthquake of January,.

3 Table Statistical information about building damages Name of Town Total Number of Number of DamagedNumber of Slightly Number of Middle Number of Heavily Damage Ratio Heavily Damage Ratio Colima Prov. Buildings Buildings Damaged Buildings Damaged Buildings Damaged Buildings (DR) (HDR) (NN) (DN) (SN) (MN) (HN) (%) (%) Armeria 6,869,5, Colima(City),5 6,8,8,, Comala, Coquimatlan,87, Cuahtlemoc 6, Ixtlahuacan, Manzanillo,66, Minatitlan, Tecoman,85 5, Villa de Alvarez,, Total,,9,89,997,9.7. DR=(HN+.5*MN)/NN, HDR=HN/NN, Number of Heavily Damaged Building : Going to Demolish Villa de Alvarez Colima City Fig. Location of cities and towns in Colima Province Table and Fig. show the statistical information about the damages of building structures. GEOGRAPHICAL AND GEOLOGICAL FEATURE IN COLIMA CITY The geological condition in Colima City and its vicinity is related to the volcanic materials from the Colima Volcano which is located at about km far from Colima City in the north-east direction. Colima Volcano is very active in the recent volcanic activity and erupted in December,, about one month before the occurrence of the Colima, Mexico Earthquake. In the south part of Colima City, there are widely distributed the basement rock constituted by limestone and the volcanic materials from Colima Volcano was stopped the flow down by the outcropped basement rock and accumulated and covered the basement rock. Basically, the ground surface is gently inclined from northeast to southwest direction influenced by the Colima Volcano s activity. Colima City was developed on the deposit of the volcanic materials from Colima Volcano. The elevation of northeast area in Colima City where is the highest point is 5m high and of southwest area where is the lowest point is m high and ground surface level in the city area and its vicinity is gently inclined along to northeast to southwest direction as mentioned above. Fig. shows the geological situation in Colima City. The surface geology is different depending on the areas in the city and they are divided into categories, (i) Acidic Volcanic Breccia (Bva), (ii) Arenaceous Conglomerate (ar-cg), (iii) Conglomerate (cg), (iv) Alluvial Stratum (al). According to the above mentioned categories indicated in Fig., the north-east part of Colima City is constituted by Andesite Conglomerate and the west part of Colima city is constituted by arenaceous conglomerate and the almost all area of Colima City is constituted by volcanic materials of Colima Volcano including Andesite Conglomerate. The area of southeast part of Colima City is developing the

4 alluvial deposit and the depth of alluvial deposit is not so thick and, basically, the main soil materials are constituted by gravel stratum. And there are many small rivers which are flowing down from northeast to southwest direction according to the inclination of landform in Colima City. Damaged Area Fig. Geological condition in Colima RESULTS OF MICROZONING STUDY The profile of deep underground structure at the area of Colima City and its vicinity is not so clear but, in general, the basement rock is constituted by tuff and the depth of the volcanic materials covered the basement rock is estimated about 8m deep. This deposit was generated by many time debris flow phenomena and was constituted from the multiple complex layers of gravel, sand and clay of volcanic rock origin. It can be seen that the upper most layer is constituted by alluvial stratum. The continuous microtremor observation related to the microzoning study in Colima City after the 995 Colima Earthquake was performed. From the reported results of observation, it s appeared two specific peaks clearly in the Fourier Spectra, one is appeared in a little bit long period range at about. sec which is generated by microseisms concerned to the deep underground structure and the other is appeared at about.. sec which are generated by microtremors concerned to the shallow soft soil structure in the surface layer. And these specific peaks are recognized in stable situation. ) ) DAMAGE DISTRIBUTION OF BUILDING STRUCTURES Many damaged buildings due to the Colima Earthquake were distributed in Colima City and Villa de Alvarez Town, located just west of Colima City. Fig. shows the damage distribution of buildings in Colima City and Villa de Alvarez Town and it can be seen that there were three regions (Region ) where the damaged buildings were concentrated and, especially, densely distributed in Region. These three regions were located at the areas along to the rivers which are flowing down from northeast to southwest direction in the city. And also, these regions were situated at the boundary of the different geological conditions, namely, between Acidic Volcanic Breccia (Bva) and Andesite Conglomerate (ar-cg) which are indicated in Fig.. Especially, Andesite Conglomerate (ar-cg) are constituted by multiple soil layers of sandy and claly soil layer including roundish Andesite Conglomerate (ar-cg) which has about.m diameter in maximum. So, the surface soil layer is constituted a relatively soft soil condition which is about m/s in S-wave velocity. In general, the characteristics of building structure existed in three regions are non strengthening masonry structures called adobe and mamposteria which are low rise and small scale building with - stories. Especially, the adobe structures are older

5 than mamposteria structures and suffered more heavy damage and the damage ratio is higher in statistic consideration. The damaged buildings are shown in Photo (a) Photo (f). Photo (a) Photo (c) show the heavy damaged building located in Region, but main destroyed building structures were already demolished. Photo (d) Photo (f) show the typical shear cracks generated in the structural walls just beside of the open space of buildings located in Region and. So, we could estimate that the seismic intensity was much larger in Region than the intensity in Region and. Fig.5 shows the damage distribution in Region, where was the most damage concentrated area, with the damage conditions. Region Region Region Fig. Location of Region of damaged areas in Colima City, Villa de Alvarez and its vicinity and Distribution of microtremor observation sites. Fig. 5 Distribution of damaged buildings in Region, Villa de Alvarez

6 (a) (b) (c) (d) (e) (f) Photo Damaged situation of buildings in damage concentrated area in Colima City and Villa de Alvarez. 5) 6) GROUND SHAKING CHARACTERISTICS BY MICROTREMOR OBSERVATION The microtremor observation was performed at about 5 sites during days from 8th of February to rd of March,, densely setting about 5m interval in the three regions (Region ) where the damaged buildings were concentrated. The microtremor observation was done by the SPC-5 (Tokyo Sokushin Co. Ltd), servo type, components velocity seismometer with specific amplifiers and observed three components, N-S, E-W and U-D component simultaneously. The frequency characteristics are flat in more than.hz. The total number of observation site is 5 and the duration of observation was 8 seconds at each site with Hz sampling rate. The stable time window with.96 seconds in the observed waveform was chosen at each site and calculated the Fourier Spectra of three components by FFT method with the Parzen s window (Bandwidth is.hz) for smoothing and finally obtained the H/V spectra which is obtained from dividing the two dimensional composed horizontal components, N-S and E-W component, by the vertical component, U-D component. Fig.6 shows the examples of observed microtremor waveforms, N-S, E-W and U-D components, respectively.,5, (a),,8 (b),,8 (c),6,6 Amplitud (mkine),5, -, 5 Amplitud (mkine),,, -, -, Amplitud (mkine),,, -, -, -, -,6 -,6 Componente N-S -, Tiempo (s) -,8 Componente E-W -, Tiempo (s) -,8 Com ponente Vertical -, Tiempo (s) Fig. 6 Waveforms of observed microtremor, (a) N-S Component (b) E-W Component (c) Component

7 Ground Shaking Characteristics of Damage Concentrated Area As mentioned above, the damaged buildings were concentrated in Region, so, we investigated the ground shaking characteristics in this region using the observed microtremor records. Fig. 7 shows the damage concentrated area in Region. Fig.7(a) shows the spectral characteristics at site E7 where was not damaged area and also Fig.7(b) shows the spectral characteristics at site M where was heavy damaged area. As indicated in the H/V spectrum at E7 and M, the clear peaks appeared at.6 sec and continued high amplitude upto. sec at site E7 where was not so heavy damaged zone but the spectral peaks appeared at about.. sec and.5.6 sec at site M where was heavy damaged zone. The difference is very clear that the area, where was occurred the heavy damage in buildings with one or two stories were concentrated, have appeared the clear peaks in spectral characteristics at short period range about.. sec. And Fig.8(a) shows the distribution of H/V spectral characteristics at several sits where were not so heavy damaged buildings and also, in comparison, Fig.8(b) shows the distribution of H/V spectral characteristics at several sites where was concentrated the heavy damaged buildings. ESPECTRO (mkine*s),, E-7 N-S E-W Vertical ESPECTRO (mkine*s), E-7 HORIZONTAL VERTICAL 5 T =,6 s E-7,,,, Fig.7(a) Fourier spectra and H/V Spectrum at non damaged zone, site E7 Site E7 Site M ESPECTRO (mkine*s),, E- M- N-S E-W Vertical, ESPECTRO (mkine*s),, E- M-, HORIZONTAL VERTICAL 5 T =,5 s T =,5 s Fig.7(b) Fourier spectra and H/V spectrum at damaged Zone, site M- M-,

8 5 T =.6 s M6, T =, s, E 5 T =, s 6 T =.6 s 5 T =,55 s M8,, E 6 5, Fig.8(a) H/V Spectral characteristics at several sites in non damaged Zone T =,58 s T =,5 s T =,6 s, E E 8 6 T =, s T =,6 s E8, 5 T =,5 s T =,6 s, Fig.8(a) H/V Spectral characteristics at several sites in heavy damaged Zone M

9 Site Effect Characteristics of Damage Concentrated Area We observed the microtremors at many sites in Region. So, we investigated the spectral characteristics for the site effect by carefully dividing the microtremor observed sites into two groups, which are the site where were not existed damaged building near the site and the site where were existed heavy damaged buildings near the site, and superposing the H/V spectrum of several sites. Fig.9(a) shows the results of the superposing H/V spectrum and the averaged characteristics of H/V spectrum at no damaged zone and also Fig.9(b) shows the results of the superposing and the averaged characteristics of H/V spectrum at heavy damaged zone, respectively. Even if in the comparison of averaged H/V spectral characteristics between the no damaged zone and heavy damaged zone, it s very clear that there appeared the spectral peak at about.. sec in the heavy damaged zone. Usually, it s estimated the natural period of buildings by using the relationship between the natural period of building T and the number of building stories N, which has represented by the relation, T=.5 or.6n. We don t have any information about such kind of relationship in Colima City but we could estimate that the natural period of heavy damaged buildings would be evaluated about.. sec in case of two storied building in this area. So, the natural period of building for the heavy damaged buildings in damage concentrated area in Colima City is very close to the predominant period of shallow surface soil and the resonant effect was generated in the damage concentrated area, especially in Region. And it can be seen that the small peak in the range of.. sec is commonly recognized at each region, respectively. The frequency characteristics obtained from H/V spectra is agreed with the reported results from microzoning study in Colima City after the 995 Colima Earthquake., T =,7 s, Fig.9(a) Site effect characteristics using by H/V spectra at several sites in non damaged zone and its average 5, T =, s T =,5 s, Fig.9(a) Site effect characteristics using by H/V spectra at several sites in heavy damaged zone and its average

10 CONCLUSIONS The damaged buildings were concentrated in three regions in Colima City and its vicinity due to the Colima Earthquake. These regions were located at areas where are caught by the small rivers flowing down from northeast to southwest direction along to the inclination of ground surface developed in the areas of Colima City and Villa de Alvarez. Taking into account the influence of surface shallow soil condition to the consideration of building damages, we performed the microtremor observation densely in three regions and compared with the former result of microzoning study. () According to the results of microtremor observation, the averaged predominant peaks generated by surface soil condition are appeared at about. and.56 sec in Region, at about. and. sec in Region and at about.7 and. sec in Region, respectively. And also, the peaks are recognized at the range.. sec in a little bit long period range, commonly, in each region and these long period characteristics is caused by the microseisms concerned to the deep underground structure in this area. Finally, the results of predominant period of soils obtained from microtremor observations are very similar to the former results in Colima City and its vicinity. () We analyzed the site effect characteristics in the damage concentrated area, Region, by dividing the microtremor observed sites into two groups, which are no damage sites and heavy damage sites. The H/V spectral characteristics were different clearly. At the heavy damage sites, there are appeared the clear spectral peaks in short period range about.. sec and, inversely, at the no damage sites, there were not appeared the spectral peaks in short period range. So, we thought that the short period component, about.. sec, generated the strong influence to the heavy damaged buildings as the resonant effect between the surface soil conditions and the building conditions. () We thought that the surface soil condition in Colima City and its vicinity, in general, is very soft condition and it means that the upper most layer of shallow soil structure is so softy and there are situated a very clear contrast between the upper most surface layer and lower layer, maybe constituted from gravel and sand. And we thought that, especially in damage concentrated Region, the depth of surface soft soil layer is very thin and we must check the borehole data for getting the evidence of the composite feature of the detailed shallow soil structure in the future. REFERENCES )Servicio Sismologico Nacional : Reporte de Sismos Sismo de Colima de Enero - ; Instituto de Geofisica, UNAM, Enero, )Sergio Alcocer, Robert Duran, Leonardo Flores, Carlos Gutierrez and Carlos Reyes : El sismo de Tecoman, Colima del de Enero de ; Observaciones Pleliminares en el Estado de Colima, CENAPRED, 7 de Enero de )J. Lermo, J. Diaz de Leon, E. Nava y M. Macias: Estimacion de periods y dominantes y amplificacion relativa del suelo en la zona urbana de Colima; Memorias del Congreso Nacional de Ingenieria Sismica (Manzanillo), 99 )C. Gutierrez, K. Masaki, J. Lermo and J. Cuenca : Relative Amplification and Dominant Period Map for Seismic Motion in Colima City, Mexico; WCEE, Paper No.65, 996 5)T. Enomoto, N. Abeki, M. Navarro and J. Lermo : Study on Distribution of Damaged Houses due to Colima Eq.(M7.6), Mexico and Ground Shaking Characteristics ; Summaries of Technical Papers of Annual Meeting, AIJ, B-, pp.8-8, 6)M. Navarro, T. Enomoto, J. Lermo and N. Abeki : Evaluacion de los efectos de sitio en la ciudad de Villa de Alvarez despues del terremoto de Colima de ; Programa y Resumenes, VII Asamblea Hispano Portuguesa de Geodesia y Geofisica, CD-ROM,