Comparison of seismic characteristics of the surface ground in Asia using microtremor observations

Transcription

1 Comparison of seismic characteristics of the surface ground in Asia using microtremor observations A. Che, T. Iwatate & Y. Oda Department of Civil Engineering, Tokyo Metropolitan University, Japan Abstract In order to investigate the local site effects for seismic vulnerability of a study area based on its ground motion characteristics of the surface ground, microtremor measurements were conducted on the ground of typical sites in Asia (Zushi, and Ogasawara- Japan, Xi an-china, Manila-Philippines, and Gujarat- India) with different soil conditions. From these observation data, the fundamental frequencies and the dynamic characteristics of the surface ground were estimated by spectral ratio H/V of Nakamura s method [1]. These results showed good agreement with those obtained from earthquake observations. Moreover, the seismic responses of the ground were evaluated using the earthquake records. This study will be useful for the seismological micro zoning and planning seismic disaster mitigation in these sites. 1 Introduction It is a well-known phenomenon that earthquake ground motion can be amplified by local site conditions, which near surface unconsolidated soil and sedimentary deposits can amplify earthquake ground motion in a selected frequency band and increase the duration of shaking depending on the thickness and elastic properties of the surface layer. It is therefore extremely important for seismic design to estimate the dynamic characteristics of the surface ground. However, it is not easy to do this precisely, for example the method of investigations from borehole data is well used, but suffers from vast expense. Microtremor observation is one of the most convenient methods to investigate the dynamic characteristics of the surface ground. The applications of microtremor WIT Press, ISBN X

2 Geo-Environment observations to characterize site effects have been used by many authors and in a large variety of seismic environments [3]. These microtremor measurements are used to obtain the predominant frequencies and the dynamic characteristic of the research area by Nakamura s method H/V [1]. However, the wave of microtremor measurements is considered as a body wave in the method of H/V, which has no clear background theory. Thus, it is necessary to examine the relationships between the dynamic characteristics of the ground for an earthquake and the H/V method. In order to clarify the relationships and the seismic micro zoning and seismic risk assessment of the surface ground, we have engaged in detailed research in Asia (Zushi and Ogasawara - Japan, Xi an - China, Manila - Philippines, and Gujarat - India) using microtremor measurements since 199. In the Zushi area in particular, the seismic response of the surface ground was evaluated using the earthquake records. Geological conditions and microtremor observations of research areas Microtremor measurements were conducted on the grounds of the typical sites in Asia (Zushi and Ogasawara - Japan, Xi an - China, Manila - Philippines, and Gujarat - India) with different soil conditions. The measurements were recorded for 5 minutes at a sampling rate of 1 Hz, for a total of 3 measurements per shot. The records of the horizontal and vertical components of short-period microtremors were obtained, using a three-component high-sensitive seismometer, which has a natural period of 1 second. We tried to avoid punctual noise sources such as machinery, vehicle traffic or pedestrians near the seismometer during the measurement time..1 Research area in Zushi and Ogasawara (Japan) (1) The city of Zushi is located in Kanagawa Prefecture, in the southern part of Tokyo Metropolitan. The Zushi area is a particularly high seismic area, which suffered severe damage during the 193 Great Kanto Earthquake (M=7.9). Two major rivers run through the city, the Tagoe River and the Ikego River. Geographically, the city can be divided into three zones based on ground characteristics: the Holocene lowland zones along the banks and mouths of the main rivers; a zone consisting of reclaimed land near the coastline; and a hill zone (Figure 1). Microtremor measurements were conducted at the earthquake observation stations (K1~K5), and also along the North-South (NS) direction (9m) on the surface of K1, spaced approximately five meters apart in July, 199. The total number of observation points is. () The Ogasawara islands are located about 1km south of Tokyo Metropolitan on the Pacific. The Chichi-island is the greatest among the Ogasawara islands, with an area of about.5 km. The east coast of Chichiisland consists of steep cliffs, and the west coast consists of many gulfs with WIT Press, ISBN X

3 Geo-Environment 3 sandy beaches. Microtremor measurements were spaced at representative spots over the whole the Chichi-island with 1 observation points in Aug. 1999, and also in the Miyanohama area (1.5km.5km); the area is subdivided into square grids of 5m, thus summing up to 3 observation points (Figure ). N Ikego River K K1, K: Zushi K: Kotubo K3: Center A K3 Tagoe River K K1 A K5 K: Numama - 1 K5: Numama - 1km km A-A Figure 1: Location of observation stations at Zushi (K1~K).. Research area in Manila (Philippines) Manila and its immediate vicinity used to be a submerged area with the sea extending to the mountains on the east. Volcanic activities followed intermittently and, after which, deposition of volcanic materials occurred. During the intervening periods of inactivity, transported sediments were deposited on top of the previously laid volcanic materials. Thus, interbeds consisting of tuff beds and transported sediments became a characteristic feature of the deposit [, 5, ]. This research project limits the study area with a 1.5 km x. km land area in the western part of Greater Manila (GMA) located in the vicinities of Malate, Ermita, Intramuros, and some parts of Paco and San Andres. The area was selected given that the site has diverse constructions of buildings from residential (apartments and hotels), government facilities, hospitals, churches, academic institutions, to commercial (malls and office buildings) structures. Thus, it could be viewed that the area has the most growing demand of urbanity within the City of Manila. The area is subdivided into square grids of 5m, thus summing up to 3 observation points (Figure 7). The measurements were conducted in April 1999 []. WIT Press, ISBN X

4 1 3 5 Geo-Environment.3 Research area in Xi an (China) Xi an city is located in the central part of China, it is important for the politics, economy, culture and southwest traffic in Shanxi-ministry, China. In the past, a lot of earthquakes (Shanxi-Huaxian Earthquake, Ninxia-Haiyuan Earthquake) occurred in the area, and significant damage was caused. The geological conditions of Xi an area are constructed from loess soil. The ground of loess soil is constructed from Q1 (73,-5,, years), Q (13,-73, years), Q3 (1,-13, years, 1-15m depth), Q (-1, years, -5m depth). The microtremor measurement project in Xi an City was conducted in collaboration with Xi an Jiaotong University Department of Civil Engineering in Sep. 1. This research project limits the study area with a 1. km x. km land area in Xi an Jiaotong University located in the western part of Xi an city with a total of 3 points, and the centre of Xi an city with a total of points (Figure ). 1.5m 1.5m 35m N value Q Q3 Q 9m Photo of Loess soil N Borehole data 1m Figure : The research area shown with observation points and borehole data.. Research area in Gujarat (India) Gujarat State of India was severely shaken by a powerful earthquake (M7.9) at : am on, January 1 at India Standard Time, which was the most damaging earthquake in the last five decades in India. The state of Gujarat is largely barren except for a fertile band along the Gulf of Kachchh in the Arabian Sea. It was the scene of major border disputes between India and Pakistan in 195 and1971. Kachchh or Kutch is an erstwhile princely state of India. It is the largest district of the state of Gujarat and the second largest district in India WIT Press, ISBN X

5 coral coral coral coral shoal approx alignment Geo-Environment 5 covering an area of 5,1 Sq kms. The land is virtually an 'island' resembling a tortoise "Katchua or Kachbo", surrounded by sea water. Kutch was also known as the kutchdweep or Kutchbet. "The Great Rann of Kutch" which dominates a major portion of the district. The Great Rann of Kutch and the Little Rann of Kutch are respectively uninhabitable deserts which during the monsoon season (June to October) are often completely submerged by floods [9]. The microtremor measurements were conducted at several locations in Gujarat State in March 1 (Figure 3). RANN OF KACHCHH RAPA KACHCHH BHU ANJAR BHACHAU GANDHIDHAM LITTLE RANN OF KACHCHH AHMEDABAD approximate alignment elect GULF OF KACHCHH KANDLA PORT RAJKOT approximate 5 Km JAMNAGAR Figure 3: Geomorphic map of Kutch region. 3 Spectral analysis of microtremor observations dates In this study, from the recorded dates of microtremor measurements, five sets of digital dates from low noise periods were selected for FFT analysis. The velocity Fourier amplitude spectra, spectra ratio and relative variation amplitude were computed. The spectral ratio (H/V) proposed by Nakamura [1] was used to identify the fundamental resonant frequency (fp: Hz) of the surface ground. About the H/V (ratio of the vertical component to the geometric mean of the horizontal component), the average type ( NS EW ) to be suggested by Japanese UD Education Ministry was used. The values were calculated for the predominant frequency of all the measurements as shown as Figure [, 7]. The geological conditions of Xi an (China) and Gujarat (India) area are different from Zushi and Ogasawara (Japan), Manilla (Phillipines) (which has soft deposits in the surface ground), it is constructed from eolian soil. There are few contrasts between the surface grounds (Figures and 3). WIT Press, ISBN X

6 Geo-Environment 1 K1 1 K 1 1 K 1 1 K5 1 (1) Zushi () Ogasawara-Japan (3) Manila-Philippines H/V () Xi an China (5) Gujarat-India Figure : Transfer function predominant frequency values (H/V) based on microtremor measurements. WIT Press, ISBN X

7 Geo-Environment 7 From the fundamental characteristics of microtremors, it is not clear how to discuss the local site conditions of the Xi an-china and Gujarat-India areas. The predominant frequency obtained from the microtremor dates, using Nakamura s method of microtremor measurements at each site, did not show clear peaks (Figure (, 5)). As a result, the predominant frequency of surface ground, which has no clear boundary to its substratum, cannot be estimated by the H/V method from microtremor data. On the other hands, when the surface layer has a sharp impedance contrast to its substratum, the predominant frequencies showed clear peaks (Figure (1,, 3)). Dynamic characteristics of surface ground evaluated from microtremor observations The purpose of our research is to clarify the dynamic behaviours of the surface ground, which is important for the seismic response of the structure. Using the fundamental characteristics of microtremor, it can be used to discuss the local site conditions of the study area. In the following consideration, the Fourier spectral ratio using Nakamura s H/V method of microtremors at each site will be used in analysing the results. 1 H(S)/H(B) K1 1 H(S)/H(B) K H(S)/H(B) K 1 H(S)/H(B) K5 1 1 Figure 5: Spectral ratio of NS component of earthquake motion between ground surface and bedrock (K1/K). WIT Press, ISBN X

8 Geo-Environment Bedrock ~3Hz ~Hz Deposit gravel Slump deposit Red soil Rock Oxbow Hyaloclastite Palagonite Palagonite Mikaduki-moutain 三日月山 Chichi-island Kiyose Okumura 奥村 Futami-por 5(m) Figure : Geological condition and microtremor observations at Chichi-island..1 Dynamic characteristics of surface ground in the Zushi area In the Zushi area, not only the earthquake observation dates but also the microtremor observation dates estimated seismic characteristics of the surface ground. Figure 5 showed the spectral ratio of the NS component of earthquake motion between ground surface and bedrock (K1/K). From the results, the predominant frequency of the surface ground was examined [1]. Table 1: Comparison of the Transfer function predominant frequency values from H/V of microtremor measurements and seismic array observations. Earthquake records Microtremor observations K1 K K3 K K5,7Hz Hz -7Hz Hz Hz -7Hz Compared with the predominant frequency values from H/V of microtremor measurements and seismic array observations, it shows a good agreement WIT Press, ISBN X

9 Geo-Environment 9 (Table 1). Therefore, it can be considered that the seismic characteristics of the surface ground in the Zushi area can be evaluated by the spectral ratios (H/V) from microtremor measurements.. Dynamic characteristics of surface ground in Ogasawara and Manila The local site conditions of Ogasawara Chichi-island and Western Manila were also evaluated using the H/V spectral ratio from microtremor measurement dates. In Ogasawara area, the predominant frequency obtained from the microtremor data shows a peak of ~ Hz and no peak correspondence to the bedrock ground, which is in good agreement with its soil conditions (Figure ). In the Western Manila area, the predominant frequency obtained from the microtremor data shows a peak of 1~3 Hz which demonstrates that the soil layer of the site is of generally soft sediments. This is true of all lowland area in the whole National Capital Region (Metropolitan Manila) compared to highland areas that may consist of rock sites. The map with the distribution of predominant frequencies is shown as Figure 7. Pasiq River INTRAMUROS Manila South Harbor PACO ERMITA 1.~1.5Hz 1.5~.Hz.~.5Hz.5~3.Hz MALATE SAN ANDRES 1 Km Figure 7: The map with the distribution of predominant frequencies from Fourier spectral ratio (H/V) for each site. WIT Press, ISBN X

10 9 Geo-Environment 5 Conclusions (1) The microtremor measurements are useful to estimate the dynamic characteristics of surface ground, where the surface layer has a sharp impedance contrast to its substratum. The predominant frequencies of surface ground are obtained from the H/V spectral ratio. () In the case of the Zushi area, the results show a good agreement with the transfer function obtained from the earthquake observation dates. (3) In the case of the Manila area of the Philippines, and Ogasawara area, the dynamic characteristics of the surface ground are estimated by the H/V spectral ratio from microtremor measurements. () In the case of Xi an (China) and Gujarat (India), the dynamic characteristics of the surface ground cannot be estimated by the H/V spectral ratio from microtremor measurements. It can be considered that the condition of the surface ground is very hard and has not a sharp contrast to its substratum. (5) These results will be useful for seismological micro zoning and for planning seismic disaster mitigation in these sites. Acknowledgements The authors would like to thank civil engineering, Manila s De La Salle University, and Xi an Jiaotong University, for cooperation and giving us useful data. Special thanks are extended to Professor Maohong Yu of Xi an Jiaotong University, Mr. Shigeo Aoki of Hokuto institute and all the members of our group for the great assistance in the microtremor measurements. References [1] Nakamura, Y., A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, OR or RTR1, 3,1,199 [] Takahiro Iwatate, Study on seismic site response characteristics of Tokyo Metropolitan University (TMU) area, Proceeding of the second international symposium on the effect of surface geology on seismic motion, 1, 199 [3] Dowrick, D.J. Earthquake Resistant Design for Engineers and Architects Second Edition. John Wiley & Sons, 197 [] Dungca, J.R. Liquefaction Potential Map of Manila. De La Salle University Graduate Thesis, 1997 [5] Tan, R. Engineering Properties of Manila Subsoil. University of the Philippines Graduate Thesis, 193 [] Takahiro Iwatate, Stephen Santos Dy, Seismic Response Evaluation of Surface Ground of Western Manila, Philippines ICSZ,Palm Springs CA, USA, November, WIT Press, ISBN X

11 Geo-Environment 91 [7] Iwatate, T. et al. Seismic Site Response Characteristics of Tokyo Metropolitan University (TMU) Area. Memoirs of Graduate School of Engineering Tokyo Metropolitan University No., 199 [] Matsuda, I., T. Enomoto, E.L. Banganan, and I.C. Narag. Regional Division of Metro Manila on the basis of Geological and Geomorphologic Conditions. Bulletin of Institute of Science and Technology. No. 5, pp 11-11, 199 [9] Japan Society of Civil Engineers (JSCE), The 1 Kutch earthquake, Gujarat state, India-Investigation into damage to civil engineering structures-, 15, March. 1~. March. 1, CD-ROM [1] Takahisa Enomoto and Takahiro Iwatate, Site-effects evaluation H/V spectra comparing microtremor with strong motion records observed at ground surface and basement using borehole, 1th European Conference on Earthquake Engineering, WIT Press, ISBN X