HR: 1340h
AN: S23B-0246    [Abstracts]
TI: A Study of Seismicity and Subsurface Structures by a Temporary Seismic Network in Northwestern Taiwan
AU: * Lee, C
EM: cplee@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No. 128, Section 2, Academia Road, Nankang, Taipei, 115 Taiwan
AU: Tsai, Y
EM: yibentsai@gmail.com
AF: Geosciences Department, Pacific Gas and Electric Company, Geosciences Department, Pacific Gas and Electric Company, San Francisco, CA 94177 United States
AU: Huang, B
EM: hwbs@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No. 128, Section 2, Academia Road, Nankang, Taipei, 115 Taiwan
AB: The seismicity is low in northwestern Taiwan recently. However, recent studies found special geological characteristics in this area. Besides, many important facilities, and big cities are in this area. Significant shaking intensities were recorded at strong motion stations in the Hsinchu area during the 1999 Chi-Chi earthquake. The disastrous Hsinchu-Taichung earthquake in 1935 occurred in the southern part of this area. Thus, recurrence of potential large earthquakes in this area becomes an important topic. In response, the seismicity and subsurface structures are analyzed in this study. A temporary seismic network consisting of ten seismic stations was deployed in Taoyuan, Hsinchu and Miaoli counties in northwestern Taiwan since January 2001. Each station has one triaxial accelerograph, three external one-component velocity sensors, global positioning system for timing, and a data storage device. Seismic records with absolute timing are critical to obtain accurate earthquake locations. Dense station distribution is necessary to locate earthquakes of lower magnitude from clear seismic signals recorded locally. Based on careful considerations of available instruments and recording sites, the stations were deployed uniformly in the study area. In this study, the arrival time data of the temporary seismic network and CWBSN are combined to locate earthquake. Two dense earthquake clusters in the study area were relocated to compare with several geological cross sections. In order to study the regional stress patterns, numerous focal mechanisms were determined by waveform inversion. By combining the temporary seismic network and CWBSN data, the results of earthquake location show significant convergent in focal depths by adding of near-source arrival time data. Most of the hypocenters are located shallower than 15 km at depth. Relocation of the two dense clusters using the JHD and DD methods was able to remove systematic bias due to one-dimensional velocity model. The events were shifted toward northwest in the horizontal direction and became more tightly clustered at depth from 5 to 10 km. The station corrections of JHD reflected the difference in geology of the northwest and southeast parts of the study area. It is also consistent with distinct topographic features. By comparing the relocated events with several geological cross sections, we found that the seismicity and subsurface structures are related. To determine the focal mechanisms by waveform inversion, we used the acceleration records. The acceleration records of individual stations with three-component sensors are doubly integrated to get displacement waveforms. Then the focal mechanisms are determined by waveform inversion. In total, 88 focal mechanisms were determined with local magnitudes from 1.35 to 3.33. The widespread presences of varying types of focal mechanisms imply that the microearthquakes might be associated with many subfaults.
DE: 7200 SEISMOLOGY
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: Fall Meeting 2005