[S-SS04] New trends in data acquisition, analysis and interpretation of seismicity

[SSS04-07] Site Effects Analysis of Shallow Subsurface Layer Using Borehole Seismic Arrays in Taiwan

*TZSHIN LAI^1,2、Wei-An Chao^3,4、Yih-Min Wu^2,5,6 (1.Seismological Observation Center, Central Weather Administration、2.Department of Geosciences, National Taiwan University、3.Department of Civil Engineering, National Yang Ming Chiao Tung University、4.Disaster Prevention and Water Environment Research Center, National Yang Ming Chiao Tung University、5.Institute of Earth Sciences, Academia Sinica、6.Research Center for Future Earth, National Taiwan University)

キーワード：Site effects, Amplification factors, Kappa, Borehole seismic array, Local magnitude

Since the inception of 62 borehole seismic arrays deployed by Central Weather Administration (CWA) in Taiwan in recent years, a large quantity of strong-motion records have been accumulated from frequently occurring earthquakes around Taiwan, which provide an opportunity to understand the site effects (e.g., amplification) caused by the subsurface materials. Each borehole array includes two force balance accelerometers, one at the surface and other at a depth of a few ten-to-hundred (30-492) meters, as well as one broadband seismometer is below the borehole accelerometer. . In general, the background seismic noise level are lower at the borehole stations than surface stations, facilitating to detect microseismicity. The resulting of site-specific spectral decay parameter κ0 (Kappa value) has been derived by our pervious study, and relative higher κ0 could be observed at surface station, indicating a strong attenuation effect. In this study, records of 19,079 earthquakes were collected to determine site amplifications corresponding to site corrections of the surface-to-downhole. The results reveal that amplification factors ranging from 1.10 to 6.06 can be used to describe site effects. These amplification factors are strongly correlated with Vs30 (average shear-wave velocity for the top 30 m of strata). Furthermore, we conduct a series investigation of borehole-based and surface-based M_L values for a purpose of providing the comprehensive earthquake catalog.