Site Effect Study in Taiwan Using the Surface-Downhole Seismic Stations

Abstract eng:
A high-quality surface-downhole monitoring network is under construction in Taiwan by the Central Weather Bureau (CWB) since 2008. Twenty-nine surface-downhole stations had been installed in Taiwan until the end of 2013. Each station includes a pair of force balance accelerometers at the surface and the downhole, respectively, as well as one broadband velocity seismometer at the downhole. Earthquake records of those stations are very helpful in studying site effects in Taiwan. This study analyzes site effects including characteristics of Peak Ground Acceleration (PGA) and empirical transfer functions at fifteen of those stations using ground motion recordings from earthquakes with local magnitudes (M L ) of lager than four during 2012 and 2013. Moreover, all strong motion recordings of four large earthquakes (M L > 6) are utilized to illustrate shake maps and magnitude differences for the surface and downhole locations. P-wave and S-wave velocity profiles at thirteen stations measured by a suspension PS-logging system are compiled by this study to calculate Vs30 at the sites. Site amplification factors of PGA are calculated by the ratio between the surface and the downhole recordings. The mean borehole amplification factors of PGA vary from 2 to an exaggerated value of 20 at different stations. In addition, a power-law relationship between PGAs at downhole and surface is evaluated to understand amplification variation with increasing PGA. Strong ground motions with and without site effects throughout Taiwan are able to be seen via the differences of the shake maps illustrated using the surface and downhole accelerations from four significant earthquakes with magnitude of larger than six in 2012 and 2013. The intensity differences at the surface and the downhole evidently indicate the site amplifications caused by the sediments during the large events. Empirical transfer functions derived by the single-station and the two-station methods at same stations show comparable dominant frequencies; however, the empirical transfer function from the two-station method shows clearer resonant peaks not only at fundamental frequencies but also at higher-mode of resonant frequencies. Moreover, this study further indicates that the local magnitudes calculated by surface recordings are obviously larger than those by downhole recordings. The differences are believed caused by the amplification of the sedimentary layers between the surface and downhole accelerometers. The difference of estimated local magnitudes is 0.38 in average for those large events.

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