||Both TAP056(Chutzuhu) and TAP066(Anpu) strong motion stations were installed in the Tatun area by Taiwan Strong Motion Instrumentation Program of CWB since 1992. It is discovered that unusual PGA value recorded at TAP056 over the years. In this study, local site effects are analyzed by using earthquake and microtremor survey data for understanding why TAP056 had this unusual phenomenon.|
First of all, we analyze the 18 earthquake records at TAP056 and TAP066 during the period from 1995 to 2002. In time domain, PGA ratios between TAP056 and TAP066 are: EW: 7.57±6.25, NS: 5.45 ±4.51, and V: 3.89 ±2.86. In frequency domain, dominant frequency recorded at TAP066 (rock site) varies with events. But for different earthquake events, dominant frequency at TAP056 is always located at around 8Hz. The spectra ratio of TAP056/TAP066 shows the amplification frequency at 8~10Hz. Second, we performed 17 microtremor measurements in the lawn which the TAP056 seismometer is located. According to the contour of the H/V Fourier spectrum ratio at different frequencies, we can see the position where the TAP056 is located higher than others. All of the microtremor measurements show the same dominant frequency at 5~6Hz. But only measured points near TAP056 station can found an amplified peak at around 8~10Hz. This result demonstrated that the microtremor measurements can be applied in small scale. Third, In order to check local site effect on this grassplot, we set up the other seismometer(056A) about 20 meters away from the grassplot. The spectra ratio of 056/056A also shows the amplification frequency at 8~10Hz. After the refraction survey, we can sure that the local back filled or top soft soil effects controlled the local site amplification at station TAP056.
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