Analysis of Nonlinear soil response during the HengChun Earthquake Using Horizontal-to-Vertical Spectral Ratio Method

Shin-Wen Chen

Abstract

On December 26, 2006 at 20:26 and 20:34, two successive earthquakes with the same magnitude 7.0 on the Richter scale occurredin the southeast Taiwan Strait near HengChun peninsula, with one epicenter approximately 38.4 km southwest of HEN station, and another is located in 33.1km west of NEN station. The Central News Agency is reporting that it is the strongest earthquake to hit HengChun in one hundred years. The earthquake not only caused casualties and building damages, but also damaged several undersea cables, disrupting telecommunication services in various parts of Asia. The aim of this study is to research whether the nonlinear soil response occurred or not during the HengChun earthquake. The data we use include weak motion records observed by KAU stations during the period from 1999 to 2004 and 2006 HengChun earthquakes, which were collected by the Taiwan Strong Motion Instrumentation Program (TSMIP) of Central Weather Bureau (CWB). We use the Horizontal-to-Vertical(H/V) Spectral Ratio Method to conduct the analysis in this study because a good rock referent site is hard to find in PingTung alluvial plain. Two characters of nonlinear soil response from earthquake spectrum between weak and strong motions :(1)the dominant frequency of weak motion is larger than strong one; (2) the soil amplification factor have the same result as former. From our initial result, we find the nonlinear soil response in some stations, and most of all stations, the nonlinear soil response are not obviously under about 10Hz, and above 10Hz, we can see the phenomenon of deamplification from the spectra between weak and strong motions. Besides, the stations, KAU044 and KAU042 show the apparent characters of nonlinear soil response. The spectrum of KAU042 we can see the dominant frequency which shift forward to low frequency which compare with weak motions. And the spectrum of KAU044, the phenomenon of deamplification exists between 0.5Hz between 2Hz.

 

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