dc.description.abstract | Kaohsiung city is the most important harbor and second large city in Taiwan. Recently, there are many high-rise buildings and public transportation system are under construction in this area. Therefore, it is very important to know the surface geological conditions for many practical reasons especially after the strikes of 1999 Chi-Chi, Taiwan earthquake. The shear wave velocity is a very important parameter in earthquake engineering problems and essential for studying earthquake characteristic.
This study estimates the S-wave velocity structures from the surface to a depth of 3 km by six microtremor arrays. The shallow S-wave velocities also compare well with which get by P-S logging. To serve the purpose of earthquake hazard mitigation, it would be better to understand the soil amplification effect of the Kaohsiung-Pingtung area. We then conducted a research to study the site effects of the area, which includes analyze three installed borehole seismometer arrays, and perform very dense microtremor measurements in the study area. After carefully selection, we pick 705 records and use the H/V ratio method to get information of soil amplification. In this study, we select several frequencies to plot out the contour map for understanding the frequency responses in this area. For the 0.6 ~ 1.5 Hz, the contours show that main amplification effects occurred at the southern part of Kaohsiung area and most Pingtung plain. With the frequency increasing to 2.0 Hz, the main amplification area move from the plain area to the hill area. For the higher frequency (3.0 Hz), there are no obvious high contour areas in the plain area. We pick the dominant frequency of each record and plot out the contour map. At the plain area, the dominant frequency is about 0.6 ~ 2.0 Hz, and the hill area is great than 2.0 Hz. We found that the basement structure can explain the contour very well. Yet, the H/V dominated frequency distribution map reveals more detail features. The Vs30 can’t exactly respond the site effects in the plain area stations.
On December 26, 2006 at 20:26 and 20:34, two successive earthquakes with the same magnitude 7.0 on the Richter scale occurred in 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 H/V method is extended to identify nonlinear soil responses. From our result, we find most of the nonlinear soil response usually occurred in soft alluvial plain near shore area of Kau-Ping plain. The liquefaction site, Nanwan area, the phenomena of liquefaction was caused by the local refill soil of ground surface.
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