3-D Shallow Shear-Wave Velocity Structures around the Source Area of the 1999 Chi-Chi (Taiwan) Earthquake

• Main Authors: Tseng , Yu-Ting, 曾渝婷
• Other Authors: Hwang , Ruey-Der
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/27181768982206146650

碩士 === 中國文化大學 === 地學研究所地質組 === 103 === To understand the ground-motion after an earthquake is helpful for seismic hazard, especially for shallow-depth S-wave velocity structure which is a key factor to predict the ground-motion. The 1999 Chi-Chi earthquake occurred in central Taiwan and resulted in great casualties and numerous buildings destruction. For this reason, it is still worthy to focus on the seismic hazard in central Taiwan. The purpose of this study was to use short-period Rayleigh-wave dispersion curves to investigate the 3-D shallow S-wave velocity structure around the source area of the Chi-Chi earthquake. The short-period Rayleigh waves were generated from 19 earthquakes, located around the source area of the Chi-Chi earthquake. First, we analyzed the group-velocity dispersion curves of short-period Rayleigh waves at periods of 1.5-7 seconds; subsequently, the study area was divided into 56 sub-regions with each size of 0.150.15 to image 2-D group velocity maps using a smoothing tomographic method; finally, we inverted the 1-D S-wave velocity up to 10-km depth for each sub-region. Combining the 56 1-D S-wave velocity structures reconstructed a 3-D S-wave velocity structure for the source area of the Chi-Chi earthquake. Results showed an obviously lateral variation, related to the geological features. The region with low S-wave velocities was due to the thick sediments in Western Coastal Plain; the high S-wave velocities were related to the Western Foothills and metamorphic rocks. Generally speaking, the S-wave velocity systemically increased from west to east, especially for the top 2-km structures. The Changhua fault, Chelungpu fault and the Shuangdong fault were three main boundaries to clearly divide the source area into different geological features. To the west of the Changhua fault, the S-wave velocities were lower than 1.5 km/sec; the S-wave velocities were 1.25-1.75 km/sec between the Changhua fault and Chelungpu fault; there was an average S-wave velocity of about 2.0 km/sec between the Chelungpu fault and Shuangdong fault; to the east of the Shuangdong fault, the S-wave velocities were higher than 1.5 km/sec. For depths greater than 3 km, the velocity variation was unobvious for the study region other than the northern region of Cho Shui His and western region of Central Range. In the future, we will increase the seismic data to improve the resolution in space so that the 3-D S-wave velocity structure can provide useful information about seismic hazard assessment in Central Taiwan