若能了解地震發生時的地表振動，對地震防災是有幫助的，尤其淺層S波速度構造則是預估地表振動的重要參數之一。1999年9月21日台灣中部車籠埔斷層活動所引起的集集地震使台灣中部災情慘重，因此，中部地區的地震防災工作是刻不容緩，值得保持關注。本研究利用位於台灣中部強震站所收錄之19個發生於台灣中部的地震之短週期雷利波群速來研究集集地震震源區的淺層三維S波速度構造。首先，分析這些通過震源區的雷利波群速頻散曲線；接著研究區以0.150.15分成56個區塊並進行層析成像分析，得到各區塊週期介於1.5-7.0秒的短週期二維雷利波群速成像；最後，逆推各區塊的一維S波速度構造，再組合所有區塊而成三維S波速度構造。結果顯示此集集地震震源區有明顯的速度側向變化，且淺層S波速度構造變化指出與地質特徵有高度相關性：低速的部分則與西部平原有關，因為有較厚的沉積層；高速的部分則與西部麓山帶與變質岩區有關。一般而言，S波速度的變化由西向東增加，在深度2km以上，彰化斷層、車籠埔斷層與雙冬斷層是個重要的地質特徵分界，在彰化斷層西邊S波速度約小於1.5 km/s；而彰化斷層與車籠埔斷層間的S波速度約介於1.25-1.75 km/s；在車籠埔斷層與雙冬斷層間的S波速度平均約2.0 km/s；在雙冬斷層東邊的中央山脈西翼S波速度約大於3.0 km/s。在深度3km之下，這樣的速度差異變化則不明顯，除濁水溪北邊仍存在較低速度及中央山脈西翼的高速外，大部分區域的速度約在2.5 km/s左右。爾後，將增加此分析在空間的解析度，以作為日後地動預估的參考。
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 0.150.15 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.
