本研究主要藉由微地動陣列資料之分析探討台中-苗栗間鐵砧山區域之淺層S波速度構造,首先在該區域選取19個場址,並在每個場址施行4個不同尺寸的陣列,為解析較為深部(4公里)之地下構造,將陣列之最長展距擴大為800公尺。關於微地動陣列資料的分析,主要利用頻率-波數分析法(F-K method; Capon, 1969),求得不同頻率之能量來源及其方位,並推估其雷利波之相速度頻散曲線;再者利用Herrmann(1991)所提之表面波逆推技術,反演此區的淺層S波速度構造。 根據F-K方法之分析結果,低頻段(0.2~1.0 Hz)訊號之能量來源主要集中於西北至西南方,推測為台灣海峽的海浪拍打海岸所造成,而高頻段(3.0~5.0 Hz)訊號之能量來源指向各交通要道,推測為交通與人為因素造成。 根據逆推方法所得之S波速度構造,若假設第三紀岩盤之S波速度為2200 m/s,則此區域之第四紀沖積層厚度約為1300~2800 m,再者,當深度為300~1000公尺,速度隨深度由東向西遞減;當深度介於1200~3000公尺,東西兩側為速度低區,南北兩邊為速度高區;當深度介於3200~4000公尺,東西兩邊的速度低區範圍擴大,南北兩邊的速度高區範圍縮小,並以CHC及ZTO測點分別具有速度最低與最高值。 將本研究逆推所得之速度構造與前人研究結果進行比對,於構造剖面(Hung and Wiltschko, 1993)上,不論深度是2000或4000公尺,皆可看到明顯的通霄背斜與鐵砧山背斜構造,二者比對良好。再者,與震測資料(吳,2010)及(莊,2011)做比較,不論是深度或速度的觀點,皆可看到相似的背斜構造形貌及錦水頁岩層。由此顯示本研究結果具相當可信度。因此微地動陣列量測法亦可應用於探討較深之地下速度構造,並獲得良好的結果。
Shear-wave velocities have widely been used for earthquake ground-motion site characterization. Thus, the shallow S-wave velocity structures of the Tiehchenshan region in the Tiachung-Miaoli area are investigated using the array records of microtremors at 19 sites. The dispersion curves at these sites are calculated using the f-k method (Capon, 1969); then, the S-wave velocity structures of the Tiehchenshan region are estimated by employing the surface wave inversion technique (Herrmann, 1991). At frequencies lower than about 3 Hz, the propagation directions are concentrated between the northwest and southwest quadrants. The generation of these may be attributed to the ocean waves of the Taiwan Strait. If the S-wave velocity of the bedrock is assumed to be 2200 m/s, the depths of the alluvium are between 1300 m and 2800 m. S-wave velocity gradually decreases from east to west at the depths less than 1200 m. At the depths between 1200m and 4000m, the higher Vs values appear at the north and the south of the region while the lower values are at the east and the west. Our results are in good agreement with the available geological and geophysical information at the Tiehchenshan region.