臺灣造山帶地震活動的頻率規模分布及其與地殼變形和應力架構之相關性研究

Abstract

我們使用臺灣中央氣象局地震觀測網地震目錄的343,581地震與2,640個地震震源機制解，用以描繪臺灣地震頻率規模分布與地震b值之空間變化，研究其與斷層錯動類型與地殼應力型態的相關性。地震b值的估計結果顯示，在95％的信賴區間內，各類震源機制的地震b值顯然存在著明顯的差異，逆斷層b值最低(0.82±0.02)，走向滑移斷層b值居於中間(0.89±0.03)，正斷層b值最高(1.03±0.09)，所得結果與全球和其他區域得到相當一致的地震觀察結果。 我們估算臺灣三種震源機制型態的地震b值結果顯示，具有非常顯著的差異，其中逆斷層錯動型態的震源機制具有最低的的地震b值，走向滑移斷層錯動型態具有中間值，正斷層錯動型態具有最高的地震b值，這個結果與全球性或區域性地震活動的相關研究都有很好的一致性。在地震b值的側向分布研究顯示，其與主要的斷層錯動機制、地殼變形及應力型態都有非常良好的相關性。臺灣在強大的東西向縮短與差異應力作用下，造成在東臺灣和西臺灣的兩個南北走向的逆衝斷層帶，其具有較低的地震b值；在中臺灣狹長南北向的山脈內或山脈間，受到較小的拉張應力作用，主要受到走向滑移和正斷層作用，其具有較高的地震b值。在地震b值的深度分布研究顯示，地震b值隨深度單調遞減至大約於15-20公里深度終止， 顯示地震b值與應力呈現反比關係，並且證實在臺灣造山帶底下具有弱質中部地殼，並存在一層脆塑性的轉變帶。 簡言之，本研究首先驗證在臺灣造山帶之地震b值與斷層錯動型態及地殼應力同樣具有通用的對應相關。區域地震b值的變化與地殼變形及應力架構的良好相關性，充分顯現了區域的震源構造特性。隨震源深度變化的地震b值顯示，臺灣底下的弱質中部地殼，在約15-20公里深度，存在一層脆塑性的轉換帶。

We analyze 343,581 earthquakes and 2,640 focal mechanisms from reprocessed CWB event catalog of Taiwan to map spatial variations of their frequency-magnitude distribution, i.e., b-value, and explore its possible dependence on faulting type and stress pattern within the crust in and around Taiwan. The b values estimated from three basic faulting types of the seismicity in the entire investigation region show significant differences in a 95% confidence interval, with the lowest of 0.82±0.02 for thrust, the intermediate of 0.89±0.03 for strike-slip, and the highest of 1.03±0.09 for normal events, which are consistent with those observed for worldwide and regional seismicity. We investigate the correlation of the earthquake frequency-magnitude distribution with the style of faulting and stress in Taiwan. The b-values estimated for three types of focal mechanisms show significant differences with the lowest for thrust, intermediate for strike-slip, and highest value for normal events, consistent with those found in global and other regional seismicity. Lateral distribution of the b-values shows a good correlation with the predominant faulting mechanism, crustal deformation and stress patterns. The two N-S striking thrust zones in western and eastern Taiwan under the larger E-W shortening and differential stress yield the lower b-values than those in the in-between mountain ranges subject to the smaller extensional stress and dominated by strike-slip and normal faults. The termination of the monotonically decreasing b-value with depth at ~15-20 km corroborates its inverse relationship with stress and the existence of the brittle-plastic transition in the weak middle crust beneath the Taiwan orogen. In summary, general dependence of the b-value with the style of faulting and stress is first verified in the Taiwan orogen. Regional b-value variations correlate well with the crustal deformation and stress regimes manifested from the local tectonics. The depth-varying b-value reveals the brittle-plastic transition at ~15-20 km and a weak middle crust under Taiwan.