利用重複地震監測地殼速度變化：以集集地震的斷層復原行為為例

Abstract

重複地震因有幾乎相同的震源位置，故具有高相似度的波形、相同的規模與 震源機制解。本研究系統性搜尋全台ML>4 之重複地震序列，比較各個重複地震 至相同測站之走時差，以推求震波速度變化，並探討其與大地震的關係。本研究 利用中央氣象局短週期地震觀測網(CWBSN)與中央研究院地球科學所之台灣寬 頻地震網(BATS)的地震資料，搜尋到10 組重複地震序列，並具有超過三年的生 命期，其空間分布於中央山脈東翼及東部外海，深部滑移速率為3.4-7.9 cm/yr， 此時間跨距夠長之序列可供長期地殼特性變化之監測。進一步利用發生在花蓮 ML4.6 的重複地震序列之走時差分析，我們發現在集集地震前後S 波波速有明顯 變異，對應到S 波速度變化由1999 年之前的~0%，到2001 年的4-6%，至2007 趨勢減緩至1%，而P 波速度變化皆低於0.5%。和P 波相比，S 波具有更顯著的 速度變化，此差異可能指示孔隙流體效應造成震波速度之突降；而S 波速度變異 集中在近車籠埔斷層的下盤測站而非上盤區，則說明此震波速度變化不僅只肇因 於淺層地表破壞，還包含深部斷層破裂帶之效應。至2007 年為止，此效應仍然 顯著，說明破裂帶附近之地殼特性隨時間的復原仍在進行。由重複地震資料的更 新和累積，我們可追蹤大地震後地殼物理特性之長期變化，期能提供地震週期和 深部地殼物理特性的連結，以裨益大地震再現週期的評估。

Repeating earthquakes with nearly identical waveforms are regarded as useful tool for measuring the temporal changes of wave propagation in Earth’s crust. They have a common source and path such that changes in their waveforms can be attributed to variation in material properties. By searching for ML>4 repeating earthquakes in Taiwan using 1991 to 2011 short-period seismic data, we have found 27 repeating earthquake sequences composed of 3 to 7 events. Among the 27 sequences, 17 are short-lived (i.e., lifetime shorter than 3 yrs) while 10 are continual-type (lifetime longer than 3 yrs). Most of the continunal-type repeating sequences are distributed along the east coast of Taiwan with the derived fault slip rates ranging from 3.45 to 7.85 cm/year, which can be used to monitor the change in crustal properties. Among the 10 continual-type sequences, a ML4.6 repeating sequence in Hualien is characterized by largest event number and therefore, regarded as a target sequence for further analysis. We have observed remarkable delayed arrivals in repeating events that occurred after the 1999 M7.6 Chi-Chi mainshock, suggesting a sudden change of seismic wave propagation properties. The effects of near-surface damage and fault zone damage can be roughly separated by the hanging wall and footwall stations based on distinct travel path (not-across vs. across fault). Using spectral coherence derived time-lag between the pre- and post-1999 repeating events, we have identified 4-6 % delay of S-wave velocity for fault zone damage, which is 3-4 fold greater than near surface damage. The reduced velocity in S waves gradually increase to ~1% in 2007 but has not recovered to the pre-Chi-Chi level.