臺灣多重斷層破裂情境初探

Abstract

多重斷層破裂為兩條或以上斷層同時破裂的情境，造成比區域裡所有斷層單一破裂更大的地震規模。儘管歷史地震紀錄中早有多重斷層破裂的出現，過去對於其發生的控制條件仍未有完整的討論，且現有多數的地震危害度評估模型裡亦未充分考慮此破裂情境。為改善臺灣地震模型裡對於多重斷層破裂的地震危害度分析，本研究聚焦於分析斷層間的空間關係，透過收集世界各地的多重斷層破裂地震事件資料，從已知的地震事件中找出多重斷層破裂的可能行為模式，並將其應用到臺灣地震模型裡的孕震構造，以找出臺灣本島可能產生的多重斷層破裂情境，從而估算它們的規模以及再現週期。 本研究收集的資料顯示多重斷層的同震破裂會受控於斷層的滑移類型、三維斷層面間的距離以及其相對排列關係。從收集的資料可發現斷層同震破裂能跳躍至五公里以外的斷層後繼續其破裂行為，且逆斷層可以影響的距離比走滑斷層為大，此現象可能歸因於逆斷層傾角較走滑斷層緩。此外，結果顯示走滑斷層間和逆斷層間只有在走向差異介於0-40度與80-90度之間才能發生多重斷層破裂。 依上述的條件，本研究基於臺灣地震模型裡現有的孕震構造資料庫列出臺灣可能出現的多重斷層破裂情境，並在考慮多重斷層破裂情境後，計算臺灣本島孕震構造的發震地震規模與頻率之間的關係。本研究結果顯示多重斷層破裂可能會對長時間尺度的地震危害度評估造成一定的影響，此類型破裂除了會造成比只考慮單一斷層破裂更大規模的地震外，孕震構造的破裂再現週期也可能在考慮多重斷層破裂情境後變長。因此在未來的地震危害度分析中，應考量斷層面間的距離以及其相對排列關係對模型所產生的影響。

Multi-fault rupture earthquake is an event which involves near-simultaneous or cascading rupture on two or more faults in one single earthquake, and results in a larger magnitude that exceeds the capable magnitude of any single seismogenic fault involved in the earthquake. Although the unexpected earthquake magnitude may have high impact on the seismic hazard assessment, most seismic hazard models include only single-fault rupture scenarios, and the geological condition to promote multi-fault rupture in single earthquake is yet well-studied. Hence, we collected worldwide multi-fault rupture cases and built up a database focusing on spatial relationships between contributing faults and their physical characteristics (e.g., fault types, geometries), in order to find out the possible multi-fault rupture scenario in Taiwan. In terms of the structural discontinuities that may arrest the coseismic ruptures, our data suggest the “5-km” surface fault separation may not act as the termini to stop all the coseismic ruptures. For reverse faulting cases, the coseismic ruptures could jump further on the surface, compared to strike-slip faulting cases. This difference is likely resulted from the three-dimensional geometries between the adjacent faults since reverse faults tend to have gentler geometries at depth than the strike-slip faults. Moreover, the fault alignment also affects the jumping behaviour. For both strike-slip faults and reverse faults, jumps would not occur within the strike variation between 40-80°. We provide possible multi-fault rupture sets from the empirical observations, their corresponding capable magnitude and recurrence interval, and present a magnitude-frequency relationship under the architecture of the seismogenic structures in the Taiwan Earthquake Model. Since the multi-fault rupture would result in larger magnitude events with lower frequency than the ordinary fault ruptures, we suggest the future analysis need to consider its impact on the seismic hazard assessment.