| 摘要: | 台灣島位處於歐亞板塊與菲律賓海板塊的聚合邊界，在東部的縱谷區域，作為兩板塊的碰撞縫合帶，成為台灣地震活動最頻繁的區域。在2022年的9月17號和18號，縱谷區域相繼發生兩起強烈地震，芮氏規模分別為6.6(關山地震)和6.8(池上地震)。根據氣象局的地震報告以及餘震分布的結果，我們注意到兩個值得關注的現象: (1) 關山地震與池上地震在時間與空間上存在密切的關聯性。具體來說，這兩起地震事件時間間隔不到24小時;空間上僅相距7公里。(2)餘震分布顯示，關山地震與池上地震可能發生於過去地震活動度相對較低的中央山脈斷層(Bruce et al., 2006)。本研究首先透過聯合逆推多種地球物理資料包括:遠震資料(P波、SH波、 Rayleigh波和Love波) 、強地動資料、靜態GNSS和高取樣率GNSS，以建立關山地震與池上地震有限斷層模型，嘗試了解中央山脈斷層在縱谷中南段的區域破裂特性。依據逆推得到的有限斷層模型，本研究進一步計算了庫倫應力，以檢驗關山地震與池上地震之間是否存在靜態誘發現象。有限斷層的逆推結果顯示，關山地震與池上地震展現不同的破裂特徵。關山地震主要呈現往傾角方向破裂，並具有往淺部區域及南邊延伸的趨勢，最大滑移量近1.9公尺。相較之下，池上地震主要向北方破裂，且滑移主要發生在較靠近地表的區域，其最大滑移量約為5公尺。本研究根據有限斷層逆推的結果，嘗試解釋池上地震在玉里鎮造成的速度脈衝訊號。基於正演模擬的結果，速度脈衝可能是來自兩個不同斷層系統的破裂所致。其中，部分脈衝訊號是由玉里斷層的近斷層破裂方向性效應再加上測站場址效應所產生，另一部分則可能與縱谷斷層的破裂有關。此外，我們以靜力學與動力學兩種不同角度，分別探討關山地震與池上地震之間的交互關係，並得出以下結論: (1)根據庫倫應力的計算結果，關山地震靜態地誘發池上地震的發生。(2)池上地震震源區域的破裂速度低區(1.0至1.5公里/秒) 提供存在障礙體的證據，阻礙了關山地震的破裂前緣往北擴展。;On September 17th and 18th, 2022, two strong earthquakes with moment magnitudes Mw 6.5 (referred to as the Guanshan earthquake) and Mw 7.0 (referred to as the Chishang earthquake) struck the Longitudinal Valley (LV), where the arc-continent collision occurs between the Eurasian plate (EP) and the Philippine Sea plate (PSP) sutures. Two intriguing phenomena were observed from the Central Weather Bureau (CWB) reports and the distribution of their aftershocks.Firstly, the Guanshan earthquake and Chishang earthquake had a close spatial and temporal relationship, occurring only within 24 hours apart and several kilometers away from each other. Secondly, the aftershock distributions suggested that both events ruptured on the west-dipping Central Range Fault (CRF), which is considered to have smaller earthquake production compared to the Longitudinal Valley Fault (Bruce et al., 2006).To understand the rupture behavior of the CRF, we first determined the finite fault models of both events using a joint inversion approach that considered teleseismic waveforms (P, SH, Rayleigh, Love), strong-motion records, static, and high-rate GNSS data. Then, the Coulomb stress change is calculated to examine the triggering effect between the two earthquakes. The results showed that the Guanshan earthquake and Chishang earthquake exhibited different rupture characteristics. The Guanshan earthquake mainly ruptured in the down-dip direction, with a slight extension towards the shallow part and to the south, reaching a maximum slip of approximately 1.9 meters. On the other hand, the Chishang earthquake′s rupture primarily propagated northward and was closer to the surface, with a maximum slip of about 5 meters. Additionally, the interesting observation related to velocity pulse produced by Chishang earthquake is also investigated. Our forward simulation suggested that the velocity pulse observed in Yuli township may have resulted from the rupture of two different fault systems. One could be due to the combination of the near-fault directivity of the Yuli Fault and the amplification effect of shallow low-velocity layers, while the other could be caused by the rupture of the Longitudinal Valley Fault. We also discussed some issues related to the interaction between the Guanshan earthquake and Chishang earthquake, both statically and dynamically. Firstly, based on the inverted finite fault model, the Coulomb stress change increased on the Chishang mainshock, indicating that the Guanshan earthquake statically triggered the subsequent Chishang earthquake. Secondly, the inverted lower rupture velocity (ranging from 1.0 to 1.5 km/s) near the source region of the Chishang earthquake may provide evidence of the existence of a barrier that prevented the Guanshan earthquake from rupturing northward dynamically. |
