利用GPS資料分析縱谷中段中央山脈斷層的震間變形

Abstract

菲律賓海板塊每年以約8公分速度向西北方的歐亞板塊移動，並造成臺灣東部的花東縱谷地區極為活躍的構造活動。透過地質與地球物理等研究方式，我們對位於縱谷東側的縱谷斷層之活動特性已有相當程度的了解，但對於位在縱谷西側的中央山脈斷層系統來說，我們對於其運動速率、斷層鎖定深度等特性仍所知有限。為瞭解中央山脈斷層系統於間震期的活動特性，本研究利用中央研究院佈設於縱谷中段區域GPS連續與移動站的觀測結果，求得各站點於間震期的移動速率，分析縱谷中段區域於2022年關山—池上地震前的間震期速度場，並結合震後地表破裂調查成果與孕震斷層模型後，利用彈性半空間模型推算中央山脈斷層系統於縱谷中段於2022年以前的間震期活動特性，以瞭解該斷層在間震期時斷層鎖定區域的分布，以及中央山脈斷層系統與鄰近斷層活動的關聯特性。 本研究結果顯示，相對於縱谷中央，縱谷斷層上盤海岸山脈側的運動速度可達20-30 mm/yr，方向為320-340°；而中央山脈斷層系統上盤相對縱谷中央的速度則為7-12 mm/yr，方向為160-190°，速度越靠近下盤有越慢的趨勢。在二維模型的模擬中，中央山脈斷層系統在走滑方向上的速度為15 mm/yr，沿傾角方向則為25 mm/yr，鎖定深度約為10公里。三維模型結果同樣顯示斷層鎖定深度約在10-15公里之間，且靠近瑞穗似有鎖定深度變深的趨勢。此結果證實中央山脈斷層中段淺部在2022年關山—池上地震前30年間處於震間閉鎖狀態，且閉鎖區間與2022年地震的破裂區間約略相符，與其東側的縱谷斷層活動特性有明顯的差異。雖其閉鎖深度因GPS測站分布限制而難以精確控制，但與2022地震破裂帶深度應可相互映證。此一結果顯示中央山脈斷層系統為縱谷中極為重要的活動與孕震構造，建議須對其上盤中央山脈東麓區域進行長期的大地變形監測。

The Philippine Sea Plate moves towards Taiwan at a rate of about 8 cm/yr. Most of these deformations were accommodated by active faults in western Taiwan, and the active Longitudinal Valley Suture (LVS), eastern Taiwan. Through geomorphic, geodetic, and paleoseismological studies, we have gained a considerable understanding of the east-dipping Longitudinal Valley Fault (LVF), which is one of the most important seismogenic sources within the LVS in the eastern side of the valley. However, our knowledge of the west-dipping Central Range Fault system (CRFs), the other major active fault in the LVS in the western side of the valley, remains limited, even though the fault ruptured in Spet-2022 producing Mw 7.0 earthquake. To understand the CRFs’ active behavior, including its slip rate and the locking depth over the past few decades, we analyze campaign GPS and continuous GNSS data in the central segment of the valley. We calculate the horizontal and the vertical velocity of each station before the Mw 7.0 Chihshang earthquake in 2022, and use the coseismic surface rupture of the Chihshang earthquake to constrain the fault location. We use both the 2D and 3D half-space elastic dislocation models to obtain the CRF’s fault slip rate and its locking depth prior to the 2022 earthquake. Our results from a series of velocity profiles across the central valley suggest the shallow portion of the CRFs is locked from the surface down to 10 km at depth during the interseismic period. The CRFs’ geodetic fault slip rate may reach to 32 mm/yr, with a dip-slip rate of 25 mm/yr and a left-lateral slip rate of 15-20 mm/yr. Our 3D back-slip model also suggests the slip rate of CRFs is approximately 25 mm/yr, with the fault locking depth plausibly down to ~20 km at depth. Both results coincide to the coseismic rupture zone of the 2022 earthquake, even though our model resolution is severely limited by the spatial distribution of GPS stations. Our result indicates that the activity of CRFs is comparable to the LVF in the past few decades. Hence, it is recommended to establish additional GNSS stations in the Central Range area, to assess the slip behaviors of the CRFs for the future seismic hazard assessment.