2018年花蓮地震中嶺頂斷層北段之淺部破裂

Abstract

2018年2月6日時，一場規模6.4的地震發生在花蓮外海，並伴隨兩條陸上活動斷層(米崙斷層和嶺頂斷層)的同震地表變形與破裂。在地震後對於此一地震事件的地表變形相關調查和研究主要集中在米崙斷層上，而嶺頂斷層則較少被提及和討論，歸咎其部分原因可能與嶺頂斷層主要位在較偏僻的花蓮溪中，致使相關的調查和研究不易有關。為了瞭解嶺頂斷層在本次地震事件中的破裂行為與分佈，本研究使用震前、震後的航空照片與光學衛星影像探討該斷層在本次地震中的同震地表破裂行為。本研究除了透過影像判識來描繪花蓮溪床上相關的地表破裂外，次像素匹配的方法也用來計算地表位移量和空間上的變化。這兩種方法都揭示斷層破裂主要位在花蓮溪中，並在本次地震中呈現左移的走向滑移運動型態。透過次像素匹配所測得的近斷層地表位移以南北向的位移分量為主，從北邊的花蓮溪口附近(>1.0公尺)往南邊的木瓜溪(<0.4公尺)遞減，其量值與GNSS RTK以及花蓮大橋上量測的位移量相近。本研究將所得到的地表變形進行一系列的半空間斷層滑移模型測試，結果顯示嶺頂斷層可能為一條高角度的斷層，且在本次的花蓮地震中，破裂集中在較淺部的位置(<5公里深)。從本區域過去三十年的地震分布推論，本研究認為此地區存在兩條主要的斷層系統，斷層面分別向西和東傾，而結合2018花蓮地震的主震和餘震位置，我們認為這次的地震主要和向西傾的斷層活動有關，並導致位在較淺部的嶺頂斷層錯動。綜上所述，在2018花蓮地震中，嶺頂斷層北端有發生淺部的破裂，地表的變形以左移的走向滑移為主，並在花蓮溪中上形成一些地表破裂。

On 6th February 2018, the Mw 6.4 Hualien earthquake occurred near Hualien City, accompanied with ground deformations along the Milun and the Lingding Faults. While the post-event field survey provided a detailed description and data of deformation along the Milun Fault, there were not many reports about the ground deformations along the Lingding Fault since the main trace of the Lingding Fault passed through the Hualien River bed. In order to investigate the rupture behavior of the Lingding Fault during the 2018 earthquake, the pre- and post-earthquake aerial photos are used in this study. We not only map the plausible coseismic surface rupture on the Hualien River bed by visual inspection of the images, but also obtain the surface displacements to determine the spatial pattern and magnitude of offset across the Lingding Fault by Co-Registration of Optically Sensed Images and Correlation (COSI-Corr). Both approaches reveal clear deformations along the Hualien River bed, suggesting the 2018 coseismic rupture along the Lingding Fault is dominated by the left-lateral slip, where the displacement decreased from north to south, from >1.0m near the Hualien River mouth to <0.4 m south of the Mugua River. Such a pattern agrees well with the GNSS RTK field survey result. After that, we conduct a series of half-space fault slip models to fit the sub-pixel correlation result and GNSS RTK survey data. The result indicates the slip on the Lingding Fault concentrated in the shallow depth (<5 km depth) and the fault plane dips at a high angle. The largest fault slip happened at the northernmost tip of the Lingding Fault. The thirty-year seismicity reveals two major fault system in this area, with their fault planes dipping to the east and west, respectively. From the distribution of the mainshock and aftershocks of the 2018 Hualien earthquake, we suggest the motion of the west-dipping fault caused this event, and triggered the movement of the Lingding Fault. To sum up, the shallow portion of the northernmost Lingding Fault did rupture during the 2018 Hualien earthquake, forming a series of left-lateral surface ruptures with limited fault-normal movement.