利用海底地震儀資料探討 2018花蓮地震之餘震及地體構造;Using Ocean Bottom Seismometer Data to Explore the Aftershocks of 2018 Hualien Earthquake and the Related Structure

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Title:	利用海底地震儀資料探討 2018花蓮地震之餘震及地體構造;Using Ocean Bottom Seismometer Data to Explore the Aftershocks of 2018 Hualien Earthquake and the Related Structure
Authors:	李宜玲;Lee, I-Ling
Contributors:	地球科學學系
Keywords:	2018花蓮地震;一維速度模型;碰撞-隱沒過渡帶;海底地震儀;2018 Hualien Earthquake;1D Velocity Model;Collision-Subduction Transition;Ocean Bottom Seismometer
Date:	2022-07-20
Issue Date:	2022-10-04 11:11:42 (UTC+8)
Publisher:	國立中央大學
Abstract:	台灣位在歐亞板塊與菲律賓海板塊的交界處，菲律賓海板塊以每年8公分的速度向西北方移動，並在台灣東北部外海向北隱沒到歐亞板塊下方。而呂宋島弧也在約六百萬年前在台灣北部附近和歐亞板塊發生碰撞，進而開始一系列的台灣造山活動。花蓮不只位在板塊交界帶上，更是鄰近碰撞到隱沒的過渡帶，因此花蓮附近構造複雜，地震活動也相當活躍。在2018年2月6日於台灣東部花蓮近海發生規模6.2、震央6公里深的淺層地震，對花蓮造成嚴重災情，中央地質調查所的地表變形顯示災損點主要沿著向東傾的米崙斷層，然而此地震由震源機制解來看為一東北-西南走向、向西傾的左移斷層，因此有許多前人研究著重在發生主震之向西傾的構造。此外，此地震位於板塊碰撞縫合帶的縱谷北端，是屬於碰撞與隱沒系統的交會處，地體構造較為複雜。本研究分析海底地震儀（OBS）以及中央氣象局（CWB）和台灣寬頻地震網（BATS）的陸上測站在2018年2月13日到3月5日所紀錄的地震，目的是利用餘震來解釋此區域之構造。由於資料量大，此研究使用Earthquake Transformer自動偵測地震和挑選波相，得到約2,600個地震，再進行人工品質篩檢至約1,858個地震，並藉由地震資料以及VELEST逆推此區域一維P波速度模型，並重新定位地震。結果顯示一條鄰近2018主震的西北-東南向剖面，10-15公里深的地震為向東傾、6-10公里深的地震為向西傾。推測應為向西傾的主震破裂影響到此向東傾的斷層面，並且向南破裂到淺層的米崙斷層，與花蓮的災損點皆沿著米崙斷層分布結果相同。另外在北緯24°6’以南的西北-東南向剖面，餘震分布皆是向東傾，但在以北的剖面卻具有向西傾的趨勢。而東北-西南向剖面看到在北緯24°12’因傾向不同而分隔成南北兩個構造。因為兩個方向的剖面皆可看到明顯分界，所以推論在北緯24°6’到24°12’附近為板塊碰撞與隱沒的過渡帶。;Taiwan is located at the boundary between the Eurasian Plate (EP) and the Philippine Sea Plate (PSP). PSP moves northwestward with a convergence rate of about 8 cm/yr and subducts northward beneath EP in the northeast of Taiwan. The Luzon volcanic arc, which is produced by the eastward subduction of EP and belongs to PSP, colliding with EP in the late Miocene. As a result of this oblique arc-continent collision between two plates, the Taiwan orogen is known for its active and complicated structure. Hualien is at the western end of the Ryukyu Trench and the northern end of the Longitudinal Valley which is considered as the collisional suture. Therefore, the seismicity rate of the region is high, showing the complicated structure. On 6 February 2018, a M6.2 earthquake occurred at the offshore Hualien and caused severe damage along the east-dipping Milun Fault. However, based on the focal mechanism, this is a left-lateral earthquake and dips toward the west. Hence, it is important to understand the relationship between two different structures. In order to understand the aftershocks, we combine 5 OBS with 12 onland stations from CWB and BATS for a detailed earthquake analysis, and we also generate the 1D P wave velocity model to relocate aftershocks. In total, 1,858 events are observed the NE-SW trending along the east coast. On the NW-SE cross-section near the 2018 Hualien earthquake, there is an east-dipping distribution at depths between 10-15 km and a nearly horizontal and west-dipping cluster at 6-10 km depth. We consider that the initial rupture started from the west-dipping plane of the 2018 Hualien earthquake and affected the east-dipping plane on its west side. The rupture propagated southward later and affected the shallower east-dipping Milun fault. Another feature is the different structures between 24°6’N and 24°12’N. The earthquake distribution dip to the north and east in the south and change to the opposite direction in the north. Because of this obvious transition, we suggest the Hualien area is the transition structure between the collision and subduction.
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