台灣東部花蓮地區米崙活動斷層之古地震研究;Paleoseismic Study of Milun Active Fault in Hualien, Eastern Taiwan

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Title:	台灣東部花蓮地區米崙活動斷層之古地震研究;Paleoseismic Study of Milun Active Fault in Hualien, Eastern Taiwan
Authors:	曾雅筑;Tseng, Ya-Chu
Contributors:	應用地質研究所
Keywords:	2018花蓮地震;米崙斷層;古地震;2018 Hualien earthquake;Milun fault;Paleoseismology
Date:	2019-07-24
Issue Date:	2019-09-03 16:59:25 (UTC+8)
Publisher:	國立中央大學
Abstract:	台灣花東縱谷地區位於歐亞板塊與菲律賓海板塊之板塊縫合帶，地震活動頻繁，而近百年內至少有兩起地震造成花東縱谷向北延伸之米崙斷層沿線出現地表破裂，並導致房屋損害與人民傷亡，此兩起地震分別為1951年10月22日ML 7.3的縱谷地震及2018年2月6日MW 6.4的花蓮地震，顯示米崙斷層為致災性活動斷層。米崙斷層過去並未有斷層露頭的報導，缺少斷層相關的資料，本研究選定2018花蓮地震出現雁行破裂跡之花蓮空軍基地內米崙線型之斜坡坡腳進行槽溝開挖，並以4口地質鑽井為輔，藉以獲得斷層位態與解析米崙斷層古地震事件，並進一步推估地震再現週期與斷層垂直錯移速率。本研究共開挖三個槽溝，由兩個相互重疊槽溝合併之主要槽溝共二階，長約36米，寬約4米，深約7米，底部另加挖1.5米深，約3米寬的坑，槽溝剖面出露米崙斷層之主斷層以80度向東傾，更西側的分支斷層則以36度、56度及70度向東傾。槽溝出露之楔形崩積體、褶曲生長地層及液化現象為古地震相關之現象，將剖面進行回復與剝除並配合定年結果，可推得過去三千年至少有五次的古地震事件紀錄於此槽溝內，除2018花蓮地震及1951縱谷地震外，尚有790-144 cal yr BP、2360-1301 cal yr BP、3980-2306 cal yr BP有地震紀錄，而五次古地震事件之時間間隔顯示米崙斷層發生地震之時間可能較不規律，說明此斷層可能未具有規律的地震再現週期。本研究中4口鑽井岩芯經岩性對比後推測主斷層垂直錯移量約17.9-18.6米，並估算出近米崙斷層的最大垂直錯移速率約為2.9-3.4 mm/yr。;The Milun fault is the northern extension part of the Longitudinal valley which is the suture between the Philippine Sea plate and the Eurasian plate on the Taiwan Island. The Milun fault relates to two destructive earthquakes on October 22nd, 1951, and February 6th, 2018, respectively. The 1951 ML 7.3 earthquake and the 2018 MW 6.4 earthquake both produced surface ruptures along the Milun fault, causing casualties and damage on buildings in the Hualien city. Although the Milun Fault is surely a hazardous fault, the subsurface configuration and history of this active fault remain little exposed. The purpose of this study is to uncover the fault at shallow depth, reason the activity and seismic history of the Milun Fault and estimate the recurrence interval. Three trenches and four boreholes were carried out at the Hualien Air-Force Base, where surface ruptures were reported during the 2018 earthquake. The main trench with two stages is 36 m in length, 4 m in width, and 7 m in depth. The Milun main fault revealed on the trench wall dips 80° to the east, and its branch faults dip 36°, 56°, and 70° to the east, respectively. Some clues are related to paleoseismic events, including the colluvial wedges, folded growth strata, and liquefaction. By compiling the results of restoration and stripping of strata in conjunction with the age constraints, five paleo-earthquake events associated with the Milun Fault are resolved: (1) the 2018 event (2) the 1951 event and the earlier ones occurred at (3) 790-144 cal yr BP (4) 2360-1301 cal yr BP (5) 3980-2306 cal yr BP. The intervals between each paleo-earthquake events indicate that the recurrence interval of the Milun fault might be irregular. Further investigation will be needed to evaluate the recurrence interval of the Milun fault. Based on the borehole data, the throw component of the Milun fault is 17.9-18.6 m. Combined with the dating result, the maximum uplift rate of the Milun fault is 2.9-3.4 mm/yr.
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