利用高倍率地震網探討臺灣地區ML > 6.0的地震構造特性

A Seismotectonic Study of ML > 6.0 Earthquakes in Taiwan Using High Resolution Seismic Network

李伊婷(Yi-Ting Lee)

指導教授 ： 吳逸民

國立臺灣大學/理學院/地質科學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201703476

摘要

中央氣象局監測臺灣地區之地震於2012年開始使用新一代觀測系統，本研究將採用其2012至2016年的地震目錄，討論期間ML > 6.0的地震及其在構造上的特性。研究得知新觀測系統的更新，添補了中央氣象局地震目錄的微震資料（ML 2.0以下），讓島內最小完整地震規模(Mc)可達到更小，本研究5個案例的震源區，其Mc就下降為0.8至1.2。在b值方面，新舊系統不會對其造成太大的差異，但在小規模地震的數據更為完整。
（1）2012年02/26霧台地震序列：主要餘震序列集中在一個7.5公里乘10公里的橢圓型範圍，震源深度集中在20到30公里，且該區為1994年至2011年地震缺乏區域。本研究求得的主震震源機制解為向北傾約55度的逆斷層，並沿著一個近北北西-南南東走向的破裂面滑動，這和淺部地殼南北走向的拉張應力構造截然不同，但其發震構造和深度與2010年的甲仙地震相似，認為本次地震與甲仙地震有關，皆為板塊擠壓應力造成的下部地殼破裂。（2）2013年的03/27及06/02南投地震序列：兩群序列的主要餘震皆分布於20公里乘20公里，深度10至20公里間，主震震源機制解皆為近南北走向且向東傾30度的逆斷層。1999年921集集地震規模6以上的餘震幾乎坐落在這兩次南投地震群的東側和深度上界，推測深度於20至30公里間應存在一個破裂面，且該破裂面可能與921集集地震中較深的餘震有關。（3）2013年10/31瑞穗地震序列：由一個高角度約60度向西傾的構造面破裂所致，餘震序列位在1994年至2011年的地震空白帶，係作為縱谷的南北構造的分界。透過速度構造模型，推測本案例是未破裂至地表的邊界斷層錯動造成，該斷層可能為中央山脈系統斷層之一。（4）在2016年02/06美濃地震序列：此餘震序列分布廣泛，然中央山脈淺層所發生的正斷層地震群，應為被觸發，而非美濃地震相關構造。本研究求得的主震震源機制為低角度向北傾、近東西走向的解，但餘震分布較為複雜，還需要更進一步的研究。從主震區餘震位置和震源機制，推測可能與附近的甲仙地震構造有關，也就是臺灣南部較深層的應力釋放造成此次地震。
上述5個案例中，a值無特定規律。震前1年的b值，除了霧台地震因反應淺層拉張的背景地震，使得b值高達1.44外，其他案例都在1.0以內。震後b值落在0.62至0.73：霧台及美濃都在0.7以上，東部瑞穗是0.7，中部兩個南投是0.7以下。震後b值相對震前的變化皆為下降，下降幅度從0.12到0.71，b值變化量因部分案例發生在背景地震缺乏區域，故無法看出與地區的相關性。

關鍵字

地震觀測網 ； 災害地震 ； 地震序列 ； 地震重定位

並列摘要

A new of seismic network of the Central Weather Bureau (CWB) has been used to monitor the seismicity in Taiwan since 2012. The new seismic network includes more stations than the old system. Thus, it can detect much more small earthquakes. The magnitude completeness (Mc) of the old system is about 2.0. The Mc of the new system can research 0.8 to 1.2. It provides a nice dataset to study the seismotectonics of earthquake sequence. In this study, the seismic catalog of the new system was used to discuss the characteristics of the ML > 6.0 earthquakes during 2012 to 2016.
(1) 2012/02/26 Wutai earthquake sequence: The distribution of earthquakes concentrate in the 7.5x10 km region and depth from 20 to 30 km. This zone is a low seismic activity zone during 1994 to 2011. The fault plane is a nearly SE-NW, dip angle is 55o to the northeastward. It is a blind fault in the lower crust and does not correlate to the surface geological structures.
(2) 2013/02/27 and 2013/06/02 two Nantou earthquake sequences: The focal mechanisms of both earthquakes are thrust with strike nearly N-S direction and dipping to east with an angle about 30o. These two events are located within regions of the 1999 Chi-Chi earthquake aftershocks. These two events could be aftershocks of the 1999 Chi-Chi earthquake.
(3) 2013/10/31 Ruisui earthquake sequences: Fault plane of this event strike is parallel to the Longitudinal Valley and dips to the west. It could be correlated the Central Mountain fault system and should be a boundary fault.
(4) 2016/02/06 Meinong earthquake sequences: Aftershocks of this event distribute at a wide region. However, shallow earthquakes in Central Mountains could be triggered by Meinong earthquake. The fault plane of this event dips to north with a low angle and strike is EW. This event is also a blind fault. It could not be correlated to surface structures.
The b value of G-R Law is analyzed in this study. The most of the b values before the occurrence of main shock is around 1.0. The b values of aftershock are between 0.62 to 0.73. The decreasing of the b value could be a physical condition. It also could be contributed by small events did not fully be identified of the aftershocks sequence.

並列關鍵字

Seismic network ； damage earthquake ； earthquake sequence ； earthquake relocation

參考文獻

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