本研究探討大地震發生前後應力之時空變化，分別以1999年台灣集集地震與2011年日本東北地震為例。就台灣集集地震而言，我們根據1991年3月至2010年10月、規模1.8～7.3共7126筆震源機制解，選取集集地震震源周圍，深度30km內的七塊地震密集處，進行不同時期的應力反演。我們繪製研究範圍內震源機制解的P、T軸時空序列，將應力變化各自分成三個時期。第一階段為1991年至震前、第二階段因各區P軸或T軸改變的時間長短不同，分別介於43～116天、第二階段後至2010年屬於第三階段。接著利用應力反演，我們發現每一個時期，各區域之間因地質構造的不同，而有不同的地震機制。如位於震源區西側下盤震後主要為東西伸張的正斷層為主；而位於震源區東側震後則以東北-西南伸張的正斷層為主。另外，對於日本東北地震，我們根據1997年2月至2012年3月日本F-NET、規模2.7～9.0共8269筆震源機制解，選取日本東北地震震源周圍，深度30km內的八塊地震密集處，進行不同時期的應力反演。採用相同的研究方法，將應力變化各自分為三個時期。第一階段為1997年至震前、第二階段因各區P軸或T軸改變的時間長短不同，分別介於95～263天、第二階段後至2012年3月屬於第三階段。經由應力反演，東北地震東側震後以東南－西北向的正斷層為主。而東北地震西側數處地震密集處，其震後應力反演結果皆存在著東北－西南向的伸張應力。由此次的研究結果我們可以得知，集集地震震後至今，震源區、震源區南側及東側已經回復到震前的應力狀態，而震源區西側、西南側及震源區北側則處於應力持續累積的過程。比較集集及日本東北地震震後的應力行為，可以發現震後應力持續釋放的時間除了受到主震規模大小的影響之外，離主震的距離也是影響因素之一。規模越大、距離越遠，都會造成應力持續釋放的時間增長。另外，由於台灣與日本在地體構造上的相似性，其震後的應力行為也有類似的表現。

This study investigated the spatial and temporal stress variations before and after a large earthquake, respectively, 1999 Chi-Chi earthquake and 2011 Tohoku earthquake. For the Chi-Chi earthquake, according to the 7126 focal mechanism solutions from March 1991 to October 2010, we selected seven regions in the Chi-Chi earthquake source area and analyzed stress inversion for different periods. The first period started from 1991 to the time right before Chi-Chi. According to the changes of P axis or T axis, the second period ranged from 43 to 116 days. The end of the second period to 2010 is third period. We analyzed P and T axe of the focal mechanism solutions in the spatial and temporal sequences. Stress inversion results are divided into three periods. The stress inversion results between the various regions are due to different geological structure and earthquake mechanisms. For the Tohoku earthquake, we selected 8269 focal mechanism solutions from February 1997 to March 2012. We used the same approach as the Chi-Chi event to the eight regions of the Tohoku earthquake and analyzed stress inversion for different periods. The first period started from 1997 to pre-Tohoku. According to the changes of P axis or T axis values, the second period ranged from 95 to 263 days. The end of the second period to March 2010 is the third period. The results show that Taiwan and Japan after a large earthquake, their stress behaviors have a similar performance. In addition, since the Chi-Chi earthquake, there are three regions had returned to the state of stress before the earthquake and three other regions are in the ongoing stress accumulation process. Comparison of the behavior of the stress after the earthquakes of Chi-Chi and Tohoku, we find that the stress release process is affected by both the magnitude of the mainshock and the distance from the source.

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