台灣的地震活動相當頻繁，平均每年發生超過15000次地震，屬於地殼活動劇烈之區域，然而地震常伴隨著重大的災害，若能了解地震活動率(seismicity rate)的時空演化及影響因素，將可提供地震災害分析一些重要的依據。大地震發生後所釋放的應力，我們稱為庫倫應力，庫倫應力隨時間變化後沿著斷層或斷層附近進行轉移與累積，這樣的應力轉移將會改變鄰近地區的地震活動度(seismicity)。本研究藉由探討主震發生後所產生的庫倫應力與餘震分佈的相關性，期望能了解台灣地區的地震特性。 本研究選用三個台灣本島的地震，分別為1998年7月17日的瑞里地震(Mw=5.84) 、2003年12月10日的成功地震(Mw=6.8)、2010年3月4日的甲仙地震(Mw=5.96)，瑞里地震使用地震波形逆推得出斷層面上的錯動量分佈、成功地震使用GPS觀測到的地表變形資料逆推得出斷層面上的錯動量分佈、甲仙地震使用地震波形逆推得出斷層面上的錯動量分佈，使用上述同震斷層錯動量模型將格點切成2km x 2km的網格進行計算庫倫應力變化，並將深度0公里到30公里一共分為五層，探討庫倫應力變化與地震活動在時間和空間分佈的相關性。 震後庫倫應力變化上升的區域餘震分佈的比例確實比應力變化下降區域來得高。瑞里地震震後一年的餘震分佈在庫倫應力變化上升與下降的區域分別為61%和1%；成功地震震後兩年的餘震分佈在庫倫應力變化上升與下降的區域分別為66%和16%；甲仙地震震後一年的餘震分在庫倫應力變化上升與下降的區域分別為35%和1%。 Taiwan is located on the boundary between the Eurasia Plate and the Philippines Sea Plate, where large earthquakes occur frequently and can induce lots of disasters and casualties. Thus, it is important to understand factors that would influence the temporal and spatial evaluations of seismicity in Taiwan, and this analysis will provide critical view for studies of earthquake hazard. Earthquake forecast has yet developed completely, but some preliminarily characters can be studied by the relationship between Coulomb stress changes and rates of earthquake occurrence. The positive correlation between increase of static Coulomb stress and high occurrence of aftershocks based on many researches has thus provided evidence that stress increase promotes seismicity. In this study, we chose three large earthquakes in Taiwan, including the 1998 Ruey-Li earthquake (Mw5.8), the 2003 Chen-Kung earthquake (Mw6.8) and the 2010 Jia-Shian earthquake (Mw6.0) to study the relationship between static Coulomb stress changes and seismicity rate changes. We estimated Coulomb stress changes on a optimal-oriented faults implied by regional tectonic stress fields. Results show that most of aftershocks are located in the area where Coulomb stress increases. Accordingly, about 61%, 66% and 35% of aftershocks located in the area of positive Coulomb stress changes for the earthquake sequences of Ruey-Li, Chen-Kung and Jia-Shian earthquake, respectively. In addition, we observed that the seismicity rate is higher after the occurrence of three main shocks than before. For the Chen-Kung earthquake, increased seismicity rates in area of positive stress changes decayed to background level five years after the main shock, and the trend of decay follows the Omori’s law.
