臺灣地區潛在致災地震(規模 ≥ 6.0)之研究：發震機制及地震統計觀點

Abstract

經濟部中央地質調查所於2010年公布之臺灣陸上33條活動斷層，集中在西部麓山帶、西部平原及花東縱谷區；相對中央山脈地區，由於受到調查能量及區域複雜程度之限制，活動構造不易從地表直接觀察到。然而，不可忽視的在過去一個世紀以來，島內發生的地震紀錄顯示，超過5成數量規模大於6.0的地震，發生在中央山脈地區，這也促使本論文利用地震地質學，探討與釐清內麓山帶地表地質構造與地下地震構造之關係，以及規模6.0以上之可能致災地震在未來可能之發生機率。地震地質學為利用地質科學之理論和分法來研究地震發生之成因與行為，其主要研究之範疇有地震構造、活動斷層、古地震研究、誘發地震及地震災害等研究，尤其是第四紀以來之地殼新期構造運動。本論文聚焦在地震構造及地震統計，探討臺灣中部及南部地區，於2010年~2013年6月，在中央山脈地質區發生4次規模大於6.0以上之震源參數及可能之地震成因。研究結果顯示此4次分別發生在雪山山脈和脊梁山脈規模6.0以上之地震，與地下隆起之高速構造沿著斷層系統上拱有關，並可延伸到中至下部地殼，且震源參數可能對應到重要地質意義之界限斷層。論文最後復以長期地震發生機率之預測，藉以提醒在未調查清楚的活動地質構造環境下，應隨時做好地震防災之準備。 論文主要由3大部分所組成，第一部分為重新檢視2012年屏東霧臺地震之震源參數，其為繼2010年3月甲仙發生規模6.0以上的地震再一次撼動南臺灣，甲仙地震之斷層面解，並無對應到相近的地表構造斷層且未在地表造成明顯之破裂，以致當時推論為地下盲斷層系統，2010年又陸續於附近發生規模接近6.0的桃源地震及震源機制相仿的2012年霧臺地震，提供研究此區複雜地震成因之良機。藉由利用不同定位方法所得之地震深度顯示霧臺地震之深度可能深超過30公里，地殼變形的行為包含更厚的地殼範圍，顯示斷層系統可能延到更深。 第二部分主要探討2013年3月及6月兩次規模大於6.0之南投地震成因。所採用之方法為地震層析速度構造，藉由速度構造之異常可以進一步討論其可能代表物質及地下不均質構造。前人研究在解算速度構造通常都是直接逆推Vp或聯合逆推Vp 與 Vp/Vs，主因為S波到時通常較難判斷導致於往後演算有較多誤差，然而若能求得清楚之Vs，在配合Vp/Vs判釋物質材料會較為敏銳。因此，本研究在保留足夠逆推數據的前提下，透過一些嚴格條件之篩選，使得P波到時及S波到時有同樣之射線包覆率及資料品質。逆推結果顯示在雪山山脈區呈現南北連續之冒起構造(pop-up structure)，速度抬升區之東翼被梨山斷層所侷限住，顯示雪山山脈掘升(exhumation)的過程可能受梨山斷層所控制。梨山斷層往東有另一高速隆起區位於脊梁山脈與大南澳變質雜岩底下，速度構造之性質與雪山山脈下之性質相仿，因此推論者兩區之造山運動除了時間上有先後順序外，其異常速度構造之形成機制可能相似。 第三部分為進行臺灣地區地震發生機率統計。過去一個世紀以來，中央氣象局累積收集非常龐大數量之地震目錄，這對於統計分析是一大好處；然而在規模中型(6.0)以上之地震，所累積之記錄時間仍然不足，過去前人研究顯示不管小規模或中、大規模的地震，其地震間隔發生時間存在統一的分布曲線，本部分嘗試利用此一特點，先推求臺灣地區較小規模之地震間隔發生時間之分布模型，再利用分布曲線對應參數應用到中、大規模地震，求取未來地震在某段時間間隔內可能發生之機率。研究結果顯示，規模6.0以上，從2011年起算，1年內之發生機率超過70%；規模6.5以上1年內發生機率為33%；規模7.0以上地震，1年內發生率為15%，10年內發生率為78%。

There are 33 active faults announced by Central Geological Survey, Taiwan (CGS, 2010), and they almost located in the Western Foothill, Western Coastal Plain, and Eastern Rift Valley area. Due to the limitation of advanced surveys and the complexity in mountain area, the evidences of active faults never easy to discovered, however, over 50 % of ML ≥ 6.0 inland earthquakes in the past century occurred in the Central Range area of Taiwan. To better understand the seismogenesis, we attempted to distinct the relationships between surface structure and seismological structure by earthquake geology in this dissertation. Earthquake geology is the study of the causality, source parameters, deformation feature, and recurrence of earthquakes within the crustal scale. In this study, we found the four ML ≥ 6.0 earthquakes from 2010 to June 2013 in Central Range correlated with the uplift of velocity structures. The seismic clusters extend to mid-lower crust represents the crust is undergoing thick-skinned deformation in Central Taiwan. Furthermore, a statistic study was also carried out to calculate the recurrence probability of ML ≥ 6.0 earthquakes. We ultimately summarized the achievements for respective projects, and their important for seismic hazard assessment. The dissertation mainly consists of three projects. In the first, we re-examined source parameters of the 2012 Wutai earthquake in southern Taiwan, which is essential for earthquake geology study. After the 2010 ML 6.4 Jiasian earthquake caused extensive damage in southern Taiwan, deep, complicated structures were revealed. This earthquake provided us an opportunity to further investigate related tectonic issues and hazard mitigation. Together with recent relative large earthquakes (e.g. 2010 Jiasian and 2010 Taoyuan earthquakes) in southern Taiwan, we also delineated a picture of their tectonic environment. The more deeply relocated Wutai earthquake implied the involvement of the lower crust and that the blind thrust system in this region could go deeper. In the second project, we studied the velocity structures of Central Taiwan by using the newly developed joint-inversion algorithm and seismological data. Although the previous tomographic results in Taiwan have been compatible with the tectonic interpretation, seldom local scale structures are discussed in advanced. Most tomography were P-wave velocities, or with Vp/Vs ratio jointly, but few for S-wave velocities. Therefore, we attempted to provide a new velocity structure of Vp, Vs, and Vp/Vs. The results show that seismicity is bounded by the pop-up velocity structure. The spatial geometry of Lishan Fault in the crustal scale was first revealed by the retrieved tomography. A high velocity body is exhumed to the west under the eastern Hsuehshan Range and right stop by the Lishan Fault on the east, demonstrating that the orogenic thrust system developed in central Taiwan is controlled by the Lishan Fault. In the third project, we further expand the long-term forecasting model in Taiwan area. The time intervals between a series of events (the inter-occurrence periods) are stationary with an identical temporal distribution after an elimination of aftershocks. Based on the goodness-of-fit testing between a few models and observation, we suggested the use of the Gamma distribution in modeling this variable, earthquake inter-occurrence periods, for the study region. Accordingly, unified relationship was constructed, and statistical limitations of sparse sampling for devastating earthquakes (such as M ≥ 6.0 or 7.0) could be resolved.