台灣海峽震源機制及其應力狀態

Abstract

台灣海峽位於歐亞大陸邊緣，屬於被動性大陸邊緣。而由地體構造之觀點切入，其坐落於台灣弧陸碰撞帶和穩定之歐亞大陸板塊之間。雖然主要地震能量釋放發生於板塊邊界，但由過去紀錄顯示，一向被認定為板塊內部地區的台灣海峽卻仍有為數不少的地震活動發生。然而，因爲觀測資料的不足，有關台灣海峽地殼構造研究的工作和文獻相當有限，缺乏系統性之震源機制解研究。本研究使用海峽兩側區域寬頻和短週期地震測站之波形資料求解出自1991年以來55筆台灣海峽中小型地震震源機制解，並利用所求得之解進一步估計區域構造應力狀態。 本研究採用Zhao & Helmberger [1994] 和Zhu & Helmberger [1996]所建立之Cut-and-Paste (CAP)波形逆推法以求解震源機制解，並應用Gephart & Forsyth [1984]所發表之Focal Mechanism Stress Inversion (FMSI) 法和Michael [1984, 1987]的Linear Stress Inversion with Bootstrapping (LSIB)法來估計區域應力型態。 本研究斷層型態結果顯示，整個台灣海峽區域地震絕大多數為深度30公里內之淺層地震且以走向滑移斷層為主，並伴隨著多處張裂活動，僅在海峽東北部擁有少數逆斷層地震活動。究其原因，也許暗示著此區域是介於台灣地區板塊隱沒與碰撞造山帶之碰撞壓縮環境與中國大陸東南沿海之伸張應力環境的轉換帶。而海峽南部以1994年9月為首之地震餘震序列，則呈現較為一致之正斷層特性。

應力結果顯示，台灣海峽區域無法視為單一均勻應力場，較爲合理的解釋是將海峽區域劃分為四個應力均質區。台中、新竹外海區域由σ1主軸方向顯示，即使位於變形前緣西側，此區仍受到板塊碰撞效應影響。而台南陸上、沿海區域顯示在變形前緣兩側，空間上應力變化十分高頻。整體而言，台灣海峽就應力結果表現出張裂應力狀態，且σ3由南而北呈現由南北向往東西向偏轉之現象。針對兩應力方法結果之比較，認為雖然兩方法所得之應力主軸方向大致相同，但就可信度範圍和集中程度，本研究區域目前以LSIB法較為適用。

The Taiwan Strait is situated on the Eurasia continental shelf, and, from a tectonic perspective, it sits between the arc-continent collision zone of Taiwan in the east and the stable Eurasian continent in the west. Although the majority of seismic moment release occurs at the plate boundaries, there are still seismic activities in the Taiwan Strait, within the Eurasia Plate. Seismic studies have been very limited in the Taiwan Strait, and the crustal structure and the earthquakes in the strait have been poorly understood. In this study, we determine the depths and focal mechanisms of 55 small and moderate earthquakes in Taiwan Strait since 1991 using regional broadband and short-period waveform records from stations on both sides of the strait, and then estimate orientations of the principle stresses from these focal mechanism solutions. In this study, the focal mechanisms are inverted by the cut-and-paste (CAP) of Zhao & Helmberger [1994] and Zhu & Helmberger [1996]. The focal mechanism stress inversion (FMSI) method of Gephart & Forsyth [1984] and the linear stress inversion with bootstrapping (LSIB) method of Michael [1984, 1987] are adopted to estimate the stress orientations in the region. Our result suggests that most events in the Taiwan Strait occurred in the crust above 30-km depth, and strike-slip faulting mechanism dominates in most of the region, although there are some extensional activities and a few thrust events in the northeast of the Taiwan Strait. This may be interpreted as resulting from the transition between extensional and compressional regimes. Focal mechanisms in the south show that strike-slip and normal faulting mechanisms dominate, consistent with the aftershock behavior of the largest event occurred in the region in September, 1994. Our tests on stress-field orientation estimation indicate that the Taiwan Strait as a whole cannot be represented by a single homogeneous stress orientation. A more reasonable interpretation of our tests is to divide the region into four stress regimes. For the stress field in the northeastern part of the Taiwan Strait, the azimuth of σ1 is consistent with that in previous studies carried out on the island that reveals the variation pattern determined mainly by the direction of the plate motion of the Philippine Sea Plate relative to the Eurasia Plate. Our result also shows a rapid spatial change in stress orientation off the coast near Tainan within a short distance about 30 km, indicating very different tectonic regimes on the two sides of the deformation front. Base on the results of both FMSI and LSIB stress inversion methods, the western part of the Taiwan Strait experiences an extensional stress, and the direction of σ3 rotates clockwise from north-south direction in the south to east-west direction in the north. Although results for stress orientations from FMSI and LSIB methods are compatible in regions of relatively homogeneous stresses, based on the confidence ranges of the two results, we believe that the LSIB method is more suitable for the stress-field estimation in the Taiwan Strait.