| 摘要: | 本研究利用格點搜索應力逆推法和阻尼應力逆推法配合全球及區域性地震網所提供的震源機制解，逆推菲律賓海板塊西緣的大地應力場。結果顯示，在琉球隱沒帶弧前位置的壓應力場有清楚分段且相對應於海桌山延伸方向有壓應力軸發生偏轉現象，同時，張應力場主要彰顯於整個琉球隱沒帶的弧後位置，暗示弧後張裂作用可能延伸至日本九州附近一帶。 整體菲律賓海板塊西緣應力場變化最為劇烈的位置莫過於琉球島弧的最南端，於此，壓應力軸從平行於板塊相對運動方向，突然大幅度轉成南北向，然後逐漸偏轉，直到進入台灣北部後，回復成原本的西北-東南方向，相同的偏轉現象也發生於張應力軸，推測此現象反應出菲律賓海板塊與歐亞板塊之間從隱沒過渡到碰撞的作用，因此本研究將此地區定義為「琉球-台灣應力轉換帶」(Ryukyu-Taiwan Stress Transition, RTST)。值得一提的是，此應力轉換帶的東界位於123°E經線，為加瓜海脊(Gugua ridge)向北延伸與琉球海溝交會之處，而其西界則與前人研究提出的後碰撞(post-collision)和弱碰撞(waning-collision)的分野位置吻合。 台灣碰撞帶的壓應力軸方向，主要反應板塊的擠壓作用且大致與前人所提出的扇形分佈吻合，然而以本研究更細緻的解析程度顯示，壓應力軸的變化並非由南而北逐漸呈順時鐘方向旋轉（典型的扇形分佈），而是局部性地沿著組成相對較為堅硬的鹿港磁力高區(Lukang Magnetization High, LMH)變化。因此，除了過去所認知的北港高區，本研究認為鹿港磁力高區對於區域應力場也有重要的影響，應於未來防災評估時納入考慮。 依據自由重力異常值的分佈將呂宋島弧區分成六個區塊，然後各別逆推全區和各區塊所對應的應力場。整體而言，呂宋島弧的壓應力軸方向與板塊相對運動方向是一致的。但若比較分區壓應力軸方向、理論板塊相對運動方向和全球定位系統水平位移可發現，除蘭嶼和宜蘭地區外，其餘區域的壓應力軸方向都與理論板塊運動方向一致。宜蘭區域的差異歸因於受沖繩海槽弧後擴張影響，然而在蘭嶼地區的差異，可能與東北-西南走向的花東斷層有關。根據前人研究推估，花東斷層的滑移量為8 mm/yr，然而不管是以理論板塊運動(71 mm/yr, N310°)或者以壓應力方向(N245°)為約制，花東斷層的位移量都應該遠高於8 mm/yr。因此，本研究推論蘭嶼地區的壓應力偏差可能肇因於下部地殼與上部地幔解耦合作用(decoupling)或者是因為尚有許多應變能尚未釋放，未來值得關注。另外，壓應力軸相對於板塊運動方向的偏轉於19.5°N以南便已消失，推測呂宋島弧與台灣之間的碰撞或許於此已開始發生，而張應力場主要於22°N以南才有觀測到，因此，此位置可能是從隱沒開始轉為碰撞的位置。 除此之外，本研究指認出十六組發震位置相近、規模大小接近、發生週期近似固定、波形相似且震源機制為逆衝型態的重複地震群，利用其重複出現的特質來研究琉球隱沒帶的孕震特性。各重複地震群的地震矩釋放率分佈雖然有點凌亂，不過大致而言，地震矩釋放率在北琉球島弧相對地較南琉球島弧偏高，而在琉球隱沒帶的中段和北段接近海溝的位置，其地震矩釋放率又相對高於弧前地區。這樣局部性的變化，或許與板塊介面的摩擦性質有關。另外，根據前人發表的重複地震的地震矩與滑移量經驗公式，本研究估算菲律賓海板塊相對於歐亞板塊的滑移量落在5.3與8.6 cm/yr之間，同時，配合理論板塊相對運動速度的估算，琉球隱沒帶的南段耦合強度是相對較強的。最後，基於大地震將會發生在遠離重複地震和重力異常低區位置的假設，並配合地形與震測資料所描繪的破碎帶與分叉斷層(spray fault)分佈，本研究提出在兩處位於琉球隱沒帶南部的地區將是地震潛能的危險高區。To understand the convergent characteristics of the westernmost plate boundary between the Philippine Sea plate and Eurasian plate, we have calculated the stress states of plate motion by focal mechanisms. Cataloged by the Harvard centroid moment tensor solutions (Harvard CMT) and the Broadband Array in Taiwan (BATS) moment tensor, 251 focal mechanisms are used to determine the azimuths of the principal stress axes. We first used all the data to derive the mean stress tensor of the study area. The inversion result shows that the stress regime has a maximum compression along the direction of azimuth N299°. This result is consistent with the general direction of the rigid plate motion between the Philippine Sea and Eurasia plates in the study area. To understand the spatial variation of the regional stress pattern, we divided the study area into six sub-areas (blocks A to F) based on the feature of the free-air gravity anomaly. We compare the compressive directions obtained from the stress inversion with the plate motions calculated by the Euler pole and the Global Positioning System (GPS) analysis. As a result, the azimuth of the maximum stress axis, σ1, generally agrees with the directions of the theoretical plate motion and GPS velocity vectors except block C (Lanhsu region) and block F (Ilan plain region). The discrepancy of convergent direction near the Ilan plain region is probably caused by the rifting of the Okinawa Trough. The deviation of the σ1 azimuth in the Lanhsu region could be attributed to a southwestward extrusion of the Luzon Arc block between 21°N and 22°N whose northern boundary may be associated with the right-lateral NE–SW trending fault (i.e. Huatung Fault) along the Taitung Canyon. Comparing the σ1 stress patterns between block C and block D, great strain energy along HF may not be completely released yet. Alternatively, the upper crust of block C may significantly have decoupled from its lower crust or uppermost mantle. We also applied an improved stress inversion method to a comprehensive dataset of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the Philippine Sea plate. Our results indicate that the crustal stress along the Ryukyu forearc is segmented with boundaries at or near the places of seamount subduction, including the Tokara channel. An extensional stress regime is observed along the entire Ryukyu backarc, implying that backarc-rifting may have extended northward to Kyushu. A triangular area near the southernmost terminus of the Ryukyu arc is characterized by a unique stress signature. The eastern boundary of this Ryukyu-Taiwan Stress Transition (RTST) coincides with the 123°E meridian where the Gugua ridge intercepts the Ryukyu trench; whereas its western boundary agrees remarkably well with the border between the post-collision and waning-collision domains in northern Taiwan. The Taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the Lukang Magnetization High (LMH), suggesting that the LMH may be critical in controlling the local stress distribution. The stress signature of the Luzon arc–Taiwan collision reaches as far south as 19.5°N. The tectonic stress along the Manila trench–Luzon forearc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in-slab membrane stress. Since this extensional regime is observed only south of ~22°N, it probably marks the northern limit of the contemporary boundary between the subduction along the Manila trench and the collision in Taiwan. We used repeating earthquake sequences (i.e., a series of earthquakes occurring on the same patch of fault regularly with the similar seismic waveforms) to estimate the aseismic slip rate and seismic coupling along the Ryukyu arc-trench system. To detect similar events, we used the broadband seismograms from the National Research Institute for Earth Science and Disaster Prevention of Japan (NIED) from January 1, 1997 to December 31, 2009. The events that occurred in the Ryukyu subduction zone (23°-35°N, 120°-133°E) were sorted out for further analysis. A total of 10657 earthquake pairs with hypocenter separations less than 10 km were selected. The vertical-component seismograms were band-pass filtered with 1-8 Hz. Only stations with epicentral distance less than 400 km are used. For each pair, we then determine the maximum coefficients of cross-correlation for waveforms of the same station using the 40-second time window centered on the timing of the maximum amplitude (i.e., the direct S arrival). Repeating earthquakes are identified based on the cross-correlation coefficients, focal mechanisms and locations. Different pairs of repeating earthquakes are combined to form a group if they share an earthquake in common. As a result, we detected 16 sequences distributed along the Ryukyu forearc region. Most of those events show thrust faulting mechanism and may be linked to the relative plate motion between the overriding plate and subducting slab. Moment release rate for each repeating sequence is estimated by least-square fitting to a straight light of the upper envelope of the moment release plot. It appears to have large scatter in the distribution of moment release rate, but the overall trend is clear. To north of the Ryukyu arc, the moment release rate is relatively larger than the south. Interestingly, the moment release rate near the trench is apparently larger than that in the forearc region of the central and northern Ryukyu subduction zone. Such a locally spatial variation of moment release rate may be related to the frictional characteristics of the plate interface. Furthermore, on the basis of the empirical scaling relationship between seismic slip and seismic moment as well as the recurrences of similar events, the slip rates for repeating earthquake sequences are calculated and used to estimate the coefficients of seismic coupling. Consequently, the strength of seismic coupling is relatively strong in southern Ryukyu trench. Besides, according to the premises that the next great earthquake could probably occur in the regions of low gravity anomalies and no repeating earthquakes, two high potential seismic risk areas are identified in the Ryukyu subduction zone. |
