| 摘要: | 本論文以近期收集的地震資料，針對越南北部的兩個顯著地震序列及菲律賓北部呂宋島的板塊構造進行地震源特性及地震層析成像之分析研究。東南亞是一個地震活躍度高且持續板塊運動活躍的廣大區域。在越南北部，雖然該地區目前的板塊運動活動相對不活躍，但本地區存在著大尺度地震斷層，並且過去曾發生過中大規模的地震。2020年在越南北部發生了兩個中等規模地震，分別為木州5.0級地震和芒特4.9級地震。此兩個地震序列都對震源區域的基礎設施造成災害損失。我們通過定位地震分析、確定主震和餘震序列的震源機制。對兩地震序列震源過程進行研究，通過應力反演推斷現地發震斷層。莫州主震及其餘震主要為右行走滑型事件，與活躍的紅河斷層有關。應力反演結果顯示，最大水平壓縮方向為南北方向，最大擴張方向為東北-西南方向。芒特地震序列發生在一個發育良好的斷層系統區域，與主要的延邊附近斷層和紅河斷層平行。震源機制表明主要為走滑型結合部分正斷型。地震位置表明，震源斷層是兩個共軛斷層，分別平行於延邊附近斷層和紅河斷層。從震源機制反演的應力狀態顯示，最小壓縮軸σ3的方向穩定為東北-西南方向，最大壓縮軸σ1和σ2的方向在西北-東南方向上交替。本論文的推論基本上支持現有區域應力的壓縮變化模式。本論文針對菲律賓北部地區，進行了P波和S波的共同地震層析成像分析，目的為約束該區域的淺層地質結構並建構板塊撕裂模型。層析成像顯示，在40公里深度處存在一個低速板塊撕裂區，本研究解釋其為板塊撕裂的位置。通過該地區的震源機制和地震活動對比，提出板塊撕裂從老海脊延伸到薩邦和其北部地區下方，並產生區域運動擾動。本研究認為板塊下插形成地幔楔並可能產生部分熔融，俯冲板塊可能在擾動下插過程中造成撕裂破損，這種撕裂在地殼中產生淺層上升岩漿並形成板塊低速區。研究還揭示了在層析成像中，菲律賓斷層及其分支之間的速度異常存在著很高的對比度，這表明這些斷層是切割穿過地殼的岩石結構，深入至少30公里深度。;This dissertation presents an analysis and research on the seismic source characteristics and seismic tomography of two significant earthquake sequences in northern Vietnam, as well as the tectonic structure in the Luzon Island, the northern part of the Philippines, based on recently collected seismic data. Southeast Asia is a vast area with high seismicity and currently tectonic process. However, this region is characterized by large-scale seismic faults, and moderate to large earthquakes have occurred in the past. In 2020, two moderate-scale earthquakes occurred in northern Vietnam, namely the Moc Chau earthquake with a magnitude of 5.0 and the Muong Te earthquake with a magnitude of 4.9. Both earthquake sequences caused damage to infrastructure in the source area. I investigated these sequences by locating the earthquakes, determining the focal mechanisms of the mainshock and the largest aftershocks, stress inversion, and finding out the seismogenic fault. The Moc Chau mainshock and its aftershocks were determined to be mostly right-lateral strike-slip events that related to the active Da River fault. The stress inversion results in maximum horizontal compression direction in north–south and the maximum extension direction is northeast-southwest. The Muong Te earthquake sequence occurred in a region with a well-developed faults system, parallel to the major Dien Bien Phu fault and Red River fault. The focal mechanism indicates a combination of mostly strike-slip with some normal faulting. The earthquake locations suggest that the seismogenic faults are two conjugate faults parallel to the Dien Bien Phu fault and Red River fault, correspondingly. The stress regime inverted from the focal mechanism shows a stable northeast-southwest direction of minimum compression axis σ3, while the alternating direction of immediate and maximum compression axis σ1 and σ2 in the northwest-southeast direction, which support the stress permutation model of the regional stress. For the North Philippines region, I carried out a simultaneous P- and S-wave seismic tomography to constrain the shallow geological structure in the area and refine the slab tearing model. The tomographic images indicate a region of low-velocity slab window from a depth of 40 km, which is interpreted as the site of slab tearing. Compare with focal mechanism and earthquakes occurrence in this region, I propose the slab tearing extends from the fossil ridge to the area beneath north of Zambales and creates regional kinematic perturbations. The tearing produces shallow upwelling magma staying at the crust while may disrupt mantle wedge partial melting due to the absence of contact with mantle and subducting slab. The study also reveals the high contrast in velocity anomalies across the Philippine Fault and its branches in tomographic images, which suggests that the faults are lithospheric structure that cuts through the crust to approximately at least 30 km depth. |
