| dc.description.abstract | 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. | en_US |