The 3D Velocity Structures beneath the Song Ma Fault Zone, North Vietnam

Tuesday, 16 December 2014
Wei-Jhe Wu1, Strong Wen2, Chi-Chia Tang3, Yulien Yeh1, Lai Hop Phong4, Dinh Van Toan4 and Chauhuei Chen1, (1)Graduate Institute of Seismology National Chung Cheng University, Chiayi County, Taiwan, (2)NCREE National Center for Research on Earthquake Engineering of Taiwan, Taipei, Taiwan, (3)Academia Sinica, Taipei, Taiwan, (4)Institute of Geological Sciences, Vietnamese Academy of Science and Technology, Hanoi, Vietnam
The Indochina area is a tectonic active region where creates complex topographies and tectonic structures. Especially, the Ma River shear zone plays an important role in understanding the mechanism and revolution of the escape process between the Indian plate and the Eurasian plate. In order to have better understanding the seismotectonic structures around the fault zone, this study has carried out a project to deploy 12 temporary broadband seismic stations around/near the Ma River area to record high quality waveform data. In this study, we adopt damping least-square inversion method to investigate the Vp structures and Vp/Vs ratios of the crust and upper mantle beneath the Ma River fault zone, northern Vietnam. From many studies, the velocity structure can be used as an indicator to identify the geometry of fault and precise the earthquake location. Therefore, the goal of this research is to analyze the degree of correlation between the velocity structure and the characteristics of the seismicity, and its tectonic implications. Finally the distribution of Vp/Vs ratio and its association with fault activities is also investigated. Our results indicate that the variation of velocity structure beneath the Ma River fault zone is caused by local geological structures, and the earthquake clusters are located between the Ma River fault and the Song-La fault in the northern Vietnam. Besides, the obtained focal mechanisms from the tomography inversion also exhibit that the orientations are northeast-southwest trending with normal faulting nearby the Ma river fault and should be highly correlation with local geological structures. Based on the analysis of focal mechanism solutions, this area is in extension status which is consistent with the geological survey. Finally, we find a dense cluster occurred in the bending segment of the Ma River fault, and according to the distribution of seismic events, there may exist a south-dipping fault in the southern part of Ma River fault. However, this argument needs to be investigated more in future study.