Three-dimensional velocity structure beneath the Song Ma area, Vietnam

The Song Ma fault zone, located in northern Vietnam, plays a crucial role in understanding the mechanism and evolution of the collision zone between the Indian Plate and the Eurasian Plate. To investigate these mechanism and their evolution, we deployed a temporary broadband seismograph network surrounding the Song Ma fault to examine the body wave tomography and ultimately obtained the velocity structure. The results suggest that the variation in the velocity structure beneath the fault zone is due to the local geological structures and that earthquake clusters are situated between the Song Ma fault and the Son La fault in northern Vietnam. Our research findings demonstrate the transformation of the region from a collision zone to a strike-slip fault zone in terms of the velocity structure. This reflects the geological and geophysical evolution of the Song Ma area. This study is crucial for comprehending the active mechanism of the main shear zone in northern Vietnam and the crustal structure in the surrounding area.