High-Resolution 3-D Crustal Velocity Structure Imaging in the Middle Eastern Boundary of the Sichuan-Yunnan Rhombic Block

Abstract

In this study, high-resolution VP and VP/VS models of the middle eastern boundary of the Sichuan-Yunnan Rhombic block were obtained based on a long-term dense observation array, with a horizontal resolution of 5 km and a depth resolution of 2 km. The models reveal that different fault zones have complex medium structures. The high VP and VP/VS basically coincide with the distribution of the Emeishan basalt, which indicates that the ends and intersections of fault zones are the main channels for the Emeishan basalt to intrude into the upper crust after the eruption. The obviously low VP and high VP/VS anomalies in the shallow layers of the Zemuhe-Xiaojiang fault zone reflect the Quaternary sedimentary characteristics of rifted basins such as the Ningnan and Qiaojia basins. The low VP and high VP/VS at the intersection of the Zemuhe fault zone and the Ningnan-Huili fault and at the southern Sikai-Jiaojihe fault zone may indicate that cracks development in these areas contribute to fluid infiltration. The interior of the Xiaojiang fault zone is characterized by complex segmentation. In addition, our models also revealed the seismogenic environment of moderate and strong earthquakes. The Ludian Ms 6.5 earthquake and the Qiaojia Ms 5.0 earthquake occurred at the boundary zones between high and low VP or VP/VS anomalies, thus resulting in stress accumulation. The L-shaped VP anomaly at a depth of 8-12 km in the source area of the Ludian Ms 6.5 earthquake coincides with the distribution of aftershocks, which reveals that the VP anomaly controls the distribution of aftershocks. The obvious boundary zones between high and low VP and VP/VS anomalies above the source area of the Ludian Ms 6.5 earthquake may be at risk for major earthquakes in the future.

Keywords: Middle eastern boundary of the Sichuan-Yunnan Rhombic block, VP and VP/VS models, Ludian Ms 6.5 earthquake, Qiaojia Ms 5.0 earthquake, Seismic risk