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

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 obviously low VP and high VP/VS anomalies in the shallow layers of the south section of Zemuhe fault zone and the north section of the Xiaojiang fault zone reflect the Quaternary sedimentary characteristics of rifted basins such as the Ningnan and Qiaojia basins. Low VP and heterogeneous VP/VS anomalies along the Sikai-Jiaojihe fault zone may reflect differences in rock composition. The interior of the Xiaojiang fault zone and its eastern side are characterized by complex segmentation. The high VP and high VP/VS along the northern Xiaojiang fault zone and the Zhaotong-Ludian fault zone basically coincide with the distribution of the Emeishan basalt, which indicates that the two fault zones are the main channels for the Emeishan basalt to intrude into the upper crust after the eruption. 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 low VP anomaly at depths 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 low 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.