Crustal density variation and its relationship with the seismogenic structure of strong earthquakes in Sichuan-Yunnan region

In order to study the relationship between the spatio-temporal evolution characteris-tics of crustal density and the seismogenic process of strong earthquakes in Sichuan-Yunnan region, this paper obtained the three-dimensional dynamic density variation model of 0−60 km depth with the resolution of 55 km×55 km×10 km （length, width and height） based on the re-measurement gravity data in the Sichuan-Yunnan region during the period 2011−2014 by the damped least squares inversion algorithm. Then we analyzed the relationship between the three-dimensional dynamic density structure characteristics and the seismogenic characteristics of the LushanM_S7.0 in 2013, the Ludian M_S6.5 in 2014 and the Kangding M_S6.3 in 2014. Furthermore we studied the deep crustal structure, seismogenic background, and regional dynamics of the strong earthquakes. The results show that density variation model exhibited multiple density-change high-gradient zones, which is consistent with the extending direction of the main active fault zones in the Sichuan-Yunnan region. There were obvious regional density variation anomalies in the epicentral area of the three earthquakes mentioned above. The density change horizontal profiles in the depth of 15−35 km showed that strong earthquakes were likely to occur in the center of the high-gradient transition of the positive and negative density variation as well as the center of the four-quadrant distribution of the density variation in the upper-crust, and low density-change anomalies in the mid-crust is required for strong earthquakes. In the lower-crust low anomalies and the high gradient zone of the density change may be the main medium structure for the strong earthquakes. The density variation along the vertical section in the depth of 0−50 km shows that decoupling changes appeared in the shallow crust and deep crust of the epicenters and their adjacent areas. The transitional zone between positive and negative density variation in vertical direction in the crust maybe the main structure for occurrence of strong earthquakes.