The characteristics of gravity fields in Sichuan-Yunnan region and its relationship with regional earthquakes

Using the newly ultrahigh order gravity field model EIGEN-6C2,the global DEM model topo_15.1.img and the crust model CRUST1.0,we calculate three kinds of static gravity fields(Free-air gravity anomaly,Bouguer anomaly and residue gravity anomaly)of Sichuan-Yunnan region,and study the relationship between these gravity fields and regional earthquakes.As a whole,the Free-air gravity anomaly has a clear mirror-image relation with regional topography,which means that the regional topography has a big effect on Free-air gravity anomaly.By subtracting the topography gravity effect from Free-air gravity anomaly,we get the Bouguer anomaly of Sichuan-Yunnan region,which presents a characteristic of the lower northwest higher southeast;And the Bouguer anomaly basically reflect the crustal thickness changes of SichuanYunnan region,which be superposed of the intensity of various range of local anomalies.Based on the Parker method we calculate the gravity effect of regional Moho discontinuity,by subtracting it from regional Bouguer anomaly,we get the residue gravity anomaly, which can reflect the density difference of crust in Sichuan-Yunnan region.By comparing the patterns of these gravity fields with the earthquakes source mechanism which occurred after the year 2000,we found that,the thrust or normal type of earthquakes are likely to break on the high gradient zone of three kinds of static gravity fields,and the strike-slip type of earthquakes are likely to break on the smooth and steady area static of gravity fields,which may be concerned with the nature of the faults and the structural setting of the crust.