川滇地区重力场特征及其与区域强震的关系

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

本文利用最新超高阶重力场模型EIGEN-6C2、全球DEM模型topo_15.1.img和地壳模型CRUST1.0等数据,研究了川滇地区的不同类型的静态重力场(自由空气重力异常、布格重力异常和剩余均衡重力异常)分布特征,探讨了区域强震与静态重力场之间的关系.整体上看,川滇地区自由空气重力异常FA与地形镜像关系明显,说明地形效应对重力场影响较大;由于扣除了地形效应,区域布格重力异常格呈现西北低、东南高的特征,总体反映了川滇地区地壳厚度变化的基本格局,并在此之上,叠加了各种强度不等的局部异常;利用Parker方法正演了Moho起伏引起的地表重力变化,将其从区域布格重力异常中扣除,得到反映地表密度分布的剩余重力异常.通过对比2000年以来发生的区域大震震源机制与不同静态重力场分布特征可知,逆冲/正断型地震(如汶川地震、芦山地震)往往发生在静态重力异常的高梯度带上,而走滑型地震(如玉树地震、鲁甸地震)发生在重力异常变化较为平稳的区域,这可能与断裂的性质及其所处构造环境有关.

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.