华北地区地壳形变的GPS及地震矩张量反演分析

Analyze Crustal Deformation in North China by Inverting Seismic Moment Tensors and GPS Velocity

给出了在椭球面上利用GPS速度场计算地壳应变率张量的公式 ,利用华北地区GPS监测网 1992年、1995年和 1996年观测数据所得的速度场在椭球面上进行了应变率计算 ,得到了其最大剪切应变率、主应变率的图像 ;并利用 196 6年以来 2 5次大于 5 .0级地震的地震矩张量 ,由Kostrov公式计算了地震应变率 ,对两种数据所得的应变率进行了比较分析 ;结合地质构造背景探讨了华北地区现今形变特征及地震危险性地区的分布。

Repeated GPS surveys in North China provide a direct measurement of current crustal motions.GPS surveys have been carried out in the region in 1992,1995,and 1996 respectively by the First Crustal Deformation Monitoring Center (FCDMC) of the China Seismological Bureau(CSB).The velocity field are derived at 16 permanent stations in east Asia and 68 campaign mode sites in North China by using GAMIT software (King and Bock,1995) ,GLOBK software (Herring,1995) and QOCA software(Dong,et al,1998;Shen,et al,2000),and the North China region is moving at about (5～11)mm/a±1.8mm/a eastward relative to stable Eurasia.The horizontal crustal deformation and tectonic activity have been studied by using of GPS measurements.GPS measurements also offer a magnificent tool for measuring tectonic strain rates,which are assumed to be indicative of earthquake potential.The strain on the earth surface (ellipsoid surface) is calculated directly from above GPS velocity. Earthquake potential is often estimated from strain rates using Kostrov's (1974) formula,which essentially states that the moment rate inferred from geodetic straining can be equated to the moment rate to be expended in earthquakes.The seismicity in North China is of relatively high level either in its frequency or in its intensity.Since the beginning of historical records up to now,there were 127 earthquakes of M ≥6 occurred,and about 34 earthquakes of M ≥6 occurred in the Capital Circle area.The probability of earthquakes in a few years is discussed according to the geodetic strain rates and earthquake strain rates in North China area.As a result,the deformation rates from seismic moment tensors and from GPS velocities are basically agree with each other.The max principal strain rate and min principal strain rate is 2.5×10 -9 /a and -2.1×10 -9 /a from earthquake moment tensors,and 2.2×10 -9 /a and -6.1×10 -9 /a from GPS velocities,respectively.It is viable for analyzing earthquake potential by comparing with geodetic strain rate and seismic strain rate.There is no strong earthquake potential (> M 7) in the North China area,but there is about M 5 earthquake potential in few years.Maybe the earthquake will occur in Tianjing_Beijing_Tangshan deltoid region and Zhangjiakou_Datong region because there are of high shear strain rate and they are also of compression surface strain rate.