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

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 of ten 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 st rain rates and earthquake st rain 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 st rain 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 (>M7)in the North China area, but there is about M5 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.