EARTHQUAKE STRAIN RATES AND INSTANTANEOUS RELATIVE MOTIONS WITHIN CENTRAL AND EASTERN ASIA

We use the spatial distribution of moment tensors of earthquakes in this century to estimate the velocity field in Asia within a Eurasian reference frame. In a least-squares inversion, strain rates on the surface of the Earth are matched with continuous spline functions in order to recover the velocity gradient tenser associated with the seismic moment release in Asia. Earthquakes account for 40-60 per cent of the expected motion of India relative to Eurasia, with the missing component of strain rate equivalent to about 20 mm yr(-1) of N-S shortening between Siberia and India. In this solution, South China rotates counterclockwise and moves eastwards relative to Siberia. Using rigid plate constraints, we next investigate the characteristics of the complete horizontal strain held in Asia that accommodates plate motions. Our strain-rate solutions are analogous to the response of a Newtonian thin viscous sheet in which the rate of work done by the straining medium in accommodating the velocity boundary conditions is a minimum. In these solutions the Euler pole for India relative to Eurasia is constrained (NUVEL-1A; DeMets et al. 1994), but in the process of fitting the VLBI velocity at Shanghai, China (Ward 1994; Heki ef al. 1995), the Euler pole for South China is determined in the inversion. A solution that both fits the velocity at Shanghai, China and yields a strain-rate field consistent with the earthquake mechanisms is one where the South China block has a motion relative to Siberia described by the pole at (51 degrees N, 131 degrees E, 0.3 deg Myr(-1)). Comparison of the complete strain field that accommodates plate motion with the seismic strains indicates that earthquake moment release rates in this century within Mongolia are about a factor of 4 larger than the long-term rate. Within Gansu-Ningxia, the earthquake moment rates have been about a factor of 2 higher than the long-term rate. The strike-slip faulting within Mongolia, Gansu-Ningxia, western Sichuan and Yunnan is possibly a direct result of velocity boundary conditions imposed on the South China block by forces unrelated to continental collision of India and Eurasia, such as forces associated with subduction along the margins of South-eastern Asia. Verification of this requires a better understanding of the role of pre-existing zones of weakness within the Asian continental lithosphere.