A study on the mechanisms of strong earthquake occurrence in the North China Basin

Strong earthquakes often occur in the regions where the surface deformation is large. But destructive events took place frequently in the North China Basin in which the deformation is small.In the paper,we made a mechanical analysis of the possible region with seismic velocity anomaly in the North China Basin with finite element method.The calculated result shows that stresses and energy will accumulate both in the high and low-velocity anomaly bodies under the horizontal tectonic stresses.Based upon the results of this paper and previous workers,the physical mechanisms of strong earthquake occurrence in the North China Basin are as follows.In the process of break-up of the North China Craton through lithospheric thinning,the magmatic intrusions or the upwelling mantle material will evolve into low-velocity anomaly body (LVB) in the lower and middle crust.The stresses and strain energy will accumulate more quickly above the LVB underlain the seismogenic fault than in other regions under the horizontal tectonic stresses.The material above the LVB will rupture when stresses reach the failure limit,then the energy will be released.Some of the energy will be transferred to the seismogenic fault.At the same time,the locking area on the seismogenic fault will be reduced.With the loading of tectonic stresses,once the stresses in the seismogenic fault reach critical point,the fault slips,and a strong earthquake occurs.Also the result shows that the LVB plays an important role in stimulating strong earthquakes.Moreover,the analysis in the paper suggests that it is important to study the fine velocity structure in the crust in the North China Basin for earthquake prediction.