Abstract: The occurrence of earthquakes is accompanied by regional stress state adjustment. In seismology, stress drop is usually used to characterize the stress release level after an earthquake. As one of the important parameters, stress drop is widely used to judge the type of earthquake, analyze the stress state after earthquake, and predict the rupture propagation. At present, seismologists often give an average stress drop value based on the earthquake corner frequency. However, the rock strength and stress state of the seismic fault plane are inhomogeneities in real terms. Besides, a single average value is difficult to show the spatial variation in stress changes, which could not reflect the stress adjustment across fault plane. Meanwhile, there are great differences among different researches due to the limited observation stations or different source spectrum data or other related calculation parameters. In this paper, from the point of view of mechanics, we propose the method of adoption the Okada's dislocation theory to calculate the shear stress change of the fault plane, that is, based on the slip model to obtained the distribution of fault plane. From the results of numerical calculation, it is found that the occurrence of an earthquake releases the concentrated stress of fault plane, due to the presence of obstacles on fault or uneven slip distribution, the stress release in the local area with large dislocation will increase the stress concentration in the adjacent area, showing the phenomenon of non-uniform distribution of stress drop, and increasing the rupture tendency of local section. Moreover, the non-uniformity distribution of stress drop and the uneven fault geometry determine the non-uniform slip behavior of fault. Taking the 2004 M_w 6.0 Parkfield earthquake as an example, the stress drop distribution of the fault plane was calculated. The maximum stress drop was about 9.2MPa, which near the source. But the stress drop increased in some sections of the fault plane, reaching -3.5MPa. Compared with the average stress drop, the distribution of stress drop on fault plane calculated by the dislocation model can better reflect the source rupture process and predict the aftershock evolution.

Key words: slip model, Okada's dislocation theory, stress drop, parkfield earthquake