Study And Application Of Stress Field Of Aftershock Sequences Of The Yongsheng M6.0 Earthquake Through Near-source Digital-recorded Data

At 13h35m of October 27th, 2001, an earthquake of M6.0 happened in Yongsheng County of Yunnan Province. After the earthquake, the monitoring network consisted of the 6 substations constituted by mobile digital seismographs was arranged around the epicenter to monitor aftershocks. From October 28 to November 28 of 2001, 673 aftershocks were recorded, the largest was ML2.8 and the smallest was ML-0.5. By using the data obtained in near field from mobile digital observation, we studied the stress field of aftershock sequence from 3 aspects. Firstly, using the data of digital seismic wave in near field, we have studied the shear wave splitting of Yongsheng aftershock sequence. Secondly, by using a new dislocation theory, obtaining relation of stress drop using peak velocity is established. Further we have studied the feature of stress drop of aftershock sequence. Thirdly, we have studied the feature of environmental stress of aftershock sequence and realized the dynamic short-impending tracing of the environmental stress by way of estimating environmental parameters with peak velocity.The result of shear wave splitting show that the phenomena of shear wave splitting is very evidently in Yongsheng region. Extensive dilatancy anisotropy in the upper crust of Yongsheng region was exited. Generally, the shear wave has two preponderant polarizations. One is N20W oriented and another is N60W oriented. The former is in accordance with the regional predominant direction of principal compressive stress and the latter is the direction of the fracture of main shock. So, the joint effect of EDA fracture and the seismotectonic crack can be regarded as the cause of the shear wave splitting of Yongsheng Earthquake. The time delay of shear wave splitting from 2ms/km to 20ms/km and the average of the whole process is 6.7ms/km. The result is in very good accordance with the result got by the forerunners.Based on the cubic model, non-homogenous random source model, we set up the scaling relation between the peak velocity and the source parameters, which is proved to be more suitable for the study of source parameters of small earthquakes. Assuming that Q factor changes in a linear way with frequency, the relation which is able to estimate the stress drop through peak velocity of ground can be derived as. The result shows that the stress drop varies from 0.01MPa to 4.2MPa, 94.5% stress drop are no more than 1.0MPa. In the whole sequence, the stress drop attenuates very fast The low stress state is in accordance with the weak strength attenuation of the Yongsheng earthquake. The aftershock sequence reflects a low energy rupture process.The environmental stress varies from 0.5MPa to 2.0MPa and the average is 1.0TMPa. The feature of environmental stress is similar to the stress drop, attenuating very fast after mainshock and attenuating slow in aftershock sequence. Finally, by way of estimating environmental stress with peak velocity, we realized the dynamic short-impending tracing of the environmental stress in Yunnan region. It provided the new information for earthquake prediction through analyzing dynamic image of environmental stress. The result show that the stronger earthquakes are apt to occur in the area with strong stress filed.