Variations of shear wave splitting in the source region of the Madoi M(S)7. 4 earthquake, Qinghai

Based on the shear wave splitting analysis of local records waveform data at 150 temporary stations from the dense array for the Madoi M(S)7. 4 earthquake scientific expedition and a permanent station (MAD), we present a total of 22518 pairs of shear wave splitting parameters to perform a systematic analysis of the upper crustal anisotropy in the source region. The shear wave splitting results show an obvious spatial characteristics, but they don' t show regular changes with time. In the region of dense aftershock along the main rupture, the splitting results show the most obvious characteristics of shear wave splitting. The fast wave polarizations are consistent with the trend directions of the main rupture and the distribution of aftershocks. The delay times at stations in the region of dense aftershocks along the main rupture are significantly greater than these of the external. And the delay times in the eastern segment of the main rupture where the main shock occurred and aftershocks were most dense is much greater than these of the western and the middle segments, and the delay times of the north side of the main rupture is greater than these of the south side of the main rupture. Outside the region of dense aftershocks, the delay times at stations on both sides of the main rupture gradually decreases with the increase of the distance between the station and the main rupture, and reach to a certain distance, the delay times tend to stable. During the seismogenic process of the Madoi M(S)7.4 earthquake, the stress mainly accumulated in the region of dense aftershocks along the main rupture. The stress accumulated in eastern segment of the main rupture was stronger than that in the western and the middle segments of the main rupture, and the stress accumulated on the north side of the main rupture was stronger than that on the south side of the main rupture. With the distance between station and the main rupture increased, the effect of accumulated stress gradually decreased, and the effect became very weak after a certain distance. Because the dense array was not deployed until 12 days after the main shock, we hadn't obtained the waveform of the main shock and most of the strong aftershocks. The highest peak of small and medium earthquakes had occurred, and the frequency of aftershocks tended to stable during the seismic observation period. The above features show that the stress accumulated during the seismogenic process has not been completely released, and the process will continue for some time. Therefore, during the observation period of the aftershock the shear wave splitting parameters had failed to show the characteristics of regular changes with time.