Variations Of Shear Wave Splitting In The Source Region Of The 2021 Maduo Ms7.4 Earthquake,Qinghai

The effect of the sustained extrusion between the Indian plate and the Eurasian plate creates the hightest and largest plateau on the earth--the Qinghai-Tibet Plateau.At the same time,the effect leds to severe crustal deformation in the Qinghai-Tibet Plateau.The Bayankala block on the northeastern margin of the Qinghai-Tibet Plateau generates intense tectonic deformation and many faults which is the frontal zone of the plateau uplift and deformation development,gestates a series of strong earthquakes.The analysis of the seismic anisotropy,crustal deformation and geologic structural activity has important scientific research value for solving geodynamic problems such as the formation process of the Qinghai-Tibet Plateau and the seismogenic mechanism of strong earthquakes.The Maduo Ms7.4 earthquake and its rich aftershocks that occurred near the northeastern margin of the Qinghai-Tibet Plateau in May 2021 provide data protection for the seismic anisotropy research in this region.Shear wave splitting is the most obvious phenomenon of judging the anisotropy of the interior of the earth which is one of the most effective tools to study the anisotropic characteristics of the source region such as regional structure characteristics and dynamic changes of regional stress fields.Using shear wave splitting analysis and measurement to measure the seismic anisotropy in the source area is of great significance to study the deep structure and the seismogenic environment in the Qinghai-Tibet Plateau.Based on the shear wave splitting analysis of local records waveform data at 150 temporary stations from the dense array for the Maduo Ms7.4 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 Maduo Ms7.4 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.