| The intersection area of Xianshuihe fault, Aninghe fault and Longmenshan fault is a typical tectonic zone at the eastern margin of the Tibetan Plateau. It is the window to spy the seismic activity of Chundian rhombic block. Frequent microearthquakes arose in the Xianshuihe fault and relative silenct seismicity kept in Aninghe faul after the occurrence of Wenchuan earthquake (2008) and Lushan earthquake (2013).In order to detect the seismogenic mechanism in the intersection zone from the aspect of structure and texture in the crust, we inversed an 1-D initial model using VELEST method for 3-D inversion and the obtained the 3-D Vp structure, the Vp/Vs structure and the location of hypocenters in the study area at the depths of 0km-30km using Double-difference tomography method with scientific investigation data, intensive observation data and the observation data in reservoir zone. The reliability of one-dimension velocity structure was tested with stability analysis. The resolution of inversed three-dimension velocity structure was confirmed by using synthetic tests method including checkboard resolution test and restoring resolution test. Meanwhile, we confirmed the hypocentre locations are validate by statistics of travel-time residual.Combining regional tectonic background and the inverted results, we discussed the characteristics of earthquakes, interpreted the Vp and Vp/Vs anomalies in the intersection zone. The results shows:(1)The overall Vp/Vs value in the study area is 1.67,significantly less than that of normal medium.(2) In general,we have a more clustered hypocenter distribution after 1-D and 3-D inversion and the accuracy of the earthquake locations is improved.(3)In the study area, high seismic activities is find along Xianshuihe fault. The seismic activity in Aninghe fault zone and Longmenshan fault zone is relatively moderate. The deepest hypocenters are in the southern part of Shimian, which is the conjunction zone of the Xianshuihe fault, Anninghe fault and Daliagnshan fault.(4)The statistic between the magnitude of earthquakes and their depth indicates that the larger the magnitude, the greater the average depth.(5)Low-Vp, low-Vp/Vs anomalies and Intensive micro-earthquakes distributed along these anomalies in Kanging-Moxi area along Xianshuihe fault implies the weakening effects of hydrothermal activity of hot springs on the crust of rock in western Sichuan plateau. (6) Low Vp, low Vp/Vs anomaly under the Gongga mountain depicts the shape of the mountain roots and reflects the concomitant gravity equilibrium. (7) Low-Vp, high-Vp/Vs anomaly exists at depths of 0km-5km in Hanyuan. A cluster of earthquakes are located near the anomaly. We suspect this anomaly is formed by fluid permeability into shallow rock mass with impoundment in Pubugou reservoir. (8)In northeastern Kangding, high-Vp and high Vp/Vs anomalies at relatively deeper depth reflect hard rock lithology beneath the weaken layer of Jintang arc structure. These results not only outline the form of overburden layer at the end of the Jintang arcuate structural zone, but aslo depict the geometry of the high-Vp and high-Vp/Vs structure under the overburden layer in northern Kingding region. The relatively solid bedrock speculated preventing the thrust of Jintang arcuate strcture is sketched on the southern part of the overburden layer. (9)The conjunction zone of Aninghe fault and Daliangshan fault, high-Vp and high Vp/Vs anomaly at relatively deeper depth reflect hard rock lithology beneath the weaken layer of graben basin along Aninghe fault. This anomaly is surrounded by a large earthquake cluster which has the deepest hypocenters in the study area.It is necessary to explore the scientific issues metioned when we analysis the inverted results by virtue of analyzing the original travel times, mining the datas which contain the information of structure. |
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