Fine Velocity Structure And Earthquake Relocation In Source And Peripheral Area Of Eryuan Earthquake

On March 3,2013,the Ms5.5 earthquake occurred in Eryuan county,Yunnan province,causing considerable economic losses and casualties.The fault structure in this area is complex and earthquake disasters occur frequently.At the same time,The northwest of Yunnan Province,where the earthquake is located,is a region with strong seismic activity.The study on fine crust structure and earthquake relocation in this area will help us to understand the deep environent of earthquake breeding,which is useful for reducing earthquake damage.In this study,we collected the waveform data of 7597 seismic events from June12,2010 to September 13,2013,recorded by 107 temporary seismic stations in the southern segment of the North-South seismic belt(23° ? 28 ° N,97 ° ? 102 ° E)of Himalaya I,China Seismic Science Exploration Array.The deep learning method Pick Net was used to pick up 190449 of high-precision first arrival P-wave absolute arrival data,and the 199338 relative arrival times were obtained by the cross-correlation bispectrum method.The VELEST method was used to inverse and build the initial 1-D P-wave velocity model of this study.The 3-D P-wave velocity structures and the earthquake relocation in the source area of Eryuan earthquake and the northwestern region of Yunnan province were determined by double-difference tomography method with two different scales.The results show that the main earthquake of Eryuan Ms5.5 is located at 99.78 °E,25.92 ° N and the focal depth is 10.532 km.The main shock is located in the high velocity medium under the Liantie Basin,and the aftershocks are concentrated in the area where the velocity structure transits from low velocity to high velocity.We conclude the main fault of Eryuan Ms5.5 earthquake is the main boundary fault zone on the eastern margin of the Liantie Basin on the west side of Cangshan Mountain.After relocation,the aftershock sequence is concentrated in the horizontal plane near the depth of 10 km,with NNW trend,basically parallel to the strike of theseismogenic fault.There are high velocity and low velocity alternating features near the surface of the focal area,and the discontinuity of velocity structure is obvious in depth.We infer that the main boundary fault zone in the eastern margin of the iron making basin where the main earthquake occurred may be staggered laterally by multiple positive faults.The results also show that the velocity structure of the upper crust in the northwest Yunnan region reflects the thickness distribution of sedimentary cap well,and the characteristics of low velocity are very prominent in this region.The aftershocks after the relocation were concentrated along the fault zone.In the depth range of 10 ? 30 km,there is a large range of low velocity regions,and we infer that there are obvious middle and lower crustal plastic layers in the northwest Yunnan region.The discontinuous characteristics of low velocity medium appear at a depth of15 ? 20 km in Tengchong and Longling area,which proves that there may be two crustal circulation channels in the southeastern margin of the Tibet Plateau.Large earthquakes tend to occur in the high velocity medium above the low velocity zone or in the region where the low velocity medium is transitioned to the high velocity medium.We conclude that the existence of low velocity layers in the middle and lower crust is likely to cause stress concentration in the upper crust and cause strong earthquakes.