Double-Difference Tomography In Northeastern Margin Of The Tibetan Plateau Constrained By The Observations Of High Density Seismic Array

The double-difference tomography method is a tomographic imaging method developed in recent years,which utilizes the travel time information of seismic events to jointly invert the three-dimensional velocity structure and seismic event locations.The arrival information of the seismic phases are the relatively most stable and the largest amount of observational information in the current seismic observation and record,which has provided natural rich information for studying the structure of the crust and the seismic source distributions.This method has been widely used in the Pacific plate subduction zone in northeastern Japan,Longmenshan and its neighboring areas,Zhaotong,Lushan,Ludian and Tianzhu-Gulang and other areas.And a series of important results of crustal structure and the hypocenter distributions in different research areas have been obtained.At present,the northeastern margin of the Tibetan Plateau is the focus area for many scholars.Many problems to be clarified in this region attract many researchers' attention.The northeastern margin of the Tibetan Plateau has extremely complex geological structure.And the strong earthquakes occurred frequently in this area.So far,it has accumulated rich observation data of seismic wave for our research.These data lay a solid foundation for our research on the fine deep structure in this area.On the basis of previous research,we have used the data from the high-density mobile seismic array and permanent seismic stations on the northeastern edge of the Tibetan Plateau and combined with the double-difference tomography methods to attempt to obtain the high-resolution tomographic results for this region.Based on a large number of numerical experiments,we determined the optimum resolution by continuously dividing the grid.According to the L curve,damping value and smoothing weight for actual inversion are determined.Finally,we used 311538 absolute arrival times and 2671254 relative arrival times of 9644 seismic events to jointly invert the earthquake locations and the crustal velocity structure in the study area.The results indicate that the velocity structure in the northeastern Tibetan Plateau has visible lateral heterogeneity.As a whole,the Tibetan Plateau shows low-velocityanomaly,while the Ordos area shows high-velocity anomaly.At the same time,the Yangtze block also shows high-velocity anomaly.The velocity structures at different depths behave differently.And the velocity structure of P-wave and S-wave of the same depth also have significant differences.In the region with intense seismic activity enclosed by the fault of northern margin of the West Qinling Mountains and the Lintan-Dangchang fault and the Lixian-Luojiapu fault,the velocity structure of P-wave changes from a low-velocity anomaly at a depth of 0 km to high-velocity anomaly and low-velocity anomaly interphase distribution at a depth of 5 km.And in this region,the S wave velocity structure is characterized by high-velocity anomaly and low-velocity anomaly interphase distribution at a depth of 0km near the surface.While it is almost characterized by low-velocity anomaly at the depth of 30 km.The P wave velocity structure has changed obviously at the depth of 20 km in the area surrounded by Minjiang fault,Tazang fault and Xueshan fault where the Jiuzhaigou earthquake of magnitude 7 occurred in August 8,2017.The hypocenter of Jiuzhaigou earthquake lies in the transition zone between the high velocity anomalies and low velocity anomalies.In addition,the Minxian-Zhangxian Ms6.6 earthquake,which occurred on July 22,2013,is located in the northeastern Tibetan Plateau.The P-wave velocity structure around the Lintan-Dangchang fault,where the hypocenter was located,began to change significantly at a depth of 15 km.The Minxian-Zhangxian earthquake is also located on the transition zone between the high velocity anomalies and low velocity anomalies.This high and low velocity transition zone is relatively easy to accumulate stress.And it may be an important reason for occurring moderately strong earthquakes in this area.