Three-Dimensional Crustal P-Wave Velocity Structure Around Longmenshan And Aninghe Fault Zones

Longmenshan and Anninghe fault zones are located on the eastern margin of the Tibetan Plateau where the junction of the Bayankara block,Sichuan-Yunnan block and Yangtze block.Influenced by the expansion of the Tibetan Plateau,the crustal structure around the Longmenshan and Anninghe fault zone has been changing dramatically laterally.The area is characterized by frequent seismic activity and a complex seismogenic environment.Since historical records,nearly 100 moderate and strong earthquakes with ML≥6 have occurred.These earthquakes have caused significant human casualties and economic losses.Therefore,these areas have been the focus of attention in earth sciences.In recent years,geophysicists have deployed many broadband temporary seismic arrays on the Longmenshan Fault,the Anninghe Fault,and surrounding areas.Scholars have conducted deep tectonic studies using various geophysical tools such as seismic tomography,deep seismic sounding,and gravity-magnetic sounding.These studies have obtained important results such as the crust’s and upper mantle’s fine velocity structure,deep stress,and deformation distribution.It supports the interpretation of the deep seismogenic environment and related dynamic processes.Seismic tomography methods can effectively reveal the lateral inhomogeneous media in the Earth’s interior using seismic observation data.In areas with high seismic activity levels and dense distribution of seismic stations,local seismic body wave traveltime tomography is an important tool to obtain the crust’s and upper mantle’s velocity structure.Since the beginning of this century,the double differential tomography method has been developed based on the traditional traveltime tomography method.This method utilizes absolute and relative traveltime data to achieve a joint inversion of the relative position of the earthquake source,the absolute position and the 3D velocity structure.The three-dimensional velocity structure of the crust with high resolution can be obtained by using the double-difference tomography method,and provides an important basis for studying the inhomogeneity of the crustal medium and deep dynamic processes.The Institute of Geophysics of the China Earthquake Administration(CEA)has conducted a series of intensive temporary seismic arrays in the Sichuan-Yunnan region to provide more reliable observations for seismological studies.This paper uses the seismic phase data recorded by 120 temporary stations and 337 permanent stations of the regional seismic network by the China Earthquake Administration from October 2008 to July 2021.Finally,the double-difference tomography obtained the high-resolution three-dimensional velocity structure of the crust in the Longmenshan fault zone,the Anninghe fault zone,and the surrounding area.The main conclusions are as follows:There are apparent low-velocity anomalies in the shallow part of the Sichuan Basin,and the low-velocity anomalies can extend to about 15 km near the foreland basin,which is related to the foreland basin receiving sediments from the eastern margin of the Tibetan Plateau.Near the Longmenshan fault zone,some high-velocity anomalies in the upper to middle crust parallel to the fault zone reveal the spatial distribution of domal complexes and thrust complexes in this area.For example,The Pengguan domal located in the middle of the Longmenshan fault zone extends down to a depth of about 15 km.But Xuelongshan domal extends down to a depth of no more than 5 km.An active seismic zone with a focal depth of more than 20 km between Anninghe and Daliangshan faults is consistent with the distribution of P-wave high-velocity anomaly.We speculate that the existence of high-velocity anomalies may increase the depth of the crustal brittle-ductile transition zone and cause deep seismicity in this region due to the strong deformation at the intersection of the Anninghe and the Daliangshan faults.The research results of this paper provide new information for further understanding of the deep geological structure and seismic activity mechanism in this area.After the earthquake relocation,the earthquake source distribution is more evident in the strip distribution around the fault zone,indicating improved location accuracy.The focal spatial distribution of the northern segment of the Longmenshan fault zone is given after relocation becomes shallower from west to east,which may reflect the characteristics of thrust or thrust and strike-slip in the fault zone.The focal depth inside the plateau is relatively shallow,which may reflect the relatively weak characteristics of the medium below the plateau.The focal depth of most earthquakes in the study area is less than 20 km,but there are some earthquakes with focal depth between 20～35 km in the intersection of Anninghe fault zone and the Daliangshan fault zone.The seismic distribution reflects the complexity of seismic structure in this area...