Study On Earthquake Relocation And Three-dimensional Velocity Structure In The Maduo Area Based On Deep Learning

The Maduo area is located at the northeastern edge of the Tibetan Plateau in China,inside the Songpan-Ganzi Block,and is a key area of interest for geophysicists.The Earthquake administrations of Qinghai Province and Gansu Province installed nine temporary seismic stations and some fixed seismic stations,which together collected continuous seismic waveform data from May 20,2021,to June 10,2021.The EQTransformer algorithm was first used to pick up the seismic phase and associate the seismic phase data.Then Hypoinverse-2000 method was used to locate the earthquakes and construct the AI earthquake catalog.Then,we combined the manual earthquake catalog provided by the digital seismic network of Qinghai Province from May 20,2021,to December 19,2021,and combined the seismic events from the two catalogs with the seismic phase data to obtain the initial seismic data.The double-difference tomography was method to jointly invert the area’s crustal three-dimensional velocity structure and source location parameters.The following findings are reached after analyzing the seismogenic fault,rupture mechanism,and seismogenic mechanism of the May 22,2021 M_S 7.4 Maduo earthquake in the context of geological and tectonic background.The relocation results show that the Maduo aftershock sequence follows the Kunlun Pass-Jiangcuo Fault with a linear northwestward spreading,which has shallow source depths and is mostly concentrated in the area between 2～14 km,suggesting that the earthquake mainly faulted the medium of the upper crust.According to the aftershock sequence and the spatial spread of surface rupture,combined with the slip nature of the Kunlun Pass-Jiangcuo Fault and the Maduo mainshock,the Kunlun Pass-Jiangcuo Fault is deduced to be a seismogenic fault.In addition,the existence of northwest branch faults is inferred from the certain detachment of the 2-clusters from the Kunlun Pass-Jiangcuo Fault,and they are continuous.Three-dimensional velocity structure results show that the shallow velocity structure in the Maduo area is closely related to the surface geological structure,and the crustal velocity structure exhibits obvious lateral inhomogeneity.According to the results of the profiles,the study in this paper indicates that the aftershock energy is mainly released in the upper crust,and the mechanical properties of the medium near the surface are weak.Combined with the fact that earthquakes with unilateral rupture usually have the characteristics of high and low-velocity differentiation along the direction of the seismogenic fault,the comprehensive inference is that the Maduo earthquake is a bilateral rupture.The results also provide evidence for the large-scale rupture of the east and west sides of the Maduo mainshock.In addition,the high-velocity body in the upper right of the mainshock explains well the insignificant rupture above the mainshock.Our results combined with the results of existing studies,this paper speculates that the weak aftershock activity in the later period indicates that the energy released during earthquake rupture is sufficient and that the short-term seismic hazard is insignificant.The existence of an S-wave low-velocity layer and high wave velocity ratio beneath the Maduo area,combined with the possible existence of free water in the Triassic flysch rocks inside the Songpan-Ganzi Block and the existence of obvious high conductivity and low heat flow in the middle and lower crust of the study area,together infer that the S-wave low-velocity layer and high wave velocity ratio in the Maduo area reflect the existence of crustal fluids.The Maduo earthquake occurred at the junction of high and low velocities and is biased toward the side of the high-velocity body,where stresses are easy to accumulate and,at the same time,easy to release and rupture.By combining the two points,this paper deduces that the Maduo mainshock accumulated a large amount of stress in the source area,coupled with the fact that the crustal fluid encountered a hard slope-type high-velocity body and turned upward to flow into the seismogenic fault,resulting in the weakening of the seismogenic fault.The combined effect of both eventually triggered the Maduo M_S7.4 earthquake.