Inversion For Ridgecrest Earthquake Source And The Three-Dimensional Velocity Structure Around Its Source Region Using The Double-Difference Tomography Method

A magnitude Mw=6.4 earthquake occurred in California on July 4,2019(UTC,17:33:49.04),about 11 km away from Ridgecrest.About 34 hours later(July 6,2019,03:19:53.04),the main earthquake of Mw7.1 occurred about 35 km away from the epicenter of the Mw6.4 foreshock.This earthquake is the largest earthquake in the region after 20 years,causing serious impact on the region.A high-precision seismic catalog can reflect the fine distribution characteristics of fault structures.However,the double-difference location method assumes that the path anomaly caused by the velocity non-uniformity is independent of the location,resulting in a certain deviation in the location results;a fine velocity structure can improve the understanding of the structural background of the source area,compared with the traditional tomography method,the double-difference tomography method can simultaneously invert the seismic source position and the three-dimensional velocity structure of the seismic source area,and the model resolution is higher than the traditional tomography method.The resulting fine crustal structure and precise seismic source location can improve the understanding of the seismogenic structure,geological tectonic activities and the internal structure of the earth.The seismic phase observation reports and seismic waveform data of Mw ? 2.5within 500 km of the epicenter of the main earthquake from July 4,2019 to August 6,2019 are selected from the California Institute of Technology & U.S.Geological Survey(Caltech/USGS)SCSN and the Southern California Earthquake Data Center(SCEDC).Relocated the hypocenters of Mw 6.4 foreshock and Mw 7.1 mainshock of Ridgecrest earthquake and its aftershocks with Mw ? 2.5 within one month after the earthquake and inverted the three-dimensional velocity structure of the source area were independently and jointly inversed by using double-difference seismic tomography(tomo DD)combined with the waveform cross-correlation technique.Solve the damping factor and smoothing factor used for the best inversion by using the largescale curve method.On this basis,the maximum likelihood estimation is used to calculate the b-value of seismicity parameter,and combine the distribution characteristics of aftershocks and the focal mechanism solution and field survey reports to analyze the relationship between seismic activity and fault structure,describe the distribution of faults in the deep crust,explore the seismogenic faults of this earthquake sequence,study the relationship between velocity structure and seismic activity,and provide reference materials for subsequent seismological research.The relocation results show that four-event clusters are formed from 2253 events,and the seismic source parameters with a relocation rate of up to 92% are achieved.The travel time residual and cross-correlation travel time residual of earthquake sequences decreases to 73 ms and 30 ms,respectively.Compared with the original result,the linear features of the epicenter are more significant after relocation,with aftershocks forming an L-shaped and T-shaped pattern,respectively.The length of aftershocks is ? 90 km and the wide is ?25km.The velocity structure inversion results show that there is obvious lateral inhomogeneity in the crustal velocity structure in the focal area,the velocity distribution is closely related to the surface geological structure and the distribution of the fault strike: the velocity difference between the two sides of the Sierra Nevada fault is large,the velocity on the right side of the fault is high and seismically active.On the contrary,earthquakes occur less frequently.The earthquakes mainly occurred in the transition region of high and low velocity,and the area near the epicenter of Ridgecrest earthquake was in the low b-value region,that is,the high stress concentration region.In this thesis,the relationship between seismic activity and fault structure is quantitatively analyzed and the results state that the Ridgecrest earthquake sequence starts from the deep and propagates to the shallow surface gradually.The predominant range of depth distribution is from 4km to 10 km,showing the characteristics of segmented bands,revealing that the Ridgecrest sequence destroyed the complex "fault system" composed of multi fault structures with different scales,and it can be inferred that the Little Lake fault zone is the seismogenic fault of the Ridgecrest earthquake sequence.