A Study On The Characteristics Of Source Parameters In The Basin In Southern Sichuan

The source parameters are significant to understand the source physics and predict ground motion.This paper mainly focuses on the accurate inversion of source parameters from the theoretical inversion method and practical application.The main results of study are summarized as follows:The inversion process of source parameters is investigated in detail.(1)The two processing methods,including spectral decomposition and spectral ratio fitting are compared and analyzed in detail.The range of stress drop calculated by the two methods is close,whereas the stress drop obtained by the spectral decomposition method is slightly larger than that obtained by spectral ratio method.(2)For Brune model(γ = 1,n = 2)and Boatwright model(γ = 2,n = 2),the fitting effect of theoretical amplitude spectrum is tested respectively.Brune model is smoother at the corner,Boatwright model is sharper at the corner,which is better consistent with the processed data,the standard deviation is smaller,and the result values are more concentrated and stable;(3)The single EGF spectral ratio inversion method and the stacked EGFs spectral ratio inversion method were compared and analyzed.The stress drop and corner frequency of single EGF inversion are relatively large as well as the higher availability.We investigate the source parameters of target events in the southern Sichuan Basin,including 258 earthquakes with ML > 2.5(recorded by permanent seismic stations)from December 2018 to June 2020 and 49 earthquakes with ML > 2(recorded by transportable array)from December 2020 to July 2021.The source parameters,including corner frequency,seismic moment,stress drop and source radius of target events,are obtained by both spectral decomposition and spectral ratio fitting.From December 2018 to June2020,stress drop values estimated using seismic moment and corner frequency from single-event spectra range from 0.5 to 30 MPa,and the median stress drop is 10.8MPa;from December 2020 to July 2021,stress drop values estimated from spectral ratios range from 0.09 to 17.74 MPa,and the median stress drop is 0.8MPa.By comparing and analyzing the relationship of different focal parameters,it can be seen that there is no obvious systematic scaling relationship between stress drop and moment,corner frequency is negatively correlated with magnitude,but the source radius and slip distance are positively correlated with the magnitude.The stress drop obtained from dense transportable array shows the characteristics of temporal and spatial variation.In the target area,the overall stress drop is relatively small;On the plane,the stress drop of the southwest ares higher then the northeast;Vertically,the events are mainly divided into two swarm,and deep events release less stress than shallow events.The value of stress drop may be affected by the type and attitude of fault,the activation of detachment faults,industrial hydraulic fracturing water injection,pore pressure and so on.