Activity characteristics of the 2019 two Ms＞5.0 earthquake sequences in eastern Weiyuan(Sichuan)and body-wave velocity structure of the source region
Seismicity rates in Weiyuan(Sichuan Basin,China)and its adjacent regions have increased significantly since 2015.In 2019,one MS5.4 earthquake and one MS5.2 earthquake occurred in eastern Weiyuan on September 8 and December 18,respectively.The two events broke the record of the largest magnitude of historical earthquakes in the region.To investigate the source characteristics and occurrence mechanisms of the two events,here we collected the Pg and Sg wave travel times and continuous waveform data of 5638 ML≥1.2 earthquakes recorded by 25 temporal stations and 17 permanent stations in the study area from August 2019 to March 2020.The double-difference seismic tomography method is used to relocate the two earthquake sequences and to obtain high resolution 3D body-wave velocity structure of the source area.Our relocation results show that most events are located in the depths of 3～4 km.The spatial distribution of aftershocks occurred in 10 days after the two Ms＞5.0 earthquakes indicates that they ruptured two blind thrust faults striking to NNE and dipping to SE at an angle of～40°.The inversion results of the 3D body-wave velocity structure show that variation of VP values in the source region is more significant than Vs values,and that VP/Vs ratio is generally 4％～10％lower than the background value.Above the depth of 5 km,most earthquakes occurred within regions with low VP anomalies and weak-high Vs anomalies and transition zones from high velocity anomalies to low velocity anomalies.In the depth range of 5～7 km,earthquakes are mostly located in regions with low VP anomalies.The characteristics of velocity structure reveal that,in the source area,there are probably abundant of gas-filled pore and fractures that are favor for fluid flow.In addition,seismicity observed after 7 November 2019 shows obvious characteristics of spatiotemporal migration driven by fluid diffusion.The corresponding fluid diffusion coefficients along the strike and dip directions of the two source faults are 1.5 m2·s-1 and 0.1 m2·s-1,respectively.The major cause for the occurrences of the two events is probably pore pressure increase associated with fluid diffusion.
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