Three-dimensional fine S-wave velocity structure around the source area of the Sichuan Luxian M_s6. 0 earthquake on September 16, 2021

In recent years, the frequency of moderate and strong earthquakes and the intensity of seismicity in the southern margin of Sichuan Basin increase significantly. The occurrence of the Luxian M(s)6. 0 earthquake on 16 September 2021, located in Luzhou City of Sichuan Province, have once again attracted the attention of domestic and foreign researchers for the seismogenic environment and potential seismic hazards in southern Sichuan. In order to explore the deep structure around the source area of the Luxian M(s)6.0 earthquake, based on the dense array composed of 70 short-period seismic stations, a high-resolution 3-D S wave velocity model is constructed with the method of ambient noise tomography. The results show that the velocity varies significantly beneath Yujiasi syncline and Huayingshan fault's eastern/western branches. The velocity structure beneath the Yujiasi syncline exhibits low within the depth of 3. 0 km, and as the depth increases, the velocity gradually increases. The eastern and western branches of the Huayingshan fault zone shows high velocity in the NE-SW direction generally, except for several local low velocity anomaly zones. It shows low velocity anomaly to the west of the Luxian earthquake source area at depths of 4. 0 similar to 6. 0 km, and high velocity to the east. The Luxian earthquake epicenter is located at the boundary of the high and low velocity zone. The focal depth of the aftershock sequences are mainly between 3. 0 and 5. 0 km. Combined with the previous studies, the low-velocity zone located west of the epicenter may be interpreted as the effect of fluids. It is speculated that the occurrence of the Luxian M(s)6.0 earthquake may due to the dislocation of the pre-existing hidden fault under the action of regional tectonic stress and fluids disturbance.