Using ambient noise to study the co-seismic and post-seismic velocity changes of the 2021 Yangbi MS 6.4 earthquake in Yunnan, China

Seismic velocity change is a good indicator reflecting the stress state variation of the underground media. Therefore, seismogenic process could be effectively revealed by the velocity change calculation. Based on the seismic ambient noise interferometry, we study the co-seismic velocity change and post-seismic recovery process of the 21 May 2021 M_S6.4 Yangbi earthquake using the continuous records from 16 stations within 50km of the epicenter. The results show that the relative velocity changes between the station pairs (dv/v_pair) decrease significantly after the mainshock. The amplitude ranges from -0.29% to -0.02% and decreases with the increase of station spacing. The relative velocity changes of each station (dv/v_station) obtained by linear regression range from -0.16% to -0.02%, and are generally negatively correlated with the epicenter distance. It is notable that the measured co-seismic velocity changes are mainly originated from the shallow media (≤2km). Such changes are considered to be caused by both static and dynamic strain, but the primary controlling factor is rock fragmentation and large-scale adjustment of stress produced by strong ground motion. In addition, dv/v_station located at the northern stations far from the epicenter have the largest drop values, demonstrating that they are more sensitive to stress disturbances. This may be related to the distribution of thermal fluids below these stations. The results of velocity changes indicate that around the study area the seismic velocity reached its minimum value within a few days after the mainshock. The value then gradually recovered, reaching the pre-seismic level around August, which characterizes the healing process of broken rocks. In this study, ambient noise interferometry has been effectively applied to measure the velocity changes during and after the Yangbi earthquake, and the results show that co-seismic velocity changes are jointly controlled by various factors including the fracture degree of the fault zone, dynamic strain, and the existence of fluids.