Temporal velocity changes in the crust associated with the Lushan M_s7.0 earthquake by auto-correlation function analysis of ambient noise

According to continuous waveform recorded by three seismic stations (BAX, MDS, TQU) in the epicentral area of the Lushan earthquake, the paper studied the spatial and temporal variations of the relative velocity of the earth's crust during April 20, 2012 to April 20, 2014. The distance of the stations from the epicenter is about 15.7 km, 20.5 km, 29.6 km, respectively. Results show that the relative velocity of the earth's crust significantly decreased after the earthquake at the three stations from autocorrelation Green's function at 0.1-1 Hz frequency range. The largest velocity drop is about 0.6% at station BAX on the northwest of the epicenter, the second largest drop is at station MDS on the southeast of the epicenter, which is about 0.4%, and the minimum velocity drop is measured at seismic station TQU, about 0.2%, with a depth sensitivity range of about 10 km. In space, the relative velocity changes have good consistency with the characteristics of source rupture and coseismic volume strain changes. The maximum velocity drop appeared in 40 to 100 days after the earthquake, respectively; the recovery process of relative velocity shows a better correspondence with decay of aftershocks. The results support that the temporal change of seismic velocity may be related to the damage from fault rupture and stress change around the fault zone. For higher frequencies (2-5 Hz), there are no obvious coseismic velocity changes in all three stations, but seasonal variations are observed, especially the semi-annual variation, with an average maximum change rate of about 0.3%. The depth sensitivity range of the change is about 1 km. However, it does not have good corresponding relation with the near-surface damage caused by strong ground motion.