Seismic velocity changes in the epicentral region of the 2013 Lushan earthquake measured from ambient seismic noise

In order to investigate spatial and temporal changes of seismic velocity in the epicentral region of the M7. 0 Lushan earthquake, located in the southwestern segment of the Longmenshan fault. We used the Moving-Window Cross-spectral and Frequency-dependent polarization technique to analyze the ambient noise during the time period from April 2012 to April 2018. Firstly, we investigated the variations of amplitude and polarization of the ambient noise in the period band of 1 similar to 20 s. Secondly, we measured the seismic velocity changes at different depth during and after the Lushan earthquake. Our results indicate that (1) The seismic noise field is stable on the annual scale. The seismic records indicate a complicated, multistate wavefield with at least two noise sources in the period band of 2 similar to 10 s. The monthly and seasonal variations of the noises are different at different frequency bands. (2) Clear seasonal seismic velocity changes are observed. The amplitude of velocity change at higher frequences (1 similar to 2 s and 2 similar to 4 s, about +/- 0. 04%) are greater than velocity changes at longer periods (4 similar to 10 s, 10 similar to 20 s). This seasonal velocity changes can be explained by the precipitation induced groundwater level change. (3) The coseismic velocity changes are most prominent in the period band of 1 similar to 20 s, the coseismic velocity reduction is observed with amplitude up to 0. 08%similar to 0. 1%. And significant coseismic velocity reductions mainly contrained within 70 km from Longmenshan fault. The velocity reduction was about 0. 1% on the Sichuan Basin (east) side, which slightly higher than that on the QinghaiTibet Plateau side (0. 08%). The reduction inside the Longmenshan fault was not obvious. The results in different frequency bands show that except for 10 similar to 20 s, the relative seismic velocity in the other three sub-bands showed a sudden significant reduction after main shock; the largest reduction (about 0. 08%) was observed at the period band of 4 similar to 10 s. We suggest that the the post-seismic relaxation of the main shock and aftershocks are responsible for the observed velocity changes. (4) The seismic velocities recovery to the pre-earthquake value approzimatedly one year after the Lushan earthquake where only slight osscillations around the pre-earthquake level are obaserved until April 2018.