Ambient Noise Monitoring of Seismic Velocity Around the Longmenshan Fault Zone From 10 Years of Continuous Observation

Abstract

AbstractWe present 10‐year continuous seismic velocity changes from 2007 to 2017 around the Longmenshan fault zone, where the 2008 Mw 7.9 Wenchuan earthquake and the 2013 Mw 6.6 Lushan earthquake occurred. We collected continuous waveforms recorded by 20 broadband stations covering the entire Longmenshan fault zone and used three‐component ambient noise correlation techniques to measure the velocity changes in four different period bands. Our results show that a significant drop, 0.04–0.06%, occurred in the coseismic velocity at the time of the Wenchuan earthquake, near the rupture zone for the period bands of 1–3 and 3–8 s. In addition, only one fourth of the coseismic velocity drop has been recovered 9 years after the earthquake. We also observed a subtle velocity drop, 0.0066%, delayed by ~100 days relative to the time of the mainshock in the period bands of 6–15 s. In the source region of the Lushan earthquake, we detected a weak but reliable coseismic velocity change of ~0.01% in the period band of 1–3 s. The physical mechanism of the seismic velocity change could involve the damage in the shallow subsurface layer (<3 km), the damage in the fault zone at depth (~10 km), and the postseismic slip along the deep rupture zone up to ~20 km. The uncommonly slow postseismic velocity recovery indicates the slow healing of the Longmenshan fault zone after the Wenchuan earthquake, which may be associated with the low rate of stress loading onto the fault zone.