Seismic Velocity Variations over Four Years (2013–2016) along the Longmenshan Faults in Sichuan, China

Abstract

ABSTRACT Seismic velocity is a critical geophysical property that reflects the state of the Earth’s interior. Tectonic movements and seismic activities may affect variations in seismic velocity. Here, we report on the spatial and temporal seismic velocity changes along the Longmenshan faults in Sichuan, China, which were inferred from ambient seismic noise from 2013 to 2016, during which the Ms 7.0 Lushan earthquake occurred and introduced clear velocity changes along the southern segments of the Longmenshan faults. The Longmenshan fault zone consists of complex geological structures that were formed by the compression of the Bayan Har block and South China block. The spatial–temporal seismic velocity changes define three segments on the Longmenshan faults, which are consistent with the distributions of the three subblocks of the Bayan Har block. We also calculate the displacement field and analyze the seismicity in the same area to further understand the seismic velocity changes. The strong compression effect of the Bayan Har block and South China block caused the Lushan earthquake, and the relative movements of the blocks changed the seismic velocities and seismicity. Combined with the seismic velocity changes, seismicity, and displacement field results, the evidence suggests that the seismic velocity changes may reflect the interactions among the blocks, and earthquakes may be prone to occur on the edges of the blocks, which are the areas between the positive and negative seismic velocity changes.