The multiscale structure of the longmen shan central fault zone from local and teleseismic data recorded by short-period dense arrays

Abstract

The Longmen Shan fault zone (FZ), which consists of the back-range, the central, and the front-range faults, acts as the boundary between the Sichuan basin and eastern Tibet. In this study, local and teleseismic waveforms recorded by a 2D small aperture seismic array (176 temporary short-period seismometers) deployed by China University of Geosciences (Beijing) from 22 October to 20 November 2017 and a dense linear seismic array of 16 stations deployed by Geophysical Exploration Center, China Earthquake Administration during July 2008 are used to study the FZ structure by analyzing FZ-trapped waves (FZTWs), the radial-to-vertical amplitude ratio, and travel-time delays. Based on power density spectra analysis, FZTWs from local events with larger amplitudes and longer wavetrains are clearly observed at stations 6002–6003, 6013–6025, and W025–W032. The dispersion mea-sured from trapped waves is quite weak. The near-surface shear velocity structure esti-mated from the radial-to-vertical amplitude ratios of local initial P waves shows a low-velocity zone around the surface rupture trace. The slight time delay of direct P waves examined from local and teleseismic events indicates a relatively shallow slow structure beneath the arrays. Through the comprehensive analysis of the central FZ, our results sug-gest a shallow low-velocity zone with a width of ∼ 150–160 m along the surface rupture trace. Moreover, our P-wave receiver functions reveal that the Moho depth beneath the Longmen Shan FZ is approximately 45 km, and receiver functions at stations located within the surface rupture zone show more complicated waveforms than those off the surface rupture.