Crustal seismogenic structures and deformation styles along the Longmen Shan Fault belt in the eastern Tibetan Plateau inferred from ambient noise tomography

Abstract

A seismic array comprising 80 broadband stations with ~10–20 km inter-station distances was deployed along the Longmen Shan fault belt (LMSF), the eastern boundary of the Tibetan Plateau. The recorded ambient noise data provided densely distributed inter-station cross-correlated surface waves. A new 3-D crustal S-wave velocity model for the LMSF was constructed by carrying out ambient noise tomography. The inverted model strongly improved data fitting and decreased data misfit compared to the reference (initial) model. The model highlights several crustal structure features. The Baoxing and Pengguan Massifs on the mountain side of the southern-to-middle LMSF exhibit relatively high crustal velocities, probably indicating strong crust. Low crustal velocities that may reflect weak, deformable brittle crust, exist mainly beneath the middle-to-northern segment of the LMSF and partly around the periphery of the Baoxing and Pengguan Massifs in its southern-to-middle segment. Two SW dipping low-velocity (weak) belts approximately perpendicular to the LMSF are imaged respectively around the Wenchuan-earthquake hypocenter in the south and Beichuan in the north. The low velocities in the two belts may focus movement of the eastern Tibetan Plateau relative to the Sichuan Basin (Yangtze Craton), and the uplift of the Tibetan Plateau over long time periods. Based on the velocity and tectonic structures, the young, high topography and thickened crust but low GPS shortening rates around the southern-to-middle LMSF may be due to the dominant effect of vertical crustal deformation caused by the existence of the strong Baoxing and Pengguan Massifs. This would then be in contrast to the characteristic lateral movements due to ductile crustal flow or weak, deformable brittle crust typical of the middle-to-northern LMSF.