Crustal deformation in the eastern margin of Tibet: Implications for earthquake generation

Abstract:

The Longmen-Shan fault zone, at the eastern margin of the Tibetan Plateau, is one of the most extensively studied areas in the world, yet the deformation model and earthquake-generating mechanism remain subjects of vigorous debate. We present the three-dimensional (3-D) velocity models determined using a large volume of seismic data, together with the crustal stress and GPS data analysis, to investigate the nuclei of crustal deformation and earthquake generation along the reverse-thrust and strike-slip fault zone. It has been observed that anomalously low velocity, with low resistivity related to the Sichuan foreland basin, is in sharp contrast to high-velocity and high-resistivity anomalies in the Songpan-Ganze block in the upper crust. The tomographic models presented here reveal two crustal bodies with anomalously low velocity and high conductivity in the fault zone, separated into three contrasting segments by the two crustal bodies. The two low-velocity and low-resistivity bodies have been interpreted as being associated with extrusion of either fluids or products of partial melting from the lower crust and/or the upper mantle from Tibet. Our seismic imaging, crustal stress and GPS data analysis indicates that the 2008 Wenchuan and 2013 Lushan earthquakes occurred in the distinct areas with high-velocity, low-Poisson’s ratio and high crustal stress. The high velocity seismogenic layer that enables the accumulation of high crustal stress for large zone at the hypocenter sources may reflect the earthquake generation. We consider that the slow velocity gap zone is associated with fluid-bearing ductile flow from the lower crustal materials of Tibet being pushed into the weakened segment of the Longmen-Shan fault zone, which suggests strong variations in the rheological strength of the rock in the eastern margin of Tibet. This finding implies that coupling between these presumably fluid-bearing bodies and earthquake generation could be extremely complex.