Joint inversion for crustal and Pn velocities and Moho depth in Eastern Margin of the Tibetan Plateau

Abstract

A classical problem in seismic tomography is the trade-off between a discontinuity location and the velocity of the medium. For that reason, crustal tomography and Pn tomography are typically done separately by isolating first-arriving Pg and Pn waves. Although this avoids the contamination of secondary arrivals because of the existence of the Moho, lots of good data are discarded and, as a result, the lower crust is particularly poorly constrained. Here we design an iterative scheme to invert jointly for the velocities of the whole crust and the Pn waves as well as the Moho depth. We use a spherical pseudo-bending ray tracing method for the heterogeneous crust and include secondary Pg waves at large distances. We applied the method to the eastern and southeastern margin of the Tibetan Plateau, where this method is particularly useful because of the rapid variation of crustal structure and Moho depth. Our data include arrival times from Chinese provincial and national earthquake bulletins and our own hand picks. The results show significant features, including slow upper crust but fast lower crust under Sichuan Basin, rapid Moho variation, and slow Pn in the plateau and fast Pn in the borderland separated by major boundaries and faults. The mid–lower crust in the plateau is relatively slow compared to the average model and Sichuan Basin and Poisson's ratio is anomalous (exceeding 0.3), which is consistent with weak mid–lower crust in the plateau.