Elastic wave velocity structure of the crust and upper mantle beneath the North China Basin

Abstract

Travel time residuals from locally and teleseismically recorded earthquakes are used to determine the three‐dimensional elastic wave velocity structure beneath the central North China basin and surrounding mountains. We calculated P wave and S wave travel time residuals for 200 aftershocks of the 1976 Tangshan earthquake and P wave travel time residuals for 194 teleseisms recorded by the Beijing network between 1976 and 1983. We performed iterative one‐ and three‐dimensional block inversions for crust and uppermost mantle structure based solely on the aftershock travel time residuals. We also performed iterative three‐dimensional block inversions for crust and upper mantle structure based solely on the teleseismic travel time residuals and based on the combined aftershock and teleseismic travel time residuals. We determined that the average thickness of the crust beneath the North China basin and surrounding mountains is about 35 km. Beneath the onshore portion of the North China basin, however, the crust is closer to 32 or 33 km thick. The velocity structure of the upper 20 km or so of the crust is well correlated with the surface geology and parallels the short‐wavelength (a few tens of kilometers) features of the surface topography. Rocks with low seismic velocities lie in or beneath the basin; rocks with high seismic velocities lie in or beneath the mountains. The uppermost mantle, and possibly even the lower crust, appears to be a zone of transition in velocity structure between that of the upper crust and the mantle. The trends of relative highs and lows in the velocity structure of the mantle are nearly opposite that of the crust. The mantle material underlying most of the North China basin has higher than average seismic velocities, while the material beneath the western mountains has lower than average velocities. The velocity structure of the mantle parallels the long‐wavelength (tens to hundreds of kilometers) features evident in the gravity and topography of North China. The high‐velocity mantle material presently beneath the basin probably represents mantle processes not directly related to the formation of the North China basin. We infer that the transition zone between the short‐wavelength and long‐wavelength velocity features represents a zone of decoupling between the lithosphere and asthenosphere. Specifically, the lithosphere beneath the North China block is moving eastward with respect to the asthenosphere.