3-D velocity images beneath the Chinese continent and adjacent regions

Abstract

In this paper, 3-D velocity images beneath the Chinese continent and adjacent regions are reconstructed using P-wave traveltime residuals of regional and distant earthquakes, supplied by Chinese seismic networks and ISC. In the tomographic inversion, an orthogonal projection operator is used to separate the velocity from the hypocentral parameters, the fast Givens transformation without square-root calculations is then used in sequential orthogonal triangularizations, and the modified singular value decomposition is also utilized. Thus the difficulties of large memory demand and high computing cost in solving the system of equations with coupled velocity and hypocentre parameters are avoided. At the same time the stability of the algorithm is ensured. Resolution images are also obtained in the inversion process, based on the resolution theory of Backus & Gilbert modified for discrete models. In model parametrization, the interpolation function of grid node velocities is used as the space function of velocity. To meet the practical needs, the velocity function is allowed to be discontinuous. The consequent complications of the forward problem have been dealt with in the paper. The main results are summarized as follows. (1) The velocity images given in this paper indicate lateral heterogeneity of the crust and upper mantle beneath the Chinese continent and adjacent regions, which persist down to a depth of at least 1100 km. The correlation between the velocity image and the known surface geological features can be traced down to a depth of 110 km. Below 220 km this correlation is hardly obvious. (2) The images at 45−0 km (immediately above the Moho) and 45+0 km (below the Moho) show clearly that the crust of the Chinese continent can be approximately divided into two parts by the 102.5°E longitude. The crust to the east is thinner than that to the west. (3) The images at 400 and 600 km also display differences between the eastern and western part. This means that tectonic differences between eastern and western China may exist in the whole upper mantle.