Shear-wave velocity structure of the crust and uppermost mantle in the Shanxi rift zone

摘要

The Shanxi rift zone is one of the largest and active Cenozoic grabens in the world, studying the velocity structure of the crust and upper mantle in this region may help us to understand the mechanisms of rift processes and the seismogenic environment of active seismicity in continental rifts. In this work, using the broadband seismic data of Shanxi, Hebei, Henan, Shaanxi provinces, and the Inner Mongolia Autonomous Region from February 2009 to November 2011, we have picked out 350 high-quality phase velocity dispersion curves of fundamental mode Rayleigh waves at periods from 8 to 75 s, and Rayleigh wave phase velocity maps have been constructed from 8 to75 s period with horizontal resolution ranging from 40 to50 km by two-station surface-wave tomography. Then,using a genetic algorithm, a 3D shear-wave speed model of the crust and uppermost mantle have been derived from these maps with a spatial resolution of 0.4° 9 0.4°. Four characteristics can be outlined from the results:(1) Except in the Datong volcanic zone, in the depth range of11–30 km, the location of a transition zone between the high- and low-velocity regions is in agreement with the seismicity pattern in the study region, and the earthquakes are mostly concentrated near this transition zone;(2) In the depth range of 31–40 km, shear-wave velocities are higher to the south of the Taiyuan Basin and lower to the north,which is similar to the distribution pattern of Moho depth variations in the Shanxi region;(3) The shear-wave velocity pattern of higher velocities to the south of 38°N and lower velocities to the north is found to be consistent with that from the upper crustal levels to depth of 70 km. At the deeper depths, the spatial scale of the low-velocity anomalies zone in the north is gradually shrinking with depth increasing, the low-velocity anomalies are gradually disappearing beneath the Datong volcanic zone at the depth of 151–200 km. We proposed that the root of the Datong volcano may reach to a depth around 150 km;(4) Along the N–S vertical profile at 112.8°E, the 38°N latitude is the boundary between high and low velocities, arguing the tectonic difference between the Shanxi rift zone and its flanks, in the rift zone the seismic velocity is dominated by low-velocity anomalies while in the flanks it is high.