Shear-wave velocities beneath the Yanqing-Huailai Basin, 90–140 km northwest of Beijing, are estimated from the joint inversion of surface-wave phase velocities and teleseismic receiver functions. The data set is from a temporary broadband seismic network supported by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) in the basin and includes 34 teleseismic events from 2003 to 2005.
Receiver functions from the teleseismic events are similar for the stations around the Yanqing-Huailai Basin and exhibit little variation with azimuth. The velocity models constrained by receiver functions and surface-wave dispersion curves are also similar. The resulting models reflect the low-velocity basin sediments to 2 km followed by a positive velocity gradient to 15 km with shear-wave velocity increasing from 2.0 to 3.55 km/sec. Evidence of a midcrust low-velocity layer starts at 15 km with a shear velocity decrease to 3.3 km/sec that extends to approximately 25 km. The total crustal thickness is 38–42 km with a smooth Moho transition to an upper-mantle shear velocity of 4.3 km/sec. The low-velocity zone is consistent with recent extension, geothermal activity, and earthquake locations above this depth.
The average shear velocity model for the basin has similarities to other regional and global models but provides more detailed structure in the uppermost and lower portions of the crust. The new model includes the effect of the sediments in the basin, the low-velocity layer, and the gradual Moho transition. Predicted P- and S-travel times are 1–3.5 sec slower than the previous models at regional distances.