Quasi-waveform seismic tomography of crustal structures in the capital circle region of China

Seismic tomography is one of the main tools to explore the interior of the earth. In this study, the quasi-waveform seismic tomographic method is used for the first time to reveal the crustal structures in the capital region of China. 3-D highresolution V_P, V_S and the Poisson's ratio models are generated by inverting 29839 direct P-and 29972 direct S-wave traveltimes selected from 3231 local earthquakes. The results reveal strong crustal heterogeneities. The velocity anomalies at shallow depths are well consistent with surface geologic structures. The relatively low-velocity anomaly layer in the middle crust may be the result of multiple phases of tectonic activity. Earthquakes generally occurred on the boundaries of high-and low-velocity and Poisson's ratio anomalies. There are obvious low-velocity anomalies below the hypocenters of the Tangshan earthquake and the historical Sanhe-Pinggu earthquake, implying the existence of fluids. The similar velocity structures around the hypocenters of the two earthquakes indicate that the occurrences of the two earthquakes may be related to the same mechanism. The highresolution velocity models provide important observational constraints on the small-scale heterogeneities and dynamic mechanism of the crust in the capital region of China.