3-D P and S wave velocity structures and their relationship to strong earthquakes in the Chinese capital region

We used 33487 P and 31822 S wave arrival times from 2866 events recorded by the North China seismic telemetry network and the Capital Circle digital seismic telemetry network from 1993 to 2005. The crustal three-dimensional P and S wave velocity structures in the Capital Circle area (38.5°N ∼ 41°N, 114°E ∼ 120°E) are obtained with a horizontal resolution of 25 to 50 km. The Poisson's ratio distribution is also achieved. Our results show that there is significant inhomogeneity in the P and S velocity structures in the Capital Circle area. Shallow velocity anomalies distributions are consistent with main surface geological tectonic features. We found that strong earthquakes in the seismic zone are apt to occur between low velocity and high velocity zones as well as in low Poisson's ratio zones. Under these hypocenters, low velocity and high Poisson's ratio anomalies exist. By comparison with the results of other regions, the relation between earthquake triggering and fluids is discussed. We think that the effect of fluids plays an important role in the earthquake generation and triggering processes.