Precise relocation of small earthquakes occurred in North China and its tectonic implication

In the present study we collected 74181 P-wave and 74465 S-wave arrival times from 7519 local earthquakes in North China recorded by 166 seismic stations during October 2001 to September 2009, and relocated these earthquakes by using double-difference location algorithm, finally obtained 5511 relocated earthquakes. Our result shows that the locations of the relocated earthquakes have been improved significantly over those of original ones. In map view the relocated earthquakes are more concentrated on obvious strips around the faults, while most of them occurred at 5mdash;15 km depths, suggesting the seismogenic layer in the region being in the mid-upper crust. However, different seismic zones show obvious contrasts in predominant earthquake focal depths. Most small earthquakes around the Shanxi seismic zone occurred in upper crust, while those in continental part of the Zhang-Bo seismic zone and in Xingtai seismic zone mainly occurred in mid-upper crust. The present study obtained some results not revealed by previous studies: ① Small earthquakes occurred in NNE-SSW trend on east side of the northern segment of the Tangshan seismic zone. ② Most relocated earthquakes in sea region of the Bohai sea zone are at about 10 km depth, with a few earthquakes having a depth from a few to 10 km. ③ Small earthquakes around the fault zone in front Taihang mountain gets deeper westward under the mountain, suggesting a deeper seismogenic layer beneath the mountain. These results are helpful for understanding fault seismic activity and deep mantle dynamics of North China region.