Relocation of earthquakes in Tangshan area in the recent decade

We relocated 4 874 earthquakes totally which were recorded by Hebei seismic network in Tangshan area during the period between January of 2010 and July of 2020 by using the double-difference relocation method, the double-difference relocation method combined with waveform cross-correlation, and the double-difference tomography method combined with the waveform cross-correlation, respectively. The comparison of the location results by the three relocation methods suggests that the double-difference tomography method combined with the waveform cross-correlation could produce much higher resolution hypocentral parameters than those from the other two methods. As a result, the root-mean-square （RMS） of traveltime residuals were greatly reduced from 0.71 s to 0.27 s. In map view the relocated earthquakes are more concentrated on obvious strips around the faults, and the dominant distribution direction is NE, which is basically consistent with the fault strike in this area. The result is more reasonable in depth distribution with focal depth mainly concentrated in 5−15 km, and the distribution pattern appears as a nearly normal distribution. From the spatial distribution of the epicenters after relocation, it can be seen that there is a small earthquake dense zone with steep dip angle in NNE direction on the east of the Tangshan-Guye fault, indicating that the stress level is high and the tectonic activity is strong in this local area. Therefore it is speculated that there may be a hidden fault at the eastern end of the Tangshan-Guye fault, and the occurrence of earthquake in this region is related to its deep activities.