华北地区地震震源机制分区特征

Using waveform data we applied the FOCMEC method to calculate focal mechanisms of 918 M _L≥2.5 earthquakes occurred during the period of Jan. 2010 to Jun. 2014 in North China (latitude 36°-42°; longitude 111°-125°). Among the computed solutions, 572 were found robust based on their clear first motions and amplitudes in the event waveforms, and the similarities among solutions using different (5°, 10°, and 15°) step sizes. Using the TDMT full waveform inversion method, we further calculated the moment tensors for 14 large events which have higher signal-to-noise ratio at low frequencies, and compared the solutions with that by the FOCMEC method. In addition, we compiled focal mechanism solutions for moderate and large earthquakes since 1937 in North China. Based on the focal mechanism types and their correlation with surface tectonics, we characterized the focal mechanism solutions into several spatial clusters. Our results show the solutions for moderate and small earthquakes are generally complex in the study region, but some intriguing patterns can be recognized. The focal mechanisms of moderate and small earthquake have a clear zoning characteristic, and they are mainly normal and strike-slip fault types, suggesting that crustal deformation is dominated by translation and tension. Most of the normal type focal mechanisms are distributed in the Shanxi rift zone, Tangshan and Haicheng seismic zones. For major earthquakes (M _L≥4.0), we found that the focal mechanism type is mainly of strike-slip. We conclude that the horizontal stress is dominant in North China, while in localized regions such as the Shanxi rift zone, Tangshan and Haicheng seismic zones, and the Bohai sea (the local region of Yantai-Penglai section), the tensile stress may be prevailing.