Abstract:

Research of the focal mechanisms of the deepfocus earthquakes plays an important role in the understanding of the driving mechanism and the dynamics of the plate tectonics. In this study, we analyzed distribution characteristics and fault plane solutions of the deepfocus earthquakes in Northeast China. Our results show that: (1) Hypocentral depths of these deepfocus earthquakes become progressively deeper in the orientation from SEE (Japan Sea) to NWW (Northeastern China), but there is no obvious trend in the orientation from SSW to NNE. Furthermore, downdip compressive stress is dominant in the focal mechanism solutions for these deepfocus earthquakes, suggesting that the deepfocus earthquakes of Northeast China is the result of driving action from the Western Pacific Ocean subducting slab. (2) From JapanSea to Northeast area of China, the hypocentral depths of earthquakes exhibit a clear linear correlation with the horizontal distance of these deep focus events measured from the Japan trench and coincide with the WatatiBenioff zone under Japan, suggesting that the deepfocus earthquakes region of Northeast China, like JapanSea deepfocus earthquakes, is a part of the Western Pacific Ocean subduction zone. (3) Based on previous slab thermal structure analyses and seismological studies, we postulate that most deepfocus earthquakes in Northeast China are related to metastable olivine wedge, and their occurrence is the result of phase transformation.

Key words: deepfocus earthquakes, distribution characteristics, subduction driving force, focal mechanisms, Northeast China