Study On The Mechanism Of Deep Earthquakes In Ogasawara Region,Japan

Earthquakes with a focal depth greater than 300 km are called deep earthquakes.Although deep-source earthquakes account for only a few percent of the total global earthquakes,they provide direct information on the thermodynamic and mechanical properties of subduction lithosphere.At present,there are three hypotheses about the seismogenic mechanism of deepsource earthquakes : dehydration embrittlement,adiabatic shear instability and transition faulting of olivine.Since it is difficult to dehydrate and embrittlement when the depth exceeds 300 km,more and more experimental results support the existence of metastable olivine wedges.The study area selected in this paper is the southern Japan ' s Ogasawara area.The area is located in the Pacific plate,the Philippine Sea plate and the Okhotsk plate interaction area,frequent earthquakes,shallow-middle-deep source earthquakes have a lot of distribution.In this study,the seismogenic mechanism of deep-source earthquakes in this region is selected for research.Firstly,the waveform data of 80 deep-source earthquakes(magnitude greater than 3.5 and depth greater than 300 km)from July 2000 to July 2020 were collected from the Japan Seismic Network Fnet.Then,three aspects are carried out around the triggering mechanism of deep-source earthquakes.(1)Focal mechanism solution : The focal mechanism solutions of 80 deep-source earthquakes are solved by using ISOLA program,and the direction of stress action is analyzed.The results show that the deep-source earthquake is mainly shear fracture,and presents the stress mode of ‘ anti-volcanic '(opposite to volcanic earthquake).This is similar to the moment tensor distribution obtained by the acoustic radiation generated by the olivine phase transition;(2)The double difference positioning method is used for high-precision relocation of deep-source earthquakes,and the positioning error is within 2 km,which is helpful to analyze the spatial relationship between the source location and the subduction plate.(3)The P-wave velocity structure in the focal region of deep earthquakes is obtained by double difference travel-time tomography.The results show that the velocity anomaly distribution in the focal region is complex,and the low velocity anomaly appears in the focal region of deep earthquakes,which is consistent with the results of 5 % seismic velocity difference obtained by the thermodynamic constraint of olivine-spinel transition.Based on the existing research results of high temperature and high pressure rock physics and geophysics,this study suggests that the low velocity anomalies in the focal region reflect the possible olivine metastable phase transition in the subduction western Pacific plate.The results of focal mechanism solution show that the possibility of rock particle volume change during olivine phase change is relatively low.Comprehensive analysis shows that metastable olivine provides positive buoyancy.The overlying strata pressure is downward,and the surrounding rock pressure is concentrated inward.This stress action leads to the ‘ anti-volcanic ' mode of focal mechanism solution of deep-source earthquakes.Accordingly,this paper believes that olivine phase transition may trigger deep-source earthquakes.