Study On Focal Mechanism Solution And Stress Distribution Characteristics Of Subducted Slab In Ogasawara,Japan

The study area of this thesis is the Ogasawara region in southern Japan.It is located in an area where the Okhotsk plate,the Pacific plate and the Philippine Sea plate work together.Earthquakes occur frequently in the Ogasawara area,and there are a large number of shallow earthquakes,intermediate earthquake and deep-focus earthquakes.In order to obtain the causal mechanism of shallow,intermediate and deep-focus earthquakes in the Ogasawara region and the stress direction in the plate,this paper analyzed focal mechanism solution of the earthquake in the Ogasawara region and obtained the information of dip and azimuth angle of main tension axis and main pressure axis of the fault.The information will provide the necessary scientific basis for the study of the causal mechanism of earthquakes,for post-earthquake assessment and so on.In this paper,the Isola program is used to invert the three-component waveform data from seismic stations to obtain the focal mechanism solution.The data used are all from the Japan Seismic Network—F-net,which collected 308 earthquakes(magnitudes greater than 4.5)from 2000 to 2019 in the Ogasawara area of Japan.The Isola program was used to invert the collected data to obtain the focal mechanism solution,and then,the information of dip and azimuth angle of main tension axis and main pressure axis was obtained.After analyzing the focal mechanism solution of the Ogasawara area in Japan and the distribution characteristics of the main pressure axis,the following conclusions were obtained:(1)The shallow earthquakes are mainly caused by shear-tensioned fault movements,and the faults are mainly inverse faults.The main pressure axis is perpendicular to the strike of the Japanese trench.The internal stress of the plate is mainly extrusion stress.(2)The distribution of intermediate earthquakes on Hudson map is relatively scattered,and the cause of intermediate earthquakes is complicated.The faults are mainly inverse faults,which may be caused by the subduction of the Pacific plate.The main pressure axis is more dispersed and has obvious non-uniformity.(3)The deep-focus earthquakes are mainly distributed in the "CLVD" axis.It is speculated that the cause may be related to factors such as the metastadle phase transformation of olivine in the plate.The faults are mainly normal ones,accompanied by a large number of reverse faults.The internal stress of the plate is mainly tensile stress.