Study On The Distribution Characteristics Of Internal Stress In The Subduction Plate Of The Western Pacific Ocean In Northeast Japan

Japan is located in the western part of the Pacific Rim Seismic Zone,and its seismicity is very well developed,such as the 9.0 magnitude earthquake on March 11,2011.With the continuous subduction of the western Pacific plate to the lower part of Japan,the central and deep-focus earthquakes in Japan are more frequent.In order to study the stress distribution in the subduction plate,this paper obtains the information of fault occurrence,orientation and dip of principal pressure axis and tension axis by analyzing focal mechanism solutions of earthquakes in the western Pacific plate.This is of great significance to the analysis of deep stress state,disaster assessment and earthquake warning in plate collision zone.In this paper,the focal mechanism solution is inverted by gCAP program written by Professor Lupei Zhu.In the inversion process,the waveform is divided into five parts and fitted separately.The body wave and surface wave information of seismic waveform are synthetically used.Compared with the traditional P-wave initial polarity method and amplitude ratio method,the results obtained by gCAP are more accurate and reliable.In this paper,the focal mechanism solutions of 104 earthquakes with Mw(> 4.5)in northeastern Japan from 2006 to 2018 are inverted by using the data provided by Hi-net website of Japan.Considering that some earthquakes occurred below the sea area,the earthquakes events are not well surrounded by seismic stations,and the results of waveform fitting are poor.After eliminating these earthquake events,70 focal mechanism solutions of earthquakes are finally obtained.Through the analysis of these focal mechanism and the distribution characteristics of the principal pressure axis,the following understandings can be obtained: the Shallow-source earthquakes in northeastern Japan are mainly reverse fault-type.The principal pressure axis is perpendicular to the trend of the Japanese trench,indicating that the compressive stress is dominant in the shallow plate at 70 km,which is mainly related to the mutual compression of the Pacific plate and the Eurasian plate.Medium-source earthquakes are mainly normal fault-type and reverse fault-type.,which are caused by the subduction of the Pacific plate.During the process of subduction of the Pacific plate,the density of the medium around the plate increases,and the buoyancy of the mantle material increases gradually,resulting in the uplift of the subducted plate,which leads to normal faults.The distribution of the principal pressure axis is dispersed and has obvious heterogeneity.When h>300km,the stress is mainly compressive stress.The axis is around 270 degrees,and the dip angle is 30 degrees,which is consistent with the direction of plate subduction.