The Characteristics Of The Tectonic Stress Field In The Ryukyu Subduction Zone

The Ryukyu subduction zone is characterized by strong tectonic activity and frequent strong earthquakes,which can induce destructive tsunami earthquakes.Once a tsunami occurs,it will have a great impact on China's coastal waters and southeast coastal areas.In recent decades,a lot of geophysical exploration has been carried out on the Ryukyu subduction zone,but the research based on tectonic stress field is relatively weak According to the boundary of depth division,the seismicity and tectonic stress field in the Ryukyu subduction zone are divided into two parts:shallow source and middle source.Combined with previous research results on Ryukyu area,the stress state of subduction zone is further discussed,and the following conclusions and understandings are obtained.The normal fault type moderate source earthquakes are mainly distributed in the middle and South sections,the thrust type medium source earthquakes are mainly distributed in the north section,the depth of 30-100km is the distribution gap of normal fault type earthquakes,and the thrust type earthquakes are basically located at the depth of 0-70km.The deep dip angle of the northern segment of the subduction zone is steep(about 73°),and the dip angle of the middle and southern segments is relatively gentle(about 55°).There are many normal fault type shallow earthquakes distributed in the coastal gullies,which may be caused by the bending of the subducting plate at the trench,resulting in the extension of this part of the plate,thus producing the normal fault type earthquake with the fault plane strike consistent with the trench direction.Thrust type earthquakes are mainly distributed in the shallow part of Ryukyu Island Arc,which may be due to the extrusion of subducted plate and overlying plate.Some strike slip and normal fault type shallow earthquakes are distributed in the trough,and the focal depth is less than 30km,which indicates that there are many oblique shear faults in the shallow part of Okinawa trough.The R value of the stress field in the shallow part of the Okinawa trough is low,and the principal tensile stress is basically stable,with a NNW direction and a small dip angle(>30°).The R value of the shallow stress field in the northern offshore trench side area is higher,and the main compressive stress is basically determined,which is in the see direction,and the dip angle is larger(>30°).The R value of the deep stress field in the middle part of the study area is relatively high,and the principal compressive stress is basically determined,which is NW direction,and the dip angle is large,which is consistent with the subduction direction.It may be due to the differences in the viscosity and subduction velocity of the surrounding mantle that the middle plate encounters greater resistance when it subducts downward,so it is subject to compressive stress.