Study on mechanisms of earthquakes and inside stress characteristics of deep subducting plates beneath Izu-Bonin, Japan

Abstract:

The Izu-Bonin of Japan is located in the area where the Pacific plate, Philippine plate and Okhotsk plate interact with each other. There are many earthquake events because of the tectonic activities. In this study, the moment tensor inversion was worked out from the shallow, medium and deep source seismic waveform data. From this we discuss the focal mechanism, and the stress distribution characteristics inside the plate.

We selected seismic data above Mj4.0 during 2000 to 2018, a total of more than 200 events are selected. All data is downloaded from Japanese NIED F-net, and the latitude and longitude range is 32°-37°N, 135°-141°E.

In this study, we mainly used the Isola mechanism inversion program, the core of which is the moment tensor inversion of seismic waveform data. Focal mechanism solutions was worded out by this program and then the orientation and inclination information of the main pressure axis and the tension axis are obtained.

We find that shallow source earthquakes are mainly distributed in the "Crack" axis on the Hudson diagram. Shallow earthquakes are mainly caused by shear fault motion, due to the subduction of the Pacific plate under the Philippine plate and the interaction between the Philippine plate and the Eurasian plate. The results of medium-source earthquakes are not clear on the Hudson diagram, and the pressure and tension axis is also scattered. When the Pacific plate subducted to this depth, the temperature gradually changed and the plate curved, and the stress distribution was complex, with obvious heterogeneity. The results of deep-source earthquakes are mainly distributed on "CLVD" axis. According to the positive and negative values of CLVD components, there are two types of the stress. We conclude that the seismic mechanism is related to low-velocity fault and olivine phase transition, and the results of the pressure and tension axis show an expansion environment of the Philippine Sea.