EXPLORING THE RELATIONSHIP BETWEEN MEGATHRUST EARTHQUAKES AND INTRAPLATE STRESS FIELDS IN JAPANESE SUBDUCTION ZONES

The principal forces acting on subducting plates are slab-pull and bending/unbending, which generate the stress field and earthquakes within the plate. The focal mechanisms of intraplate earthquakes reveal the orientation of the stress field within the slab. Astiz et al. (1988) and Lay et al. (1989) observed differences in the focal mechanisms of intermediate-depth earthquakes before and after large underthrusting events. Before megathrust events, intermediate-depth earthquakes tended to be tensional. In contrast, immediately after megathrust earthquakes, intermediate-depth events tended to have compressional mechanisms and/or to occur at a decreased rate. This change in seismic behavior following a megathrust event may reflect a transient change in stress orientation within the subducting plate.

We revisit this observation with expanded datasets and new methods to calculate the orientation of the stress field. Following research performed on the Nazca slab subducting under South America, we apply the same methods to the Japan Trench subduction system. Japan is home to the best seismic monitoring systems in the world, and is also a common place for megathrust earthquakes. We utilize focal mechanism data from the global Centroid Moment Tensor project and the Full Range Seismograph Network of Japan of M_W≥4.7 and M_W≥3.5 respectively. We isolate events from within the subducting plate into different cells based on along-trench distance, depth and time, taking into account past megathrust ruptures, the degree of plate coupling, and the frequency and timing of local earthquakes. Next we invert the focal mechanisms to calculate the stress field in each cell. We damp stress variations between adjacent cells to minimize the complexity of the model and only keep changes required by the data (Hardebeck and Michael, 2006). Comparison of the calculated stress orientations and relative magnitudes of the stress axes provide clues to the nature of the interaction between intraslab stresses and megathrust earthquakes. Due to localized differences in plate geometry, subduction angle and strength and direction of applied stresses before and after megathrusts, it is difficult to determine a clear cause-and-effect relationship between large subduction events and intraplate earthquakes.