Stress Orientations in the Nankai Trough Constrained Using Seismic and Aseismic Slip

The strength of subduction thrust faults is key to understanding seismogenesis at the provenance of Earth's largest earthquakes. Earthquake focal mechanisms are routinely inverted to constrain the stress state at seismogenic depths. However, on some megathrusts, deformation is accommodated by both earthquakes and types of slow fault slip. We employ focal mechanisms of short-term slow slip events (SSEs), a type of slow fault slip, and earthquakes in a regional stress inversion to investigate the stress state of the Nankai Trough megathrust and interpret the results in the context of regional tectonics Previous studies using earthquake-only stress inversions found principal stress orientations in this region that are incompatible with thrust faulting on the megathrust. When both SSEs and earthquakes are considered, the stress state of the central and eastern Nankai Trough megathrust is well oriented for thrust faulting. Our results suggest that slow fault slip source regions may appear to have misoriented stress fields if slow fault slip constitutes a substantial proportion of fault slip and the stress field is not well constrained by earthquakes. In the SSE region, we find that faults are well oriented for failure, suggesting they have strengths similar to their surroundings. Combined with low Vp/Vs ratios and sensitivity to small stress changes, our results imply that the megathrust and surroundings operate at low deviatoric stresses in the SSE source region. Further, we show that the coefficient of friction for areas hosting SSEs is low (mu = 0.19-0.50), implying frictionally weak materials in the SSE source region.