Difference in the maximum magnitude of interplate earthquakes off Shikoku and in the Hyuganada region, southwest Japan, inferred from the temperature distribution obtained from numerical modeling: – The proposed Hyuganada triangle –

To clarify differences in the maximum magnitude of interplate earthquakes offshore of Shikoku (≈ M 8) and in the western adjacent Hyuganada region (≤ M 7.5), southwest Japan, where the oceanic Philippine Sea (PHS) plate subducts beneath the continental Amuria plate along the Nankai Trough, we calculated the temperature distribution for the plate interface using a three-dimensional thermal convection model. We attribute the observed very low heat flow values (less than approximately 45 mW/m2) in the southern part of the Hyuganada region to the subduction of the Kyushu–Palau Ridge, which is presumably older than the adjacent Shikoku Basin to the east. Trial and error methods suggest an age difference of about 20 Myr between these two areas. The heat flow calculated from the temperature distribution fits the observed heat flow in the Hyuganada region well. The results show that Hyuganada earthquakes may take place in the newly proposed “Hyuganada triangle,” bounded by a southeastern isotherm of about 200 °C on the plate interface, a northwestern boundary between the upper surface of the subducting PHS plate and the continental Moho discontinuity, a northern isotherm of about 350 °C, and a northeastern margin-normal zone with a possible barrier or large friction parameter L. Interplate earthquakes in such a limited seismogenic zone may reach a maximum magnitude of M 7.5. Our results also suggest that a seismic gap seaward of the Hyuganada region may be caused by stable sliding arising from low temperature, resulting in a deeper updip limit than that offshore of Shikoku. The seismic gap offshore southwest of southern Kyushu along the Ryukyu Trench may result from the overlapping of two stable sliding regimes on the plate interface: one with low temperature at shallower depths caused by the subduction of the old PHS plate and the other a serpentinized mantle wedge at deeper depths.