Using present-day patterns of interseismic coupling to model the C.E. 1707 Hōei earthquake and simulate tsunami inundation of Lake Ryuuoo in the Bungo Channel, southwestern Japan

Abstract:

The C.E. 1707 Hōei event is often considered the worst-case scenario for a Nankai Trough earthquake and tsunami impacting southwestern Japan, and recent estimates of the earthquake’s magnitude have exceeded M_W 9. However, when paired with tsunami simulations, previously published earthquake models for the event fail to match sedimentological and historical records of 1707 tsunami height in Shikoku and Kyushu. Specifically, models do not produce a sufficiently large tsunami in the northern Bungo Channel and Seto Inland Sea without also over-predicting tsunami heights along the open Pacific coastlines of Shikoku and Kyushu. Here, we apply a newly developed rupture model that uses present-day patterns of geodetically imaged interseismic coupling to inform patterns in coseismic slip. Along the southwestern extent of the plate interface (the Hyuga-nada area), there is a region of weak coupling up-dip along the trench axis and a region of strong coupling down-dip beneath Shikoku and Kyushu. Following this pattern, the new earthquake model produces less coseismic uplift offshore and greater subsidence in an inland region that includes the Bungo Channel. This combination of regional subsidence and a tsunami wave more focused to the Bungo Channel results in inundation patterns more consistent with historical and sedimentological observations in the Hyuga-nada area. We also run the tsunami simulation on a high-resolution grid around Lake Ryuuoo, a back-barrier lake in the northern Bungo Channel that contains a marine overwash deposit from the 1707 tsunami. We apply a simple sediment transport model to demonstrate that the coupling-based rupture scenario produces flow over Lake Ryuuoo’s barrier capable of transporting the maximum grain size observed in the lake’s 1707 deposit. These findings suggest that spatial trends in our present-day coupling model are more consistent with inundation patterns observed for large tsunamis generated by coseismic rupture along the Nankai Trough.