Extreme inundation flows during the Hokkaido‐Nansei‐Oki Tsunami

Abstract

The tsunami generated by the July 12, 1993 Hokkaido‐Nansei‐Oki Mw=7.8 earthquake produced in Japan the worst local tsunami‐related death toll in fifty years, with estimated 10–18m/sec overland flow velocities and 30m runup. These extreme values are the largest recorded in Japan this century and are among the highest ever documented for non‐landslide generated tsunamis. We model this event to confirm the estimated overland flow velocities, and we find that, given reasonable ground deformation data, current state‐of‐the‐art shallow‐water wave models can predict tsunami inundation correctly including extreme runup, current velocities and overland flow. We find that even small local topographic structures affect the runup to first‐order, and that the resolution of the bathymetric data is more important than the grid resolution. Our results qualitatively suggest that—for this event—coastal inundation is more correlated with inundation velocities than with inundation heights, explaining also why threshold‐type modeling has substantially underpredicted coastal inundation for this and other recent events.