The possibility of deeper or shallower extent of the source area of Nankai Trough earthquakes based on the 1707 Hoei tsunami heights along the Pacific and Seto Inland Sea coasts, southwest Japan

To validate the abundance of scenarios of large earthquakes in the Nankai Trough, we examined the effects of both lateral and vertical expansions of the source areas on maximum tsunami heights along the Pacific coast and Seto Inland Sea. The recently proposed Nankai Trough earthquake scenario (Mw = 9) has a maximum slip of 20 m near the trough axis. However, the predicted tsunami heights exceeded those obtained from historical records of damage caused by the 1707 Hoei tsunami event at Tosa Bay and along the Pacific coastlines near the Kii Channel, owing to the large slip on the up-dip extension of fault segments off Shikoku Island. Such discrepancy indicates that for segments off Shikoku Island, the slip near the trough axis was unremarkable, even for the 1707 Hoei earthquake event, which is considered one of the larger historical Nankai Trough earthquake events. For segments east of the Kii Peninsula, the large slip on the up-dip end might be ineffective. While the proposed Mw9-class scenario also includes large slip of several meters on the down-dip side (down to about 35-km depth), coseismic crustal subsidence reached further landward than is usual for Nankai Trough earthquakes. For the Seto Inland Sea region, this resulted in maximum subsidence of about 1 m, and such crustal subsidence effectively increased the height of the tsunamis. Furthermore, simulated tsunami heights, corrected for crustal subsidence, were in good agreement with those obtained from historical records of the damage caused in the Seto Inland Sea region.