Re-examination of Coseismic Uplift for the 1946 Showa Nankai Earthquake at Murotsu Port, Kochi Prefecture

Coseismic uplift at Murotsu Port near Cape Muroto has been a well-known tectonic feature characterizing the past M 8-class megathrust earthquakes along the Nankai trough, southwest Japan. Although the coseismic uplift at Murotsu Port for the 1946 M 8.0 Showa Nankai earthquake was often believed to be 1.15 m, the measurement of the uplift was conducted at Tsuro Port, 3 km south of Murotsu Port. We utilized leveling and sea-level measurement data from past studies to re-examine the vertical displacement and its uncertainty at Murotsu Port. We obtained four uplift estimates and their uncertainties depending on the measurement data and correction methods for the relative displacement at different locations, preseismic and postseismic displacements, and a possible difference in high-tide level. By combining these estimates with equal weight in the probability density function, we concluded that the coseismic uplift was ~ 1.02 m with estimated uncertainties of ~ 0.06 m at Murotsu Port for the 1946 Nankai earthquake.