Time Difference Between the 1854 Ansei-Tokai and Ansei-Nankai Earthquakes Estimated from Distant Tsunami Waveforms on West Coast of North America

Tsunami damages associated with the 1854 Ansei-Tokai and Ansei-Nankai earthquakes have been thoroughly investigated by historical document surveys. Kitahara et al. (2006) and Matsu'ura (2017) concluded from reliable contemporary documents that the Ansei-Nankai earthquake occurred approximately 30 hours after the Ansei-Tokai earthquake. However, the time difference may have an error because of time keeping system used in Japan at the time of the earthquakes. In this study, we estimated the time difference between these events from the comparison of trans-Pacific tsunami propagation numerical simulations and observations recorded at two tide gauge stations of San Francisco and San Diego on West Coast of North America.

Assuming that the origin times of 1854 Ansei-Tokai and Ansei-Nankai tsunamis are 0 a.m. Greenwich Mean Time (GMT) on 23 December and 6 a.m. GMT on 24 December, tsunami propagation numerical simulations were conducted. The simulation results were several ten minutes earlier than the observations. For 1854 Ansei-Tokai tsunami, cross-correlation functions between the observed and simulated waveforms showed a time gap between them of approximately 30 minutes. Similarly, it showed about 75 minutes at San Francisco or 85 minutes at San Diego for the 1854 Ansei-Nankai tsunami.

Based on the above results, the origin times of 1854 Ansei-Tokai and Ansei-Nankai tsunamis are estimated as 0:30 a.m. GMT on 23 December and 7:15 or 7:25 a.m. GMT on 24 December, respectively. It is noted that the results have an error of about several tens of minutes due to the difference between the present and past bathymetry and topography. The time difference between these events is 30 hours and 45 or 55 minutes. Difference between our result and previous studies reflects the fact that the unit of the timekeeping system used in Japan was approximately 2 hours long. Because of low cross-correlation coefficient, further examination would be also required to estimate the time difference more accurately.