Abstract
We estimated the time difference between the 1854 CE Ansei–Tokai and Ansei–Nankai earthquakes from tidal records of two tide gauge stations (San Francisco and San Diego) on the west coast of North America. The first signals of the Ansei–Tokai tsunami were apparent, whereas those of the Ansei–Nankai tsunami were obscured by the later waves of the Ansei–Tokai tsunami. Waveforms of the Ansei–Nankai tsunami simulated with nonlinear dispersive wave theory by assuming an origin time of 07:00 GMT on 24 December arrived earlier than in the observations. The normalized root mean square and the misfit between the simulated and observed waveforms of the Ansei–Nankai tsunami showed a time difference between them of approximately 0.4 h. This finding suggests that the actual origin time of the Ansei–Nankai tsunami was approximately 07:24 GMT on 24 December. A previous study estimated the origin time of the Ansei–Tokai tsunami to be about 00:30 GMT on 23 December. Thus, we concluded that the time difference between the 1854 CE Ansei–Tokai and Ansei–Nankai tsunamis was 30.9 h. Despite the significant difference in the time resolution between the seasonal timekeeping system used in Japan in 1854 and waveform digitization, our result is roughly in agreement with historical descriptions of the tsunamis, suggesting that such information can be effectively used to determine the origin times of historical earthquakes.
Highlights
The Nankai Trough subduction zone has repeatedly generated large earthquakes accompanied by tsunamis (e.g., Ando 1975; Ishibashi 2004)
We focused on the time difference between the 1854 Common Era (CE) Ansei–Tokai and Ansei–Nankai earthquakes
The first signals of the 1854 CE Ansei–Tokai tsunami were apparent, whereas those of the 1854 CE Ansei–Nankai earthquake were obscured by later waves of the Ansei–Tokai tsunami
Summary
The Nankai Trough subduction zone has repeatedly generated large earthquakes accompanied by tsunamis (e.g., Ando 1975; Ishibashi 2004). The estimated moment magnitude of the 1854 Ansei– Tokai earthquake was Mw 8.4–8.6, and that of the Ansei– Nankai earthquake was Mw 8.5–8.7 (e.g., Cabinet Office Committee for Modeling a Nankai Trough Megaquake 2015; Building Research Institute 2019) Both of these earthquakes generated huge tsunamis. The signals of the tsunamis generated by these earthquakes were recorded by tide stations on the west coast of North America using the fixed time system in which each day is divided into 24 h of equal length (e.g., Bache 1856; Satake et al 2020). Kusumoto et al (2020) estimated the origin time of the 1854 CE Ansei–Tokai tsunami to be 00:30 on 23 December by comparing the tsunami waveforms observed at stations on the west coast of North America with calculated waveforms. We calculated the time difference between the 1854 CE Ansei–Tokai and Ansei–Nankai earthquakes by using our estimated origin time for the Ansei–Nankai tsunami and the origin time estimated by Kusumoto et al (2020) for the Ansei–Tokai tsunami and compared the result with historical descriptions of the two earthquakes and tsunamis
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