The December 23, 1854 Tokai earthquake (M∼8), followed by Nankai earthquake by ∼30 hours, occurred along the Nankai Trough and caused widespread damage due to ground shaking and tsunamis. At the time of the earthquake, a Russian frigate, Diana, was anchored in Shimoda Bay and experienced the earthquake and tsunami. The frigate was damaged and eventually wrecked. Four Japanese historical documents reported that strong shaking occurred in Shimoda after 9 a.m. and before 10 a.m. in local time, reflecting the two-hour unit of the timekeeping system used in Japan at that time. The information from the frigate, reported in contemporary newspapers in English, all mentioned that an earthquake first shook the ship severely, and that they then observed the tsunami entering the bay around 10:00 in local time. The time of the earthquake, however, was reported as 9:15 in Shanghai and American newspapers (The North-China Herald, The Daily Union), and as 9:45 in the British press (The Times, The Illustrated London News). We found that the original logbook of the Diana, archived at the Russian State Archives of the Navy, Saint Petersburg, recorded that the earthquake occurred “at 3/4 of the 10th hour,” indicating 9:45. The former newspapers seemed to have been based on the information provided by the men of the American frigate Powhatan, which also arrived at Shimoda after the earthquake, but the time might have been erroneously translated into “three-quarters to ten.” More precisely, the above time expression should be interpreted as being between 9:45 and 9:59 in local time (138±b°57’E), corresponding to between 0:29 and 0:43 Greenwich Mean Time.

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