[S-SS14] Active faults and paleoseismology

[SSS14-P16] Proposal for a New Method for Determining Dates of Earthquakes Occurring Late at Midnight or Before Dawn

*Yuma Kamei¹, Kazuki Koketsu², Satoko Oki¹ (1.Keio University, 2.SFC Research Institute, Keio University)

Keywords:Historical earthquake, Historical documents related to earthquakes, Midnight, Before dawn

In pre-modern Japan, the day was divided into 12 divisions, following the Chinese system, and the time was indicated by assigning the twelve signs of the Chinese zodiac to the divisions. Although the irregular time system was mostly used, one division was approximately two hours long. The time of Rat corresponds two hours around midnight in the modern time system. In other words, the time of Rat includes the period of 23:00 to 24:00 on a day and the period of 0:00 to 1:00 on its following day. In addition, in pre-modern Japan, the sense of the time when the date changed was different from that of the present day. In order to use historical earthquakes for scientific research, it is preferable that modern date changes are applied. Therefore, Kamei et al. (2024) reexamined the occurrence dates of historical earthquakes listed in the “Comprehensive List of Damaging Earthquake” (Usami et al., 2013) (hereafter “List”) with the time of Rat, Ox, or Tiger. Their simple method is to assume that an earthquake occurred on the written day if there is a word indicating morning in the description of the earthquake, and to assume that an earthquake occurred on the next day if there is a word indicating night. Their more reliable method is to consider an earthquake to have occurred on the written day if it is mentioned before other events of the day, and to consider an earthquake to have occurred on the next day if it is mentioned after other events of the day. As a result of their reexamination, Of the 57 earthquakes (one more than in their paper), five were judged to have different dates from those in List. In this study, we investigated a method for determining the dates of earthquakes for which there are few historical documents. According to Hashimoto (1978), in ancient and medieval times, the date was generally changed at the border between the times of Ox and Tiger. In the private sector during the Edo period, the date was changed at the time of Six in the morning, which corresponded to sunrise. Furthermore, these criteria were not strict, and it is believed that the time of date change varied from region to region and from person to person. To verify this, we analyzed the relationship between the time and date written in each historical document. In addition to the earthquakes whose dates have been identified by Kamei et al. (2024), we included earthquakes at the time of Rabbit in List. First, all historical documents for all the earthquakes were analyzed. As a result, 13.0% of the documents with the time of Rat, 20.1% of the documents with the time of Ox, 76.9% of the documents with the time of Tiger, and 96.6% of the documents with the time of Rabbit state next days. This means that the date was generally changed between the times of Ox and Tiger. Next, in order to verify Hashimoto's (1978) observation of date change in the private sector during the Edo period, an analysis by social status was conducted. A single regression analysis was then conducted on the above ratios and years. Since no correlation was found, it was concluded that there were no significant differences depending on the era. The regional analysis also showed no significant regional differences. These indicate that the date was generally changed between the times of Ox and Tiger, regardless of social status, era, or region. Therefore, it can be probable that a date with the time of Rat or Ox corresponds to the day before and a date with the time of Tiger or later corresponds to the day. Based on this criterion, we reexamined the 12 earthquakes whose dates could not be determined by Kamei et al. (2024), finding that the dates in List should be changed for five of them.