Reexamining the Dates of Historical Earthquakes at Midnight or before Dawn

In pre-modern Japan, time was expressed using the 12-division system, which is based on that in China. This was generally an irregular time system, but each division can be approximately 2 hours. The divisions were called as times of animals and others. Among them, Time of Rat refers to midnight and 1 hour before and after that. This implies that Time of Rat on a certain day includes the period from around 23:00 to 24:00 on that day and the period from around 0:00 to 1:00 on the next day. In addition, in pre-modern Japan, the sense of date change was different from that in modern Japan. According to the book of Hashimoto, in the Heian period (794-1185), the date was changed between Times of Ox and Tiger, which is around 3:00. In the Edo period (1603-1868), there was a sense that the date changed at Time of Rabbit, which corresponded to the sunrise. When using historical earthquakes for scientific research, we need their dates in the modern sense. We thus reexamined the dates of historical earthquakes that were listed in the 2013 book of Usami et al. (Book) with Times of Rat, Ox, and Tiger. A simple method for reexamination is to consider an earthquake that occurred on that day if historical documents contain words related to the morning, and consider it to occur on the following day if they contain words related to the night. A more reliable method is to consider an earthquake on that day if its descriptions in historical documents come before any descriptions of human events or actions, and conversely consider it in the following day if they come afterwards. The results from these methods were compared with the dates in the Book, and the Database (DB) of Historical Documents on Japanese Earthquakes and Eruptions in the Ancient and Medieval Ages (up to February 1607), or the DB of materials for the history of Japanese earthquakes (since March 1607). Of the 56 earthquakes in total, there were four for which it was determined that the dates in the Book should have been changed, 22 for which it was determined that the dates in the DB should have been changed, and for the remaining 20, it was confirmed that the dates in the Book and DB were correct.