5:15 PM - 6:45 PM
[MIS17-P07] Diary records about aftershocks of the 1923 Great Kanto earthquake
Keywords:The 1923 Great Kanto Earthquake, Diary records, Aftershocks
We wondered how many earthquakes that actually occurred were recorded in the historical records. When analyzing past seismic activity, it is necessary to consider the completeness of the historical record, and to know the extent to which earthquakes that actually occurred are preserved in the historical record. One clue to this is a comparison of the number of earthquakes in the historical record and the observation record. Satake and Ishibe (2020) is an example of a comparison of the number of earthquakes in different era. Satake and Ishibe (2020) obtained the number of earthquakes per year in Edo using two diaries kept in the Edo domain. The number of earthquakes per year was about 11, which is about the same as the number of earthquakes of intensity 2 or greater per year in present-day Tokyo. A case study comparing historical and observational records of the same period is Shou et al. (2017), although the records are as weather records. In the case of weather records, statistical comparisons can be made with diaries and observation records of the same era due to the large number of records. However, earthquakes have fewer records than weather records, hence statistical comparisons are difficult. We hypothesized that aftershocks of major earthquakes would be more numerous, and thus more easily compared with other observational data. We focused that, in the case of the aftershocks of the 1923 Taisho Kanto Earthquake, seismic observation had already started and both diary and observation records were available. Therefore, in this study, we compare the diary records of the aftershocks of the 1923 Taisho Kanto Earthquake with the contemporaneous observation records.
The diaries used in this study include "Kawai Kiyokata Nikki" (Fujinomiya City, Shizuoka Prefecture), "Jifukuji Hinamiki" (Wako City, Saitama Prefecture), "Irako Keisai Nikki" (Hino City, Tokyo), and "Aizawa Nikki" (Sagamihara City, Kanagawa Prefecture), and so on. For "Kawai Kiyokata Nikki", Takemura (1999) compares aftershock records in the diary with the list of epicenters in Hamada (1999). Observation records include the JMA Earthquake Catalog, "Earthquake Prevention Investigation Committee Report No. 100", and "Great Kanto Earthquake Investigation Report: Earthquake Section". The "Diary of the Great Kanto Earthquake in Ubako" is the equivalent data to the observation record. This diary records earthquakes felt in Hakone-cho, Ashigarashita-gun, Kanagawa Prefecture, from September 1 to October 14 and divides them into strong, weak, and slight earthquakes. Large earthquakes tend to be recorded in multiple diaries. For example, on November 23, "Jifukuji Hinamiki" recorded "Earthquake at 11:00 a.m." and "Irako Keisai Nikki" recorded "Great earthquake at 10:00 a.m., the largest aftershock in the aftershocks." This is thought to correspond to the earthquake that occurred at 11:33, on the same day in eastern Kanagawa Prefecture, M6.3, maximum intensity 4 (cited from the seismic intensity database of the JMA). Moreover, many diaries show descriptions of strong tremors felt on January 15, 1924. This must also correspond to the Tanzawa Earthquake(western Kanagawa Prefecture, M7.3, maximum intensity 6), the largest aftershock. The number of aftershock records decreases with the passage of days after the mainshock. For example, in "Aizawa Nikki", there are descriptions of earthquakes felt until September 18. However, from September 19 to January 14, 1924, there are no earthquake records in the diary. This is thought to reflect the decrease in the number of aftershocks with the passage of time after the mainshock. However, this is not merely a reflection of a decrease in the actual number of aftershocks, but may also indicate a change in the motivation for recording earthquake experiences in diaries. As described above, by focusing on large aftershocks and the decrease in the number of aftershocks, we can compare and analyze the diary records and the observation records.
The diaries used in this study include "Kawai Kiyokata Nikki" (Fujinomiya City, Shizuoka Prefecture), "Jifukuji Hinamiki" (Wako City, Saitama Prefecture), "Irako Keisai Nikki" (Hino City, Tokyo), and "Aizawa Nikki" (Sagamihara City, Kanagawa Prefecture), and so on. For "Kawai Kiyokata Nikki", Takemura (1999) compares aftershock records in the diary with the list of epicenters in Hamada (1999). Observation records include the JMA Earthquake Catalog, "Earthquake Prevention Investigation Committee Report No. 100", and "Great Kanto Earthquake Investigation Report: Earthquake Section". The "Diary of the Great Kanto Earthquake in Ubako" is the equivalent data to the observation record. This diary records earthquakes felt in Hakone-cho, Ashigarashita-gun, Kanagawa Prefecture, from September 1 to October 14 and divides them into strong, weak, and slight earthquakes. Large earthquakes tend to be recorded in multiple diaries. For example, on November 23, "Jifukuji Hinamiki" recorded "Earthquake at 11:00 a.m." and "Irako Keisai Nikki" recorded "Great earthquake at 10:00 a.m., the largest aftershock in the aftershocks." This is thought to correspond to the earthquake that occurred at 11:33, on the same day in eastern Kanagawa Prefecture, M6.3, maximum intensity 4 (cited from the seismic intensity database of the JMA). Moreover, many diaries show descriptions of strong tremors felt on January 15, 1924. This must also correspond to the Tanzawa Earthquake(western Kanagawa Prefecture, M7.3, maximum intensity 6), the largest aftershock. The number of aftershock records decreases with the passage of days after the mainshock. For example, in "Aizawa Nikki", there are descriptions of earthquakes felt until September 18. However, from September 19 to January 14, 1924, there are no earthquake records in the diary. This is thought to reflect the decrease in the number of aftershocks with the passage of time after the mainshock. However, this is not merely a reflection of a decrease in the actual number of aftershocks, but may also indicate a change in the motivation for recording earthquake experiences in diaries. As described above, by focusing on large aftershocks and the decrease in the number of aftershocks, we can compare and analyze the diary records and the observation records.