1771 Great Yaeyama Tsunami


The 1771 Great Yaeyama Tsunami was caused by the Yaeyama Great Earthquake at about 8 A.M. on April 24, 1771, south-southeast of Ishigaki Island, part of the former Ryūkyū Kingdom and now forming part of present-day Okinawa, Japan. According to records, 8,439 persons were killed on Ishigaki Island and 2,548 on Miyako Island.

Earthquake analysis

According to the Japanese government publication Rika-Nenpyō or Chronological Scientific Tables, the epicenter was 40 km south-southeast of Ishigaki Island with a magnitude of 7.4. According to the Mamoru Nakamura Laboratory, University of the Ryukyus, the earthquake was due to the activity of the fault east of Ishigaki and it is estimated that the magnitude was 7.5. Further simulation led to the activity of faults in the Ryukyu oceanic trench and the magnitude was 8.0. Also, there is a hypothesis that claims the magnitude was 8.5. The depth was. This trench lies between the Philippine Sea Plate and the Eurasian Plate. The disparity in the amount of recorded shaking and the size of the tsunami has led to the interpretation of this event as a tsunami earthquake.

Damage

Earthquake

It is considered that the earthquake registered an intensity of 4 in the Yaeyama Islands, and the damage by the earthquake was not as serious as the tsunami.

Tsunami

The dead and missing amounted to 12,000 people, and more than 2,000 houses were destroyed on Ishigaki and Miyakojima. It has been estimated Agriculture was severely damaged because of sea water invasion and the population decreased to about one third of what it was before the earthquake. On Ishigaki island, the run-up was first estimated around 40 to 80 meters high from historical documents. However, taking into account the rough precision of the measuring instruments at this time and considering geomorphological parameters, the maximal run-up has been re-estimated to ~30m.
Following the tsunami, the damage it caused was such that it led to a famine that lasted for 80 years.

Boulders

There are many huge boulders on the coasts of the Yaeyema and Miyako islands that are believed to have been deposited by tsunamis. There was a legend that an islet disappeared, but this has never been verified. This set of rocks are called the Ishigaki East Coast Tsunami Rocks. Among them, Yasura-ufukane, Amatariya–Suuari and Taka-koru sishi have been dragged by the 1771 Meiwa tsunami.

Footnotes