Mt. Unzen, Japan, 1792 Volcanic Eruption

 Mt. Unzen, Japan, 1792 Volcanic Eruption

      In 1792, Mt. Unzen, a Composite volcano near Shimabara, Nagasaki, erupted on the island of Kyushu. This series of eruptions led to a destructive landslide and tsunami, with the death toll estimated at 15,000 people, making it the most deadly volcanic eruption in Japan's eruption (Britannica, 2023). Mount Unzen is part of the Nankai Volcanic Arc and has a summit of 1,483 meters (Smithsonian, 2024).

    The following information comes from the Unzen Restoration Office. The volcanic event is estimated to have begun in November of 1791 and comprised of 4 stages, preceding earthquake swarms reaching a seismic activity of about 5-6 magnitude.  The second stage comprised four eruptions starting in February 1792 and ending in the following March: the Fugenshi-Mae, Anasako-Tani, Hachinokubo, and Furuyake-Kashira. Followed by a lava flow at Shin'yake coming from the top of Anasako-Tani, the lava flow had an average velocity of 30-50m/day with a lava volume of about 20 million m³, which reached about 0.5km to houses. Various other phenomena accompanied these eruptions, such as a carbonated spring forming at Kureisibaru in Mie-village, new smoke from Oshiga-Tani, and new cracks forming from Hachino-Kubo to Furuyake-Kashira.  The third stage was followed by an earthquake of magnitude 5-6 on April 21, which deemed the Shimabara-Sangatsusaku earthquake as two or three large fissures that formed in the town of Shimabara castle. A landslide in the Kusunoki-Daira occurred between Mayu-Yama and Ariake Baye, which resulted in the groundwater level rising abnormally at Imamura.  The fourth stage was followed by the Shimabara-Shiatsusaku earthquake and the collapse of the Mayu-Yama sector. The earthquake was estimated to be a magnitude 6.4 and occurred on May 21. The sector (Figure 1) collapsed, and huge amounts of debris and rocks rushed down into Ariake Bay, generating a large tsunami that killed an estimated 15,000 people. (Unzen Restoration Office, 2002). 

    

Figure 1: Mayuyama landslide before and after (Higaki, 2023)

 

Image 2: Mt. Mauyama Landslide Scarp (https://www.usgs.gov)

Video 1: Simulates the tsunami that was created by the landslide caused by Mayuyama collapsing (Youtube)

Citations:

Britannica, T. Editors of Encyclopedia (2023, September 1). Mount Unzen eruption of 1792.                             Encyclopedia Britannica. https://www.britannica.com/event/Mount-Unzen-eruption-of-1792

Global Volcanism Program, 2024.Unzendake (282100) in [Database] Volcanoes of the World (v. 5.2.4;             21 Oct 2024). Distributed by Smithsonian Institution, compiled by Venzke, E.                                         https://doi.org/10.5479/si.GVP.VOTW5-2024.5.2

Gomez, C., & Wassmer, P. (2016, January 1). Evolution of the unzen volcano and the Shimabara Peninsula (Japan) ... Géomorphologie : relief, processus, environnement. https://journals.openedition.org/geomorphologie/11048?lang=en

Higaki, D. et al. (2023). Progress in Landslide Research and Technology, Volume 1 Issue 2, 2022.                 Progress in Landslide Research and Technology. Springer, Cham. https://doi.org/10.1007/978-3-            031-18471-0_22 

Unzen Restoration Office, Ministry of Land, Infrastructure and Transport of Japan. (2002). The Worst Disaster in Japan. https://www.qsr.mlit.go.jp/unzen/wlib/pdf/010101b.pdf