1792 Unzen earthquake and tsunami

Date: May 21, 1792

Place: Unzen, Japan

Result: Approximately 15,000 dead

Overview

The deadly May 21, 1792, earthquake and tsunami at Unzen, Japan, was the culmination of a series of geological events. For six months, the area had experienced earthquakes, with volcanic activity following within four months. Volcanic eruptions and resulting lava flows continued for about three months, building up a dome of hardened lava that was eventually shaken loose by a large earthquake on May 21. It was the release of this dome that caused the tsunami responsible for most of the deaths in one of the worst eruption-related events in Japanese history.

Mount Unzen.Unzen is an overlapping group of stratovolcanoes located on the Shimabara Peninsula in the southwestern part of Japan. Stratovolcanoes are composite, cone-shaped volcanoes built up of layers of lava, volcanic ash, and other material over the course of many years. These volcanoes make up about 60 percent of the volcanoes in the world and often cause the most devastation because the types of lava that form them, andesite and dacite, are thick and allow gas to build up. The lava forms a plug in the volcano known as a volcanic dome that can blow off from the pressure or break away during volcanic activity or earthquakes. This can cause landslides and lead to tsunamis, as happened in 1792.

The events leading to the fatal tsunami began in November 1791 with a series of smaller earthquakes in and around Unzen. One of these caused a rock fall that killed two people in December 1791. On February 10, 1792, a fumarole—a steam vent emitting smoke and vapor—opened in the Jigoku-ato crater at Unzen. Occasional plumes of sand, gravel, and other small matter were also ejected from the fumarole.

On February 28, a second fumarole ejecting similar materials opened up at Biwa-no-Kubi, and lava began flowing from the Fungendake portion of the Unzen volcanoes on March 1. This flow continued for two months. Within a few weeks, lava from eruptions at Mount Mayuyama and two other points in the Unzen volcanic range merged with the original lava flow, eventually covering an area about 360 meters wide and 2.7 kilometers long, or about 393 yards wide and 1.7 miles in length. This would harden and form a volcanic dome.

May 21, 1792.Throughout this time of increased volcanic activity in the Unzen area, there was also increased earthquake activity. On May 21, 1792, a significant 6.4 magnitude earthquake rocked the area, causing the collapse of the volcanic dome from Mount Mayuyama. It broke away, setting off landslides and tearing down trees, buildings, and other material, all of which landed abruptly in the Ariake Sea. This displaced large amounts of water that sent huge tsunami waves 187 feet (57 meters) high crashing into the towns of Shimabara, Higo, and Amakusa, as far as twenty-three kilometers, or fourteen miles away. While the earthquakes, volcanoes and resulting lava flows, and the landslides caused loss of life and property damage, the tsunami was responsible for most of the damage and the majority of the estimated fifteen thousand deaths that occurred that day.

The 1792 event at Unzen was neither the first nor the last volcanic explosion in the area. Researchers have determined that Unzen has been a site of volcanic activity for at least four thousand years, and it erupted at least once in recorded history prior to 1792 when about thirty people lost their lives in lava flows and resulting flooding in 1663.

The most significant events after 1792 occurred from 1990 through 1996, when increased volcanic activity led to the growth of a new lava dome as well as pyroclastic flows, or superheated masses of liquefied rock and vapor that move rapidly downhill from a volcano. However, few deaths have resulted from the more recent events because scientists have become better at interpreting the signs of volcanic activity and predicting when serious events will occur. As a result, it is often possible to warn people in advance of imminent eruptions and pyroclastic flows and move people to safety. Tsunami warning systems are also in place, especially in island areas such as Japan. These systems improve the chances of people reaching safety before disaster strikes.

One aspect of the 1792 disaster that remains difficult for twenty-first-century scientists to predict is the collapse of a volcanic dome. The collapse often happens without warning and can trigger avalanches or tsunamis with very little chance for anyone to react.

Impact

The disaster at Unzen in 1792 was important in showing the significant danger of damage and loss of life from the collapse of a volcanic dome. Since there is so little warning of a collapse and few ways to predict what the effects will be, dome collapse continues to be one of the greatest dangers of any volcanic eruption.

This caused significant concern for geologists and other scientists when Unzen became active again during the 1990s. There were a number of similarities between the 1792 event and eruptions in 1991, including repeated earthquake tremors that shook the area between eruptions. This led to fears that Unzen could once again unleash catastrophic damage in the heavily populated area where more than 150,000 people live. Nearby residents were cautioned to take steps to protect themselves from ash and falling debris, but almost two hundred years after one of the worst volcanic disasters in Japan’s history, researchers were still unable to predict with any certainty if there would be significant damage. The June 3, 1991, death of forty-three researchers, including famed volcanologists Katia and Maurice Krafft, during an eruption at Unzen indicates just how difficult predicting volcanic activity can be.

Not all of the effects of the eruption at Unzen were negative, however. The disruption of grounds and soil that accompany volcanic eruptions often create beautiful natural environments. These can include hot springs and very fertile soil that gives rise to diverse and attractive plant life, and the areas affected by the Unzen eruptions highlight many of these natural improvements. Since 1934, the area around Mount Unzen has been a designated national park, the first such park in Japan. The Unzen Volcanic Area Global Geopark has become an area where people learn how to live in the shadow of an active volcano.

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