Geology And Geophysics

Volcano Eruption Mount Unzen

Stacy Wong's image for:
"Volcano Eruption Mount Unzen"
Image by: 

Mount Unzen

The Event

After an eruption in 1792, the volcano remained dormant till November 1989, when an earthquake swarm began about 20km down and 10km west of Fugen-dake. Over the next year, earthquakes continued, with their foci gradually moving towards the summit. Tremor was first noted 4 months before the eruptions. The first eruptions began in November 1990, and after an inflation of the summit area, fresh lava emerged on May 20th 1991. On June 3rd 1991, the volcano erupted violently, possibly as a result of depressurisation of the magma column after a landslide in the crater.


From 1991, pyroclastic flows were produced as a result of landslides and the collapses of lava blocks from margins of the dome. It intensified and in May a lava dome of 50m across was formed in its crater. This lava dome grew and multiplied till 13 are now recognized, measuring 1250m east-west and 600m north-south. The pyroclastic flow from the actual eruption reached 4.5km from the crater at a speed of 100kmph and killed 43 people, including well-known volcanologists Katia and Maurice Krafft and Harry Glicken. The heavy rain (2.8 inches per hour) on June 30 triggered a mudflow of 380 000m in volume, engulfing 148 homes as it poured into the Ariake Sea. This situation worsened as the rainy season was already in with typhoons hitting the area.

The total damages caused by the Unzen eruption from 1990 to March 1995 are as follows:
Number of pyroclastic flows: At least 10 000 small ones
Number of people dead / injured: 44 dead (and missing), 10 injured
Buildings damaged: 2593 (including 1,270 residential)
Agricultural and fishery facilities: 17.4 billion yen
Public structures: 29.5 billion yen
Agricultural and dairy products: 20.0 billion yen
Commercial and service facilities (esp. tourism): 133.7 billion yen
Others: 4.1 billion yen
Total: 204.7 billion yen

The disaster devastated the area's economy. Apart from the damages listed above, the transportation system was severely damaged by pyroclastic flows and mudflows. The restrictions issued for protection affected traffic conditions in the area, adding more income losses.


The area has suffered from the disasters for more than five years, which is a relatively long time for a person to endure a natural disaster. People who lived in the area had no choice but to evacuate. All the activities were forced to halt without any guarantee for legal compensations. After the initial volcanic threat in May 1991, 12 000 local residents were evacuated from their homes. This number was reduced to 3 000 by the end of 1993.

The fundamental policy for restoration should aim to help recover pre-eruption state and include projects to provide a new comfortable living space for those who lost everything. Hence, overall measures to promote every field should be planned during disaster protection, building new towns and activating industries.

Disaster Protection

Due to the 170 million cubic meter of disposal from the eruption resting on the flank of the volcano and the fact that there could be mudflow damages for a long period of time, preventive structures were built in three rivers (Mizunashi, Nakao and Yue). These include 62 barriers to prevent landslides, 33 slit dams, 359 erosion control dams and the widening of the Mizunashi River. In spite of these efforts, there were difficulties to proceed constructions as the project was situated in restricted areas. Nevertheless, the people were determined to finish the structures as soon as possible.

Reconstructing Transportation System

The national highway (Route 57) was partly restored in 1994. The main local road that runs through the north of Shimabara Peninsula was restored completely 28 months after the disaster. The Shimabara Railway has also been elevated.

Housing Restoration

Due to some areas being reserved to establish preventive structures, a plan needs to be discussed in order to build totally new towns. The 367 lots available were prepared in three subdivisions. Sale prices were set relatively reasonable to ease the financial burden of buyers. To build subdivisions, public enterprises and The Fund for Unzen Disaster Restoration gave refugees aid to make home loans. There is also a plan to reclaim a part of Ariake Sea to build new towns. Elevation of the ground level (by 6m) was planned for the Annaka region in Shimabara City to prevent future landslides. Shelters and temporary homes, which numbered 1 445 during the peak, were gradually being removed, as public housing complex for permanent homes were prepared.

Restoration Plan for Agriculture and Fishery

In 1993, after its 307 hectare farmland was destroyed, Fukaecho's (one of the agricultural centers in Nagasaki Prefecture) gross product rate dropped to 70 million US dollars, 70% of that in 1992. By December 31st 1994, 244 out of 667 farmers went back in farming in the area. National and local governments were working on the recovery of damaged farmland and farming facilities. Special aids were prepared for farmers who wanted to return to farming in the area. With regards to fishery, management fishery was promoted.

Restoration Plan for Commercial Business and Tourism

Business transactions in Shimabara City became very inactive in 1994. The number of shops staying in business dropped by 21.6% and sales were 4.3% less than in 1988. In the downtown area, the number of companies closing down was increasing, leaving a large vacant space for business. Tourists were still comparatively low in numbers. To increase sales, various financial aids were given to local businessmen, such as interest payment, expenses for attractions and investment for redecorating shopping malls.

More about this author: Stacy Wong

From Around the Web