Geology And Geophysics

Mount Sakurajima and Volcanic Eruption



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"Mount Sakurajima and Volcanic Eruption"
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Mount Sakurajima

Since it resumed activity in 1955, the volcano has been erupting almost constantly ever since. Thousands of small explosions occur each year, throwing ash to heights of up to a few kilometers above the mountain. The current forms of explosions consist of spouting rocks, ash and gas from a crater on the top of the Minamidake peak (1040m). It generates 10 to 30 million tons of ash each year. Ash from the volcano falls on the city up to 143 days a year and 100mm in annual depth.

Monitoring and predicting large eruptions of the volcano are particularly important because of its location in a densely populated area, with Kagoshima's 600 000 people just a few kilometers from the volcano. The city conducts regular evacuation drills, and a number of shelters have been built where people can take refuge from falling volcanic debris.

Effects

There is serious agronomic damage caused by the ash, particularly when ash falls at critical points in the growing season. Pumice ash can also kill fish in fish farms. The discharge of volcanic gases such as sulphur dioxide is a significant issue as the gas can travel hundreds of kilometers and not only produce acid rain, but also can cause death in extreme situations.

The impact on human health is not as significant as it is thought out to be. People still go to work when ash falls, using umbrellas to shield them from the ash. Although no medical proof yet exists, it has been reported that susceptible people experience respiratory illnesses when exposed to the ash, and that neonatal death rates may be affected by volcanic sulphur dioxide concentrations in the city.

Measures in place

An array of mitigation measures is in place in the city, including a 220-million-Yen-per-year operation to keep the city streets clear of ash. Residents are expected to collect ash from their properties in yellow supermarket type bags and put it out at one of 5504 collection sites twice a week. Hand sweepers are used to sweep pavements and around houses. Public workers in this occupation wear eye protection, basic facemasks and helmets (to prevent ash from getting into the worker's hair).

The most pertinent problem for the city is that ash falls on the roads and obscures the road markings, causing traffic problems. It is also slippery when wet. The city has a target to sweep up the ash within three days. Ash is deposited in harbour reclamation or compacted ash landfill sites.

School children have to wear yellow crash helmets to school to protect their heads from the debris. There are many eruption shelters for people to hide in too. Additionally, each year, Sakurajima is fully evacuated so that residents are fully prepared for the day when volcanic activity may require them to leave quickly. People live in ash-proof houses which have no spouting but have special tiles to withstand the acidic nature of ash. The island is also covered by debris flow channeling devices which are immense and extremely wide in places.

Besides, sophisticated seismic, ground deformation and strain monitoring systems on the volcano provide effective warning of eruptions. Various organizations had comprehensive manuals covering all aspects of their operations. Both the Prefecture and the City Government had manuals covering their responses to volcanic and earthquake events. There are many elaborate monitoring systems in place to record daily events and pick up abnormal activity such as a swarm of "B" type earthquakes, which will give early warning to a major event.

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