In June 2000, an eruption was first observed on Miyakejima, a volcanic island in the Pacific Ocean administratively belonging to Tokyo Metropolitan Prefecture, and located 180 km south of central Tokyo. The 2000 Miyakejima eruption and resultant events had the following characteristics. Volcanic ashes covered the whole island, and disastrous mudflows occurred at various locations, especially after rain. Since late August on the same year, large quantities of volcanic gases containing hazardous sulfur dioxide have continued to be released. As a result, on September 2, 2000, all of the inhabitants were ordered to evacuate the island. Mudflows have decreased considerably, but the problem of sulfur dioxide has not yet been completely solved.
This report aims to clarify the process of disaster prevention by the related authorities for the extended evacuation and restoration of Miyakejima. Actions were taken specifically according to changes in the quantities and the concentrations of volcanic gases containing sulfur dioxide. Among these, some safety measures were taken against the volcanic gases, including those that made overnight stays on the island possible. Considering how safety measures have varied over time during the past four or five years, it is possible to establish the following four periods of disaster avoidance.
1) Period of Physical Avoidance During this period it was impossible to precisely grasp the specific characteristics of the volcanic gases, and the quantity of volcanic gases was too great to be measured. In consequence, the on-site disaster measures headquarters had to be moved offshore, and later to Kouzushima 30 km west of Miyakejima.
2) Period of Chemical Avoidance-Chemical desulphurization equipment was installed in most of the public and semi-public buildings, which were designated clean houses, to receive residents in an emergency.
3) Period of Forecasting Avoidance-Index of sulfur dioxide concentration was defined separately according to short-or long-term influence on human health. Focusing attention on this, judgments on safe areas were based on whether the sulfur dioxide concentration had decreased to the benchmark level, and some areas considered to be under a long-term influence were established. In areas under a short-time influence, a few residents were permitted to return (overnight stay permitted) provided at least one room was provided with desulphurization equipment as in the case of minshuku (pensions).
4) Period of Institutional Avoidance-With the increasing use of observation systems and information transmission systems, and the establishment of a new evacuation act (provision of clean houses, etc), it became possible for returning inhabitants to stay overnight in their houses without desulphurization equipment, with the exception of two control areas where the sulfur dioxide concentration remained high.