Topographical Survey of the Mihara Crater, Volcano Izu-Oshima
1972 Volume 23 Issue 4 Pages 411-428
Details
1972 Volume 23 Issue 4 Pages 411-428
It is suggested that, if the crater bottom shows a tendency to upheave owing to underground stress, the eruption at the crater is approaching.
At the Mihara Crater of Izu-Oshima Volcano, the crater bottom was usually deep during the dormant periods, and rose up to shallow levels before eruptions. Furthermore, great earthquakes in 1923(the Kanto Great Earthquake. Magnitude=7.9) and 1953 (the BOSOoki Earthquake. Magnitude =7.5), both of which had epicenters located near Izu-Oshima Island, occurred when the Mihara Crater bottom was shallow.
Therefore, survey of the crater bottom should be available for prediction of volcanic eruptions or great earthquakes.
At the present, the Mihara Crater bott o m is too deep to be seen from the crater rim. So topographical survey on an aeroplane was carried out on March 16 in 1972. The accuracy of location calculated for the base points of the present survey was less than ±30 cm.
As the result of the present survey, a new topographical m ap was made, and the following facts were revealed:
1). Diameter of the crater mouth was 360 m across from north to south, and 400 m from east to west.
2). Diameter of the crater bottom was about 150 m.
3). Depth of the crater bottom was about 240 m from the crater rim. The bottom is located 448 m above the sea level.
The crater bottom has changed in height since the 1950-51 Great Eruption as in the following:
Apr.1951.........680m above sea level
Nov.1951.........640m above sea level
Feb.1955......620m above sea level
Jan.1958........660m above sea level
Mar.1968...... (about 400m)above sea level
Mar.......1972-448m
4). The red-hot lava stayed at the crater bottom. The size of the lava pool was 6 m in length,3 m in width.
5). The volume of the crater (from rim to bottom) was estimated at 8.4X106 m3.
6). A sink hole,30 m in diameter and 15 m in depth, was found newly at the southwest cinder cone of the crater rim. This hole is the trace of the active pit in the 1950 Great Eruption.
