부산대학교 도서관

Kuchinoerabujima is a volcanic island, which is located 40 km south of Kyushu, Japan. In recorded history, phreatic or magma-phreatic explosion occurred repeatedly with the dormant period of about 30 years. The recent eruption occurred on August 3, 2014 at the summit crater of Shindake, which is the youngest edifice of the volcano. Various phenomena, such as the increases of volcanic earthquake, ground deformation, magnetic anomaly and discharge rate of volcanic gas have occurred from 1999. These indicate the change of underground condition before the eruption. In order to evaluate the volcanic activity, we conducted continuous GPS observation from 2004 at two stations located at 230 m northwest (SDW) and 480 m south (FDK) of the summit crater of Shindake. The data sampled every 30 seconds are sent to our institute by telemetry system using mobile phone network. For the reference station of GSI (GEONET of Geospatial Information Authority of Japan) which is located at the northwestern foot of the volcano, stations SDW and FDK show the continuous trend of westward movement at the rate of 0.4 cm/year. Intermittent events with sudden increase in the movement overlap these continuous deformations. We have observed the remarkable ground movements (2–4 cm in total) at 4 times; June in 2005, September in 2006, September in 2008 and October in 2010. These events lasted for more than a few months at respective events. They are characterized by the cumulative northwestward and upward movements of station SDW, and the slight southward movement of station FDK. The ground deformations at the summit area are explained by two sources; the intermittent ground deformation indicates the inflation of the source just under the summit crater of Shindake, and the continuous westward movement shows the deeper source which is located south-east of Shindake. The number of the volcanic earthquake increased at the same time of first and second events; however the increase of the seismic activity was sluggish at third and fourth events. The interval between the events had been prolonged gradually and another event of minute movements appeared on the continuous deformation after 2009. The movement due to the shallow source at the crater changed from dilatation to contraction, and another type of the deformations appear after 2013. These phenomena indicate the change of the deformation source condition just before the eruption.