Quaternary Volcanoes > Active Volcanoes > Sakurajima

Monitoring and Observation of Volcanic Activity / Forecasting of Future Activity

Monitoring and Observation of Volcanic Activity

   The Taisho eruption marked one of the first times in Japan that volcanic activity monitoring was performed using modern observation techniques. In addition to routine monitoring and observation of volcanic activity by the Kagoshima Local Meteorological Observatory, in 1960 AD, the Sakurajima Volcano Observatory of the Kyoto University Disaster Prevention Research Institute, (currently Sakurajima Volcano Research Center) was established. Sakurajima is one of the most monitored and observed volcanoes in Japan. Seismograph stations have been installed at 18 locations on the island by Kyoto University, Japan Meteorological Agency, Osumi River National Highway Office and Kagoshima University and there are also a large number of observation stations off the island.

   Ground deformation is under continuous observation by inclinometers and extensometers installed in tunnels as well as GPS. Kyoto University along with Osumi River National Highway Office has created a 200-m observation tunnel, and Kyoto University, Geospatial Information Authority of Japan and the Japan Meteorological Agency have installed GPS at 24 locations. Joint observation by universities and other research institutes for intensive comprehensive observations has been conducted 10 times, and crustal movement observations using GPS and level surveys as well as gravity and geomagnetic observations have been performed repeatedly.

   Among the volcanic earthquakes associated with Sakurajima Volcano, the hypocenters of A-type earthquakes caused by shear faulting similar to normal earthquakes are distributed from off the southwest coast of Sakurajima to the Wakamiko Caldera area, off the northeast coast of Sakurajima. Earthquakes have occurred multiple times directly beneath the Minamidake crater ( Fig. 9). B-type earthquakes and explosion earthquakes are produced from magma vesiculation just below the crater at depths in the 1–3 km range. In the 1970s, volcanic earthquake occurrences transitioned from A-type earthquakes to frequent occurrences of B-type earthquakes at shallow locations around the crater and summit eruptive activity became more pronounced. However, in the 1980s no A-type earthquakes occurred and B-type earthquake swarms were linked to the start of explosive eruptive activity.

   Before explosions of Minamidake, rising ground inclination toward crater and extension strain of the ground were observed for 10 minutes to several hours ( Fig. 10). This activity was a result of magma intruding into a nearby 4 km deep section of a vent, which caused the volume to expand. If an explosion occurs, the ground settles and contracts as a result of the release of magma material. Since 2009, similar ground deformation has been observed in the Showa crater where the number of explosions has been increasing. However, many of the explosions are caused by an expansion in volume at an about 1 km shallow section directly below the crater.

   The ground in the northern part of Kagoshima Bay after the Taisho eruption settled significantly; it had been found that the eruption activity of Sakurajima was related to the crustal deformation around Aira Caldera.

    Figure 11 shows the vertical displacement for Kitagawa Osaki no Hana, Kagoshima City (BM. 2474) versus the inside of Kagoshima City (BM. 2469). After the Taisho eruption, it is known that the caldera has been uplifting, thus recovering from the sinking. However, the uplift is not happening in a uniform fashion: when the Showa Lava outflow activity started, the ground settled, and when the significant eruptive activity of Minamidake started, uplift stopped briefly or slight settling occurred. On the other hand, during periods of dormancy or while there is no significant volcanic activity, uplift continues and it is estimated that the amount of magma supply is equivalent to 10 million m³ per year. The current uplift restarted around 1993 AD.

Forecasting of Future Activity

   As the magma accumulation volume can be estimated from the rising ground that followed the Taisho eruption, which is still accumulating, and because the eruption activity at the Showa crater that recommenced in 2006 AD has increased the number of explosions after 2009 AD, if the eruption activity at Sakurajima is observed over the long run, a trend for the volcanic activity can be determined. In addition, prolonged or intense eruptive activity at the Showa crater is a possible cause of the outflow of lavas like the Showa Lava. Taking into account this volcanic activity and the introduction of an eruption warning level, a new Sakurajima Volcano hazard map was published in 2010 AD by Kagoshima City. Although the explosive eruptive activity that has taken place on Minamidake since 1955 AD has subsided since 2000 AD, it is possible that Minamidake will show eruptive activity once again. The vulcanian-style eruption activity of Minamidake spread volcanic ash over a large area and discharged volcanic bombs 3 km, which fell very close to a village. In addition, debris avalanches that follow stream courses occur often after heavy rains.

   Moreover, as seen from the amount of recovery of the ground of Aira Caldera that subsided after the Taisho eruption ( Fig. 11), it can be assumed that a major eruption comparable to the likes of Bunmei, An-ei or Taisho eruptions is approaching. Currently, the volcano is being precisely and continuously monitored; so, precursors to large-scale eruptions like these can most certainly be observed and we will not be caught off guard. However, if such a large eruption occurs, extensive damage comparable to what occurred in the past may happen and new types of damage may occur in our modern and advanced society. The danger to human lives and society is greater than ever before. We must continue to conduct short and long term disaster prevention measures together with promoting innovative research into eruption prediction based on activity monitoring.

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