In 1991 there was a seismic increase at Kirishima (BGVN 25:02), a group of more than 20 Quaternary volcanoes occupying 600 km² in an area N of Kyushu island's Kagoshima Bay (figures 5 and 6). The previous eruption of Kirishima took place from 1 December 1991 to 19 April 1992, when Shinmoe-dake intermittently ejected ash (BGVN 16:11-17:04; Imura, 1992).

Figure 5. Wide-angle view of Shinmoe-dake crater at Kirishima looking NW. Prominent are both the crater wall and the aqua-blue lake. The flat-topped cone in the background is Karakuni-dake (summit elevation 1,700 m) the tallest peak in the Kirishima complex. The rubbly material draping the the lower half of the crater wall represents remnants of a lava lake formed during the 1716-17 eruption. Copyrighted photo by Keizo Morita (used with permission).

Figure 6. Maps of the South Kyushu region showing recently active volcanoes. The Kirishima volcanic group ("Kirishimayama") lies near the map's N edge. Taken from Matsumoto and others (2007).

This report notes that seismic and thermal unrest also occurred in 2003-2004. Four years later (in August 2008) Kirishma had a sudden, short-lived eruption. Although the plume seemingly did not rise above 1 km altitude, observers chronicled a thin airfall ash deposit highly elongate to the NE.

Late 2003 and early 2004 unrest. Seismicity increased from "normal" levels on 13 December 2003, and the same day observers saw new fumarole pits at the Ohachi crater. A video camera showed steam rising above that crater's rim. Observers saw two new pits that formed in the middle of that crater's southern inner wall and steam rising to ~ 100 m. Within ~ 10 m of these pits, observers saw freshly ejected mud and cognate pebbles 2-3 cm across. The seismicity peaked in mid-December, then declined somewhat, continuing at a relatively high level through at least mid-January 2004.

Multi-year seismic overview. Seismicity rose substantially starting on 19 August 2008 (figure 7), several days prior to the 22 August eruption. Japan Meteorological Agency (JMA) reported 1,005 earthquakes during August 2008. In contrast, the monthly number of earthquakes recorded during the previous 13 months ranged between 2 and 118, with only four earthquakes seen during each of the two months prior to the eruption.

Figure 7. Earthquakes and tremor duration measured at station A (1.7 km SW of the Shimoe-dake's summit) during 2003 to end of August 2008. The top two panels show daily earthquakes. The third panel down shows tremor, with circle size scaled to duration. The lowermost panel shows plume height, which was absent until the 22 August 2008 eruption. Taken from JMA (2008).

Tremor was rare during 2003-2008. There had been tremor during early 2006, and briefly in 2007, but the 2008 tremor included three episodes. During 2008 the longest tremor episode, in August, continued for 350 minutes (the full circle goes off the scale of the plot).

Eruption on 22 August 2008. The eruption began at 1634 on 22 August 2008 from Shinmoe-dake, a stratovolcano with a summit rim around 1,400 m elevation and a main 750-m-diameter crater containing a lake (figure 8). JMA noted that the tallest plume only reached ~ 850 m altitude. Post-eruption inspection found that fissures at Shinmoe-dake had recently opened both in the crater and on its W flank (figures 8 and 9). Also, observers found abundant ballistic lithics near the fissures.

Figure 8. Diagram showing key surface features found at Kirishima's Shinmoe-dake associated with the 22 August 2008 eruption. Also labeled is the upper margin of a lava lake that formed in the crater during the 1716-1717 eruption. Adapted from the August 2008 monthly report by Japan Meteorological Agency (JMA, 2008).

Figure 9. Aerial photo of the W flank of Shinmoe-dake at Kirishima showing fissures as seen on 24 August 2008. Some of these features were present prior to the eruption. An enlarged view shows numerous light colored ballistic blocks thrown out by the 2008 eruption. Some are apparently over a meter in diameter. Photo taken from JMA (2008).

Ash fell at Kobayashi City (10 km NE) and reached up to 25 km from the source (figure 10). According to Nobuo Geshi (Geological Survey of Japan), ~ 200,000 metric tons of ash was erupted. Under the microscope, the ash was composed mostly of non-juvenile materials, although some juvenile glass fragments were found (University of Tokyo - Earthquake Research Institute and Kagoshima University, 2008). As of early November 2008, authorities had not issued further reports, implying quiet conditions. Because of low seismicity and lack of ash plumes, JMA lowered the Alert Level from 2 to 1 on 29 October 2008.

Figure 10. Preliminary near-source isopach map describing ash fall from Kirishima's 22 August 2008 eruption. The map extends out to ~ 10 km from the vent (horizontal scale, bottom right) although ash also fell much farther away. On the key and enlarged views of this map one can see the sites where ash thickness data were collected, providing insight into the map's construction. Some of these points indicate the absence of detectible ash, and at two sites, density data. Field work supporting the map was conducted during 1-2 September 2008. Map was created by Nobuo Geshi (Geological Survey of Japan; original in Japanese).

Partial list of resources discussing Shinmoe-dake. Two informative reports in Japanese helped describe the eruption. The first was the report by JMA (2008), from which figures 7-9 were extracted. That report discussed pre- and post-eruption monitoring, including geophysics, geodetics, behavior of fumaroles, the development of new fissures and fumaroles (including photos and thermal anamalies). The second report, University of Tokyo - Earthquake Research Institute and Kagoshima University (2008), discussed erupted ash.

Fukui and others (2008) discussed Shinmoe-dake's deformation. Their studies employed deformation monitoring by Global Positioning System (GPS) during 2001-2007. Their data disclosed uplift starting in mid-2004.

A website mentioned Kirishima in regard to engineering approaches (sabo dams and related structures) to manage rivers and basins confronting mass wasting at volcanoes (Sakurajima International Sabo Center, 2008). The same site also shows a monitoring camera for Shimnoe-dake and posts a disaster prevention map for Kirishima (in Japanese).

In 1992, geophysicists completed a self-potential survey at Shinmoe-dake (Hashimoto and others, 1994) finding a negative anomaly over the crater basin, a result interpreted as due to streaming potential due to the crater lake and the motion of ions through porous rock. Positive anomalies were small and local and corresponded to fumaroles. Continuous self-potential monitoring during December 1991 to 1993 indicated few changes.

References. Fukui, K., Torisu, K., Tomoyuki, K., Sakai, T., and Takagi, A., 2008, Volcano deformation detected by GPS observation around Shinmoe-dake crater of Kirishima and pressure source estimation by FEM: Meeting Proceedings of the Japan Geoscience Union, Makuhari, Japan, 26 May 2008, v. 151, p. 20.

Imura, R., 1992, Minor phreatic activity of Shinmoedake, Kirishima volcano, in 1991-92: Bull Volc Soc Japan (Kazan), v. 37, p. 281-283 (in Japanese).

Hashimoto, T., Kagiyama, T., and Masutani, F., Self-potential measurements on Shinmoe-Dake, Kirishima Volcanic Group: Bull. Earthq. Res. Inst. Univ. of Tokyo, v. 69, p. 257-266.

Japan Meteorological Agency, 2008, August 2008 Monthly Report on Kirishima: Japan Meteorological Agency (URL: http://www.seisvol.kishou.go.jp/tokyo/STOCK/monthly_v-act_doc/fukuoka/08m08/505_08m08.pdf).

Matsumoto, T., Ueno, H., and Kobayashi, T., 2007, A new secular variation curve for South Kyushu, Japan, and its application to the dating of some lava flows: Rep. Fac. Sci., Kagoshima Univ., no. 40, p. 35-49.

University of Tokyo - Earthquake Research Institute and Kagoshima University, 2008, About ejecta of eruption of 22 August 2008 from Shinmoe-dake (Kirishima): University of Tokyo (Earthquake Research Institute) and Kagoshima University (in Japanese; published 30 August 2008) (URL: www.eri.u-tokyo.ac.jp/topics/Kirishima2008/Figure/kazanbai080830.pdf).

Sakurajima International Sabo Center, 2008, Volcanic Sabo in Japan: Sakurajima International Sabo Center (URL: http://www.qsr.mlit.go.jp/osumi/sivsc/home/english/j038.html).

Geological Summary. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Nobuo Geshi, Geological Survey of Japan (GSJ), AIST, (Volcanic activity research group), Building No. 7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan; Volcano Research Center (VRC-ERI), Earthquake Research Institute, University of Tokyo , Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Keizo Morita (URL: http://www.pmiyazaki.com/kirishima/tz/sinmoe/pano01.htm).