1990-1995 ERUPTION - CHRONOLOGY

PREFACE: Eruption at Unzendake (Unzen) Volcano began in November 1990, leaving 198 years dormancy. Effusion of dacite lava had continued for about 4 years, resulting in the formation of a lava dome at the summit and frequent generation of Merapi-type pyroclastic flows. Monitoring of the eruptive activity had been done by the Unzendake Weather Station of Japan Meteorological Agency (JMA), Shimabara Earthquake and Volcano Observatory (SEVO) of Kyushu University, Geological Survey of Japan, and the Joint University Research Groups (JURG) (written by Setsuya Nakada and Hiroshi Shimizu).

Unzen Volcano situated in the Shimabara Peninsula, Kyushu, Southwest Japan, initiated its activity about 500 Ka. The volcano developed in the Unzen volcanic graben characterized by EW-trending normal faults. This volcano consists of thick lava flows/domes of mainly dacite in composition, and the collapsed materials (Merapi-type pyroclastic-flow and dry avalanche deposits). The activity during recent 20 Ka has concentrated in the eastern part of the volcano, around Mt. Fugen (or Fugen-dake, 1,359 m above the sea level). Two historical eruptions produced an andesite lava flow of 2x10^6 m^3 and a dacite lava flow of 3x107 m^3 in 1663 and 1792, respectively. Earthquakes occurred before and during the 1792 eruption, and a large earthquake after the eruption triggered a large-scale collapse of an old lava dome (Mayuyama; about 4 ka); dry avalanche and the resultant tsunami took 15,000 people's lives. The present active center, Mt. Fugen, is only 6 km away from the center of the Shimabara City with 45,000 residents, the biggest city around the volcano. (written by Setsuya Nakada)

BEFORE EXTRUSION OF LAVA

The eruption was preceded by the following events. In November 1989, a swam of earthquakes occurred 15-20 km below the Chijiwa Bay, approximately 10 km west of Mt. Fugen. The hypocenters migrated toward the summit with time (JURG, Geophys. Party, 1992; Shimizu, 1992; 1993; Yanagi et al., 1992; Ohta, 1993). Isolated volcanic tremors started under the summit 4 months before the first eruption, phreatic one on 17 November 1990. Continuous tremors began in the late-January 1991. Eruption resumed on 12 February 1991, and became stronger with time, such that juvenile ash particles were included in the eruption products (Watanabe et al., 1993). Following inflation of the summit area and high-frequency earthquakes just beneath the Jigokuato crater, dacite lava started to emerge at the crater on 20 May 1991 (JURG, Geophys. Party, 1992; Saito et al., 1993; Suto et al., 1993). Deep-seated high-frequency

earthquakes suddenly stopped after May 1991 (written by Setsuya Nakada and Hiroshi Shimizu).

DOME GROWTH AND PYROCLASTIC FLOWS

Continuous lava extrusion formed a dome growing on the eastern shoulder of Mt. Fugen. Frequent collapses of lava blocks from margins of the dome generated the Merapi-type pyroclastic flows (JURG, Geol. Party, 1992; Nakada and Fujii, 1993; Umakoshi et al., 1993). During 1991-1994, approximately ten thousand pyroclastic flows were counted seismologically according to the Unzendake Weather Station, more than ninety-nine per cent of which were the Merapi-type (e.g., Ui et al., 1993). Pelee-type pyroclastic flows generated on 8 June 1991 due to an explosion resulting from sudden depressurization of magmatic conduit, which was triggered by a landslide of crater area. A vulcanian explosion occurred on 11 June 1991. Three times of landslides (3 and 8 June, and 15 September 1991) generated relatively large pyroclastic flows whose travel distances as long as 5.5 km. The 3-June pyroclastic flows killed 43 people. Furthermore, one resident became a casualty on 23 June 1993. Pyroclastic-flow deposits are widely distributed in the eastern flank and slope of Mt. Fugen. Debris flow frequently occurred in rainy seasons. More than 2,000 buildings

were destroyed by pyroclastic- and debris-flows by the summer of 1993. The number of evacuated people was approximately 12,000 in the 1991-summer, and reduced down to 3,000 by the end of 1993 (written by Setsuya Nakada and Hiroshi Shimizu).

VOLUME OF ERUPTION PRODUCTS