SEISMIC ACTIVITY AND GROUND DEFORMATION ASSOCIATED WITH 1995 PHREATICERUPTION OF KUJU VOLCANO, KYUSHU, JAPAN

Kuju Volcano lies near Aso Caldera in central Kyushu. After a few hund red years of dormancy, a phreatic eruption began with the ejection of about 20,000 m(3) ash on 11 October 1995. A number of new vents have o pened on a series of lines striking east-west on the eastern slope of Mt. Hossho, one of the domes of the Kuju complex, a few hundred meters from a pre-existing fumarolic area. After the eruption, there has bee n continuous steam emission from the new vents. There was the second a sh eruption in December 1995. Before these eruptions, seismic events w ere rarely observed, either near the site of the new vents, or elsewhe re under Kuju Volcano. In the nearly 2 years since the first eruption, several thousand earthquakes have been recorded. These events have be en very horizontally concentrated just to the north of the new vents v ertically between 800 m above sea level and 1000 m below sea level. Ve ry few earthquakes have been located on the southern side of the new v ents. There was clearly a strong high-frequency attenuation affecting the seismic waves which passed through the region beneath the new vent s to the seismometers south of Mt. Hossho. This evidence possibly indi cates a thermal fluid content beneath the new vents, suggesting that t here is a seismic attenuating zone in the feeding area of the new vent s. Nearly all the earthquake spectra were of dominantly high-frequency , but the percentage of earthquakes with predominantly low-frequency s pectra increased at times of enhanced volcanic activity. Volcanic trem ors were also observed around the times of peak activity. Slope distan ce measurements have been made since the eruption. The main results of these measurements are a contraction of more than 200 ppm in distance s between Mt. Hossho and points further north. The significant distanc e changes occurred during seismic swarms. This indicated that the seis mic activities influenced ground deformation, even though some of thes e swarms were 3 or 5 lan from Mt. Hossho. The slope distance changes i ndicate that an area near the top of Mt. Hossho has been moving to the northeast. (C) 1998 Elsevier Science B.V. All rights reserved.