Stmruture and evolution of the ash cloud derived from Unzen pyroclastic flow at 18:23, May 21. 1993 was studied using a weather-radar data. The following results were obtained. 1. The vertical motion in the ash cloud was not uniform. When downward motion dominated in the northern part of the ash cloud, upward motion was inferred in the southern part of the ash cloud. 2. Maximum echo intensity was observed at 2.5 km throughout analysis period. The ejected mass of volcanic-ash particles estimated from radar data reached the maximum 30 minuets after the occurrence of the pyroclastic flow. It was considered that some volcanic ashes were coated with water in cloud layer and aggregate growth of volcanic ashes were occurred in the ash cloud.

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An intensive refraction seismic survey was conducted in November through December 1994 around Kirishima volcanic region by a group of volcanologists from universities in Japan. About two hundred 2-Hz seismometers were deployed in the area to detect seismic signals generated by dynamites at six sites. The purpose of the experiment was to reveal velocity structure of the area through the refraction analyses. Since clear reflection phases were observed in recorded seismic signals, seismic reflection analyses were applied in this paper. However, no CDP’s could be tied to the survey line due to a small number of shot and receiver locations so a simple pre-stack migration scheme was employed to locate buried reflectors in the subsurface. Although the application of conventional seismic reflection surveys were said difficult in volcanological structural surveys, our results demonstrated that clear reflection events were identified. Beneath the second shot point, which is located in the NW end of Kirishima Volcanic Chain, deep reflectors were found at about 4 km and 10 km in depth. These depths showed the coincidence with those reported by Utada et al. (1994) from VLF-MT surveys as low resistivity layers. The results of the present analysis indicated the necessity of shorter geophone spacing and the utilization of geophone arrays in the future. Moreover, the utilization of low frequency seismic waves and appropreate Jacobian in the stage of seismic migration were proposed from the comparison of the point diffractor model (Trorey, 1970; Deregowski and Brown, 1983) with a point scatterer model (Mikada, 1995).

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There are many small inlets ranging from 100 to 200 m in diameter at the marginal of Shikinejima rhyolitic lava flow, Izu-Mariana Arc. Many volcanoclastic dykes (spiracles) are widespread on the inlet-wall, and on the lava surface, some thin pyroclastic-current deposits composed of coarce ash and lava-fragments which range widely in size and vesicularity. The deposits contain water-quenched breccia. Total thickness of the deposits is thickest in the eastern part of the island and thins toward the eruptive center (western part) of this lava flow. The deposits are divided into the following two units: massive unit which resembles the pyroclastic-flow deposits, and laminated unit which resembles base-surge deposits. The flow texture of the sedimemts and the impact structure around the inlets indicate that the explosive events occurred at the inlets. All of these geological features can be explained by explosions due to the destmction of water-steam equilibrium in the steam reservoir fomed beneath the hot lava flow.

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