An evaluation of the precursors to the 1986 eruption of Izu-Oshima volcano reveals that the long-tenn precursors are clearly divided into two, magma accumulation and ascent, stages. A gradual rise of seismic activity, infiation of the volcano and anomalous decrease in the geomagnetic total intensity, had continued for more than 10 years until around 1980. After 198 1, the volcano showed a small deflation and low seismicity at the caldera area until beginning of the eruption, while we observed anomalous changes in the subsurface resistivity and the magnetic field localized around the summit crater, that indicating a gradual rise of the temperature beneath the crater. By integrating the precursors we propose that the accumulation of magma had continued until 1980, and then the basalt magma started to rise up throuth the well-developed conduit. The magma plumbing system of the summit eruption of Izu-Oshima volcano is characterized by a continuous magma supply and an well-developed conduit connecting the magma reservoir and the summit crater. Since 1987 after the eruption, the EDM and GPS measurements have revealed a re-inflation of the volcano, suggesting a continuous magma supply. Tomographic studies on the subterranean structure also delineated a low velocity zone and a melt batch at the same location (at depths of 5-10 km) beneath the caldera as that of the inflation source. The proposed model for the magma plumbing system might provide a basis for not only the short-term but also mid-term prediction of the future eruption of Izu-Oshima volcano.
The texture of igneous rocks from the Murotomisaki gabbroic complex, Shikoku island, were quantitatively studied by means of a digital image analysis coupled with the two point spatial correlation method. The complex has a layered structure and a considerable variation of modal composition and the texture. Seven samples at different stratigraphic positions of the complex were selected for the textural analysis. Two kinds of correlation functions, σ and τ distributions, for olivine, pyroxene and plagioclase were calculated from digital images taken from photomicrographs of each sample. Significant variation of the slope of the τ distribution at zero distances for plagioclase-pyroxene and olivine-pyroxene pairs were observed among the samples. In order to interpret the textural variation in terms of the crystallization history and kinetics, a series of computer simulation based on a simple geometric model was conducted in which two new phases appear in an old phase in different sequences and with different timing. The σ and τ distributions were also calculated for simulated textures. We compared with the simulated textures and observed textures on the Murotomisaki gabbro. As a result, it was inferred that olivine preceded the plagioclase precipitation near the margin of the body while the plagioclase preceded the olivine precipitation in the interior.
The purpose of this paper is to give an analysis of the spreading of disaster information in the case of 1888 A.D. eruption of Bandai Volcano. A socio-historical approach focusing on newspaper articles, magazines, and other printed matters is adopted to analyze the disaster process. The 1888 eruption of Bandai Volcano was the first experience of a great natural disaster for the Meiji government, which was just under the process of developing the modern nation. The social structure of information in the transit time and its relation to the modernization process in the educational field of physical geography, will be discussed. The newspaper articles about the Bandai Eruption can be characterized into the following three stages in chronological order; l) Reports on the outbreak of the disaster, the initial damage, and the dispatch of the government officials for investigations of the volcano. 2) Further reports on the damage and the donation campaign raised by the newspapers. 3) More on donation campaigns for much more donations among the general public. These three stages clearly show that the social background had changed from the pre-modern society. Having gone through the contemporary understandings about volcanic eruptions and earthquakes with the age of Bandai eruption, and taking the modernization of educational system into consideration, the following reasons for the changes can be pointed out; (a) After the Meiji Restoration accompanied with the so-called “Scientific Revolution” of Japan, the young generation raised under the modern educational system had more scientific attitude to natural phenomena than those of previous generations. (b) The combined effect of the traditional system of charity activities and the new type of donation which was raised by newspapers and other media, such as disaster photographs or charity performances were quite effective in helping the people struck with the disaster under the conditions of the insufficient governmental relief.
All available historical documents, which reeord abnormal phenomena relating (or possibly relating) to the activity of Fuji Volcano, Japan, were re-examined and classified aceording to the reliability of each document. Comparisons of the reliable descriptions with geologic evidence were executed and several new correlations between historical records and eruptive deposits are proposed. Volcanic activity of Fuji Volcano was in high-level from the 9th to I Ith century; in this period at least 7 reliable and 5 possible eruptions occurred. Although only 2 reliable and 1 possible eruption records exist from the 12th to the early 17th century, this low-level activity may be apparent because of lack of enough historical records. After the middle 17th century, enough historical records suggest that the activity is generally low except for the 1707 eruption, which is one of the most voluminous and explosive eruptions in the history of Fuji Volcano. At least thirteen large earthquakes (M 8 and possible M 8 class) have occurred near Fuji Volcano (in east Nankai and Sagami Troughs) since the 9th century. Eleven of these 13 earthquakes were accompanied with volcanic events (eruption, rumbling, or change in geothermal activity) of Fuji Volcano before and/or after each earthquake (in ±25 years).
Reevaluation of places, type, magnitude, and influences of the 800-802 A.D. eruption (Enryaku eruption) of Fuji Volcano, Japan, was made through tephrochronology and analyses of historical records. The Nishi Kofuji fissure on the northeastern slope is newly recognized as a crater of the 802 A.D. flank eruption. The Nishi Kofuji fissure ejected fallout scoria toward ENE and lava flows, which can be correlated with Takamarubi and Hinokimarubi 11 Lavas on the northeastern foot. The Tenjinyama-lgatonoyama fissure on the northwestern slope probably erupted during the Enryaku eruption and ejected fallout scoria and lava fiows. A series of historical documents and paintings (Miyashita documents), which are unauthorized, personal records and are regarded to be unreliable by many historians, includes many detailed descriptions of paleogeogra-phy around Fuji Volcano and of the Enryaku eruption. Although some of the descriptions were exaggerated and conflict with geological observations, some of them are concordant with geologic data. The Enryaku eruption probably gave serious damages to ancient traffic routes particularly on the northwestern-northeastern foot of Fuji Volcano. The Gotenba area, which is located on the eastern foot, was also damaged by thin ash-fall and probably by lahars. This caused a temporal, southward relocation of the offical trafiic route, which had passed through the Gotenba area.
The eruptive sequence of the An-ei eruption of Sakurajima volcano (1779-1782) is revealed by historical records. From the evening of November 7, 1779 (the 29th day of the 9th month in the 8th year of An-ei), Kagoshima and its environs were shaken frequently. At 11 a.m. of the next day, the water in the wells in the island boiled up, spouting at several points and the color of sea became purple. On the noon of the same day, minor white plumes rose up from the Minamidake summit crater. At about 2 p.m., plinian eruption oecurred at the southern upper slope of Minamidake, and several tens of minutes later, at the northeastern flank of Kitadake. The height of eruption column reached about 12000 meters. It is estimated that a pyroclastic flow was generated at 5 p.m. The plinian eruption climaxed from the evening of November 8, to the morning of next day, and later was followed by emission of lava flows. The activity of the southern craters ceased within a few days, but lava emission from northeastern craters lasted for a long period. On November 11, the lava flow from northeastern craters entered into the sea. Since then, submarine explosions occurred repeatedly off the northeastern coast, and it continued to January 18, 1782. Nine small islands produced by this submarine volcanic activity during a year. Submarine explosions caused small tsunamis on August 6 and 15, September 9, October 3 1, November 9, 1780 and April 11, 1781.
Various suggestive documents and associated sketehes are collected for understanding the pre-eruptive and the earliest stages of the 1914 great eruption of Sakurajima volcano, southwestern Japan, in Taisho era. Based on these records, the premonitory process to the eruption was reviewed especially with respect to the occurrence of many earthquakes which resulted in repeated rock falls with dust clouds, unusual upwelling of water and hot spring, and emission of volcanic smoke in the morning of January 12. Although there are many descriptions on the beginning of the remarkable eruption, they are sometimes inconsistent with each other. In the present paper, it is proposed that the valcano started to erupt around 09 : 58 on January 12 (Japanese Standard Time) at 200 m in height in the western slope of the mountain. In several minutes, a line of craterlets was formed between 200-500 m in height in the WNW-ESE direction. The development of a subsurface fissure in this direction resulted in another outbreak in the southeastern slope probably around 10 : 05. For the better understanding of this important eruptive event, discoveries of additional references are still desired.