The Yakedake volcano is located in the southern part of the northern Japan Alps, central Japan. Yakedake volcanic hazard map was published in March 2002, and in June 2002, it was distributed to the inhabitants of Kamitakara village, Gifu prefecture, where is located 4-20km west from the volcano. In January 2003, the questionnaire survey was carried out on the inhabitants in order to know their attitudes to the volcanic hazard map and the level of their understanding of the contents of the hazard map. The Kamitakara village office distributed the questionnaires to 1,102 families through the headman of each ward, the headman collected 802 answers. The results of analysis were as follows. 89% of the respondents knew the existence of the hazard map and 35% read it well, but about 11% have not read the map at all. The elders have a tendency to have deeper understanding of the hazard map than younger ones, especially in elders who have experiences to meet some kinds of natural hazards. And the people who once attended the explanatory meeting of the hazard map, which was held for the residents living inside the disaster-prone area four times after the publication of the hazard map, also tend to have more proper understandings. The people who are engaged to the tourism give more attention to the volcanic hazard than others. The respondents have strong tendency to require more knowledge about the volcanic activities and hazards. We can say that the further activities by scientists, engineers and administrative officers are expected in order to establish an informed consent, that is, there should be a decision-making by inhabitants themselves and support by officers in charge with detailed explanations.

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Unzen volcano is a large volcanic complex which started its eruption ca. 0.5Ma at the center of Unzen graben, Shimabara peninsula, northwestern Kyushu, Japan. This volcano consists of many volcanic edifices such as Takadake, Kusenbudake, Fugendake volcanoes etc. These volcanoes are composed mainly of lava domes and thick lava flows of hornblende andesite and dacite. Volcanic history of Unzen volcano is divided into two stages : the older and younger stages. The younger stage is subdivided into three sub-stages which consist of Nodake volcano, Myokendake volcano, and Fugendake and Mayuyama volcanoes, respectively. Mayuyama is an isolated volcano on the eastern foot of Fugendake, and is the youngest among them with an age of ca. 4ka. Mayuyama volcano consists of two adjoining volcanic edifices, Shichimenzan and Tenguyama, which trend north to south. Geological data indicate that there was once a large lava dome of the same size and at the same place of the present Mayuyama. At the beginning of the Mayuyama eruption, uplift occurred around the present site of Mayuyama, and pre-Mayuyama collapsed. The Shimabara debris avalanche resulted from this movement. A small area to the west of Mayuyama tilted and formed the Taruki plateau, a flat uplifted surface. After the growth of Tenguyama lava dome, Shichimenzan, a volcanic spine, was formed at the northern slope of Tenguyama. Due to the growth of Shichimenzan, the northern part of Tenguyama suffered intense shear-stress which resulted to the formation of many faults and lineaments. During the formation of Mayuyama volcano, Mutsugi block and ash flow was generated mainly to the north of Mayuyama. On the bases of two radiocarbon dates, we estimate the eruption age of Mayuyama as ca. 4.6cal kyr BP. Summit lava domes of Fugendake were also generated shortly before the Mayuyama eruption. This means that lava domes at the summit and flank of Unzen volcano were almost simultaneously formed during the 4.6cal kyr BP eruption.

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The 1663 Usu eruption was the first and largest of all its historic eruptive activities after ca. 7000 years’ dormancy. Our recent study divided the 1663 eruption into three stages in following ascending order : Stage I, small scale of plinian and phreatomagmatic eruptions; Stage II, climactic plinian eruption (pumice fall as so-called Us-b fall); and Stage III, vigorous phreatomagmatic eruptions. Previous petrological studies mainly focused on the Stage II event. The 1663 juvenile materials are composed of three types, white (from all the stage), banded pumices (Stage I and II) and gray essential lithic fragment (Stage I). Major phenocrystic minerals (i.e. plagioclase and orthopyroxene) show nearly bimodal compositional distributions, and can be divided into two types : Type A, iron-rich orthopyroxene (Mg#〜46) and sodic plagioclase (An～42); and Type B, magnesian orthopyroxene (Mg#～70) and calcic plagioclase (An～87). This observation suggests that the juvenile materials were mixing products between mafic and felsic magmas. This observation is also consistent with linear trends in all the oxide variation diagrams for whole-rock chemistry. Based on Wo (Ca/(Ca+Mg+Fe)) content in orthopyroxene, An and FeO^* contents in plagioclase, however, the Type A phenocrysts can be further subdivided into two types : Type A₁ (lower Wo, An and FeO^* contents) and Type A₂ (higher Wo, An and FeO^* contents). Moreover, the Type A₂ phenocrysts are common in the juvenile materials of Stage I (gray essential lithic fragment) as well as in the Stage-II. Based on phenocryst size, composition and whole-rock chemistry, it can be concluded that the Type A₂ phenocrysts crystallized from the mixed magma between the mafic and felsic ones, and had grown for several years before the 1663 eruption. Considering the eruption sequence and the types of erupted magma, the mixed magma was erupted during the initial, weak eruption (Stage I), as well as the climactic, explosive eruption (Stage II). This indicates that the mixed magma of Stage I event would not stagnate between mafic and felsic magmas, as is common in a normal zoned magma chamber, but the top of the chamber. This could be explained by convective entrainment as follows. The injection of the high temperature (＞1000℃) mafic magma into the lower temperature (＜800℃) felsic magma could cause thermal convection to entrain the former into the latter. This entrainment would form the mixed magma, which could rise to the top of the chamber. In effect, the upper part of the chamber would be gravitationally stable until the eruption, because the mixed magma could be lighter than the felsic one. Our petrological analysis concludes that the 1663 eruption was derived from a compositionally reverse zoned chamber that the mixed magma had existed above the normal zoned magma, and that mafic injection had occurred not just before the eruption.

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A new fumarolic activity at Ohachi Volcano of Kirishima Volcano Group started on December 13, 2003 after about 80 years of inactivity. Two small fumarolic vents, T8 and T9, were formed on the slope within the Ohachi crater. Ejected materials were mainly fine-grained mud (altered ash), that were distributed within the southwestern sector of the crater. Small accessory lapilli, which were coated by mud, were scattered within 20 meters of T8 vent. No essential material was found. Alunite, kaolinite, 10 Å-halloysite were present as clay minerals. The volume of ejected material was very small, probably less than 10m³. At the time of this writing, the fumarolic activity at Ohachi crater is still going on. To understand the present activity, we investigated old documents of historic eruptions of this volcano, in particular those that occurred during the Meiji and Taisho eras (1880-1923), to determine the mode of eruptions during that time. The first stage of the eruption started with a fumarolic activity, which became more active. Then magmatic activities continued for about 40 years. The early phase of magmatic activities was characterized by vulcanian eruptions generating a large volume of ash, while the latter phase was characterized by sporadic but strong vulcanian explosions accompanied by ejection of volcanic bombs and blocks. At present, it would be difficult to judge whether the fumarolic activity will become more active, and proceed to magmatic stage. However, the results of our investigation of historical eruptions would still be useful to predict the nature of future eruption of this volcano.

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Ash fall deposits composed mainly of lithic fragments are often recognized in the eruptive products of Asama-Maekake Volcano. A comparative study was made on microscopic textures of coarse-grained particles of 1 to 2mm in diameter from thirteen ash-fall deposits. Average composition of the ash particles from Vulcanian eruptions after the 1783 eruption was also investigated using the cumulative ash-soil mixtures. More than 70 percent of the particles are sub-angular and blocky in shape. Crystalline, grayish, and non-vesicular grains are also abundant. These features suggest that most particles have been produced by brittle fracture of solidified magma. Individual ash-fall deposit has a particular composition. Some ash falls, which were generated after the large-scale eruption in 1108 A.D., have fragments of welded pyroclastic rock as well as those of altered lava. It suggests that they might have been generated from the specific condition inside the vent after the formation and collapse of proximal pyroclastic cone. A series of ash falls through the 2004 eruption show obvious temporal variation in their textures and compositions. Vitric and vesicular grains increased after the appearance of molten lava in the crater-bottom on 16th September and decreased thereafter. Instead, crystalline, grayish, and sub-angular particles increased with time. Oxidized particles have been recognized in the later stage. These temporal variations might correspond to the processes of the appearance and cooling of newly supplied magma. Textures of thirty ballistics including bread-crust bombs from Vulcanian eruptions between 1783 A.D. and 1983 A.D. were observed. Half of them have unbroken, euhedral phenocrysts in the homogeneous groundmass suggesting that they have been originated from magma column of the ordinal coherent lava (Type 1). Forty percent of them contain the veins filled with fine-grained crystal debris in the matrix which is similar to Type 1 (Type 2). Ten percent of them show remarkable eutaxitic texture indicating that they are fragments of welded pyroclastic rocks (Type 3). These textures of the ballisitics could account for the variety of ash particles and also imply the processes of fragmentation of magma in the conduit. Micro-faults are sometimes observed in the groundmass of Type 2. It indicates that the outer part of solidified magma in the conduit experienced the pulverization probably due to shock wave throughout Vulcanian explosion. Zigzag cracks running through a single phenocryst are also found around the veins. Characteristic vein filled with fine-grained crystal debris in Type 2 might be the evidence of the ejection of pulverized materials which occurs immediately after the explosion.

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After 31 years of dormancy, magmatic eruption started at Asama volcano on 1 September 2004. Five major vulcanian explosions, intermittent strombolian explosions and many small-scale explosions were observed from September to December. The initial explosion on 1 September was the largest explosion and the volcanic ash affected areas up to 250km northeast of the volcano. The maximum grain size and the weight of ash collected about 4km NE of the vent were 96mm and 1,000g/m², respectively. Ash from the strombolian explosions during 16-17 September covered most of Karuizawa Town, including areas SE and E of the volcano. Ash fall also affected the Tokyo District on the night of 16 September. The main axes of ash fall on 23, 25, and 29 September, 10 October, and 14 November were dispersed toward NNE to NE, NE, N to NNE, NE, and E, respectively. The distribution of ash fall deposit was thought to be influenced by wind direction and velocity. Ballistics from these explosions were thrown at least 2km away from the vent, and the nature and proportion of rock fragments deposited on the volcano flank differ, depending on the size and type of explosions. The total thickness of proximal ash fall deposits from the 1 September and 14-18 September explosions varies from less than 5mm at the northern rim of the crater to 50mm at the southern rim. Using isopleth maps and excluding ballistic ejecta, the calculated amounts of ash deposited within the area of 1×10⁹m² are as follow; 4.9×10⁷kg, 5.9×10⁷kg, 8.5×10⁶kg, 3×10⁵kg, 1.3×10⁷kg, 2.8×10⁶kg, and 2.5×10⁷kg on 1, 15-18, 23, 25, and 29 September, 10 October, and 14 November, respectively. The total amount of ash generated during the 2004 eruption is more than 1.6×10⁸kg, but still small compared to the lava that formed within the crater.

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A cloud height generated by a volcanic eruption reflects the immensity and/or magnitude of the eruption; thus a measuring of the height's temporal variation during the event is very significant in judging whether the activity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we must take information about the cloud's height by means of the pyroclastic deposits. In general, the total time taken for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T₁); the time for the pyroclastic materials to be transported laterally by the eruption cloud (T₂); and the time for pyroclastic materials to fall and be deposited on the ground (T₃). Since T₃ can be calculated from the settling velocity of pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given value for T₁ is assumed, the most suitable wind velocity to explain T₂ can be determined. Thus the height at which pyroclastic materials separate from the eruption cloud can be determined by using the vertical profile of wind velocity around the volcano. These ideas were applied to the eruption occurred at 19:44 (JST) on September 23, 2004, at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 7.2×10^6kg. Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mud raindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall was estimated to be 2.2-3.1mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at 20:03 in the Kitakaruizawa area (about 9km north-northeast from the source). Assuming 2-5 minutes for T_1 and 11.5-12.0m/s of average lateral wind velocity, the height at which the mud raindrops separated from the eruption cloud can be estimated at 3,430-3,860m (3,610m on average). From this conclusion, the transportation and depositional process of the pyroclastic materials generated on September 23, 2004, at the Asama volcano can summarized as follows : the explosion occurred at 19:44 and the eruption cloud rose to 3,610m while blowing 2.49km downwind from the source. The cloud moved laterally for 4.51km with generating raindrops. At 19:54, mud raindrops separated from the cloud 7.0km north-northeast from the source, then fell to the ground at 20:03 after being blown 2.0km downwind by a lateral wind.

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Some fundamental features of ascending volcanic clouds have been revealed using images of the clouds that were automatically recorded by video cameras for some eruptions at Asama volcano on 15 to 18 September, 2004. According to the analysis of 17 volcanic clouds that are nearly isolated and of a symmetrical shape, the radius of a cloud increases linearly with increasing height, meeting self-similarity of ascending motion nearly up to its maximum height. If the height is measured from a suitable virtual origin, the ratio of the radius to the height can be a constant that is identified with the entrainment constant. The entrainment constants determined in this way have a mean value of about 0.24 in good agreement with those obtained from previous laboratory experiments, even if the values are greater than 0.25 or less than 0.20 for 35% of the analyzed volcanic clouds. During the ascent of a cloud the height squared is nearly proportional to the time and the product of the radius and the ascent velocity is almost constant. These empirical relations are consistent with well-known characters of a thermal that moves in incompressible uniform surroundings. Coupling these kinematic features of the volcanic clouds with the Scorer's relation and the equation of state, we evaluate the total buoyancy, the total mass, the density and the mean temperature of the clouds that are regarded as thermals. The total buoyancy of most volcanic clouds did not change significantly during their ascent process. The cloud on 17 September has a relatively great density contrast and small total buoyancy, probably reflecting hot ash particles in it supplied by a Strombolian eruption at that time. The volcanic cloud discharged at 11:54, September 15 contained ash of 2,500-3,700 tons or less with the mean temperature of 310-360K or higher, and the volcanic cloud discharged at 8:38, September 18 contained ash less than 8,300-9,100 tons with the mean temperature higher than 310K or higher.

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Remote FT-IR measurements for volcanic gas chemistry were carried out twice before and four times after the 2004 Asama eruptions. In these measurements, solar infrared light scattered by higher clouds or plumes was used as a light source for the FT-IR absorption measurements. We have successfully detected 3 volcanic gas components, SO₂, HCl and HF, in the observed spectra. The HCl/SO₂ ratios observed after the onset of the 2004 Asama eruptions were between 0.17 and 0.20, which are probably reflecting the ratios not influenced by any hydrothermal interactions. In contrast, the HCl/SO₂ ratios before the eruptions were slightly lower than those after the eruptions. The slight increase in the ratio from pre-eruptive to post-eruptive periods suggests that the hydrothermal or groundwater impact to the volcanic gas emitting system was small even three years before the eruptions. The observed HF/HCl ratios before the eruptions and in March 2005 were about 0.1, whereas the ratios were higher, over 0.19, during the high activity period from mid-September to October 2004. During the continuous ash emitting eruptions on September 16, 2004, exceptionally low HF/HCl ratio of 0.03 was observed. The HF column amount on this day was probably under detection limit, possibly due to HF depletion caused by adsorption on dense ash in the plume.

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Ground deformation associated with the most recent eruptions in Asama volcano started on September 1, 2004, is reported. The ground deformation observed by continuous Global Positioning System measurements is modeled by dike intrusion for two different periods; one is between July, 2004, to March, 2005, which represents overall deformation during the unrest, and the other is between November, 2004, to March, 2005, which represents the deformation during the latter half of the unrest. We assumed a rectangular dike opening uniformly in elastic, homogeneous, and isotropic medium to model the deformation field. To solve a nonlinear optimization problem in which model parameters are nonlinear to observed deformation field, the Simulated Annealing inversion was employed. The uncertainties of and trade-offs between the model parameters are estimated by the Bootstrap method. The results show that the deformation field is well modeled by a dike striking roughly east-west, strike of which is consistent with the regional stress field. Shape of the dike, that is, length, width, and thickness, is not well constrained due to the small amount of deformation, up to 10mm, and the scarcity of GPS sites, but volume of the dike is well constrained to be 6.82 and 4.63 million cubic meters for the whole period and the latter half, respectively. The estimated depth of the dike tip is roughly 1km below the sea level; the depth of hypocenters is consistent with a theory of dike-induced earthquakes that they occur near the dike tip due to the stress concentration. However, the comparison of the location of the modeled dike and the distribution of earthquakes clearly shows that the hypocenter distribution is inconsistent with the theory described above, that is, the hypocenters are distributed only in the eastern half of the modeled dike tip. The possible reasons for this inconsistency are either 1) earthquakes exists in the west half of the dike tip as well, but they are not detected due to the sparse distribution of seismometers at the west of the flank, 2) the western half of the modeled dike is not capable of generating earthquakes because the temperature is too high for brittle failure of rocks, or 3) the differential stress in the western half is so low that the area cannot reach the critical stress field even by the introduction of dike-tip stress concentration. Current geophysical observations cannot identify the reason but future development of geophysical observations is expected to solve the puzzle.

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