Three-dimensional P-wave velocity model beneath Suwanosejima Volcano, southwest Japan, was determined by inversion of 764 rays from 9 artificial shots recorded on 97 seismic stations. We constructed 35,505 grid models in order to perform the inversion with the possible grid spaces. As a result of all inversions, the calculations of the 29,918 grid models converged. We also averaged the P-wave velocities calculated with the selected 300 models with AIC (Akaike's Information Criteria) smallness from all the resolved models to obtain the final P-wave velocity model does not depend on the grid configurations. The final P-wave velocity model is well resolved from the surface to about 0.3km below sea level. A region with high P-wave velocity up to 4.3km/s exists below sea level at 1km west of the collapse caldera, which may represent silicic intrusive rocks. Besides, another high P-wave velocity, mountain-like, body that gradually extends from sea level to the vicinity of the active crater of the volcano is revealed. The peak of the body, however, shifts southeastward about 0.5km from the active crater. We, therefore, suppose that the high velocity body is a result of the accumulation of the dense volcanic blocks and/or agglutinates in the vicinity of past eruptive vents through the growth process of the volcano edifice. On the other hand, very-long period pulses lasting before and after explosive eruptions and the explosion earthquakes occur at the inside of the high P-wave velocity, 3.0-3.8km/s, area beneath the crater. It is, therefore, concluded that the explosion field of Suwanosejima Volcano is located in the high P-wave velocity area.

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Towada volcano is an active volcano located in the northern part of the Northeast Japan arc. Ogurayama Lava Dome (OLD), which is a dacitic lava dome located near the center of Towada volcano, has been regarded as a product of the latest eruptive episode A. In this paper, the author reports that the OLD is older than previously thought and that it was formed at the end of eruptive episode D'. The OLD overlies pyroclastic deposits of the eruptive episode E and is overlain by pyroclastic deposits of the eruptive episode C. These stratigraphic relations restrict the eruption age of the OLD to 9.2-6.2cal kyr BP. Within this time interval, two eruptive episodes (D' and D) are recognized as tephra fall deposits in the distal area. The distribution of Herai Ash from the eruptive episode D' shows that the source vent is located in the vicinity of the OLD. Furthermore, the petrological features of the OLD closely resemble those of the Herai Ash. These observations indicate that the OLD is the product of the eruptive episode D' (7.5cal kyr BP). The probable eruption sequence of the eruptive episode D' is as follows. Intermittent phreatomagmatic eruptions occurred in the earliest stage. These eruptions produced the lower part of the Herai Ash. Subsequent lava eruptions formed the OLD and accompanied intermittent vulcanian eruptions produced the main part of the Herai Ash. The source vent of the eruptive episode A is not the Ogurayama, because the Ogurayama was formed before this episode. Since the only crater topography currently recognized in the Towada volcano is the Nakanoumi crater (NC), the source vent of the eruptive episode A is considered to be the NC. Since the NC has been the main crater throughout the post-caldera stage, future eruptions will probably occur in the NC. There is the current NC at the bottom of the lake of 320m in depth. A detailed examination of probable eruption style in the future will be required for predicting volcanic hazard of Towada volcano.

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Monitoring of volcanic phenomena close to active volcanic vents and inside active craters is needed to predict change of volcanic activities and to understand dynamics of volcanic eruptions. In order to carry out safe volcanic monitoring, we have developed a prototype of a mobile sensor for volcanic observation “HOMURA” which is a new robotic system that has been designed to observe volcanic phenomena inside active volcanic craters. HOMURA is a small unmanned ground vehicle (approx. 780×560×300mm in dimension and 10kg in weight) with six wheels driven by electric motors and it is operated by wireless remote control at a distance of more than 1km. Data measured by some sensors in HOMURA are sent to the base station in real time. Materials of the vehicle body and wheels are aluminum with 2mm thick and plywood with 9mm thick, respectively. HOMURA can climb up and down a rough surface with slope angle of 30 degree. In addition, HOMURA does not readily become undrivable even in overturning during climbing because it has a unique body shape with a horizontal symmetry plane. HOMURA can be made and transported to mission fields at small costs. These allow us to make a new vehicle even if HOMURA should be lost by accident during missions and promptly to explore a sudden volcanic event by HOMURA. In test campaigns at Aso volcano and Izu-Oshima volcano, we confirmed that HOMURA has planned abilities on moving on rough surfaces and wireless communication.

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Mt. Asama (2568m a.s.l.) erupted on 2^nd February 2009 at 1:51am (JST). Although it was a small-scale eruption, involving a eruption plume rising 2000m high from the summit, the ashes accidentally fell in the Tokyo metropolitan area, due to strong seasonal wind, which draw public concern. We observed migration and enlargement processes of the eruption cloud, using infrared images from Multi-functional Transport Satellite (MTSAT), and analyzed its relationship to distribution of ash-fall deposits and meteorological conditions. MTSAT is a gestational meteorological satellite of Japan Meteorological Agency and Civil Aviation Bureau and has five observation channels in the visible to infrared regions (visible: 1 and infrared: 4). Although the resolution is low (4km in infrared), the observation cycle is very high (every 30 to 60 minutes), which is preferable characteristics to eruption cloud studies. We used four MTSAT images taken on 2^nd February at 2:30am, 3:30am, 4:30am and 5:30am. Images of brightness-temperature difference between two thermal infrared channels (10.3-11.3μm and 11.5-12.5μm) were applied for detecting distribution of the eruption clouds. The results show that the eruption cloud migrated to the southeast along the line connecting between Mt. Asama and Katsuura-city on the Boso peninsula through the central part of Tokyo. The eruption cloud elongated toward the direction of the migration and the total length increased with time. The speed of the head and tail of the eruption cloud was estimated to be 135km/h and 51km/h, respectively. This relative speed difference could cause elongation of the total length. According to the meteorological data, at the altitude of 4900-5700m, wind was blowing to the southeast at the speed of 119km/h, and at 2700-3100m to the same direction at 50km/h. These values roughly coincided with the estimated speed of the head and tail, respectively, indicating the difference in the wind speed was the main cause of the elongation. Distribution area of the ash-fall deposits on the ground did not well much the area where the eruption cloud flew over, but was located 0-40km south of it. Unlike the higher altitude, in the regions lower than ～2000m, wind was roughly blowing to the south at somewhat lower speed. When falling ashes from the eruption cloud entered this region, they could be blown to the further south, which could cause the wide and biased distribution of the ash-fall deposits, as observed.

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