Archived ash fall sample with description of the first day ash from 1707 eruption of Mount Fuji was found at an old household material at a city near Nara. The sampling site of the archived ash locates downtown of the City of Yedo (Tokyo). The sample is mostly dense dacitic ash. Major element compositional variation of the ash sample supports the interpretation given by Miyaji (1984, 1993) that fall unit I was ejected during the first day of the eruption. The sampling site was locating at the north side from principal fallout axis. The grain-size distribution of the sample is enriched in finer-grained portion. Such grain-size distribution supports also the interpretation given by Miyaji (1984) that lower terminal velocity particles were selectively enriched at down wind direction of lower atmosphere.

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The 2000 eruption of Usu volcano started at 1 : 07 PM, March 31, 2000 following 3-and-a-half days precursory activities of earthquakes and ground cracking. Initial explosion took place at the western lower slope of the volcanic edifice discharging ash plume. Ballistics were thrown westward at least 1.2 km away from the source. More than 60 explosion craters were formed on the following 3 weeks. Type of the eruption cloud was similar to geyser discharging hot water vapor and smaller amount of ash. Hot lahars were directly discharged from some of the craters. A small-scale and fiat-topped cryptodome was formed during this stage. Graben-like fault swarms were formed above the cryptodome. Explosive activity had gradually declined since the later half of April. Bursting-type explosions with air shock wave frequently occurred at the water-saturated crater floors. Geothermal activity became clear on and around the cryptodome since middle May. Geothermal activity had still continued and water vapor was being discharged from three craters in summer time of 2001.

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Surface faulting is a common feature accompanied with dome (cryptodome) growth during 20th century eruptions of Usu volcano, northern Japan. Since the degree of fault development is a consequence of stress input from underlying intrusion of the magma, characterization of the faults is effective in understanding and estimating behaviors of magma. In the 2000 eruption of the volcano, the development of fault was traced by using high-resolution air-photographs. Temporal and sequential data of newly developed normal fault traces and fault-slip data were accurately measured. By using the stress inversion method, sixty fault-slip data clarified that stress regime and stress ratio (φ) for the intrusion are subvertical direction of stress σ₁ and the smallest magnitude of the φ. This indicates that a dike-like intrusion parallel to the azimuth of the faults is plausible as the intrusive magma body. The formation of initial dike beneath the Nishiyama region resulted in volcanic activity from the Nishiyama craters and induced the propagation of subsequent dike perpendicular to it. The second dike intrusion occurred beneath the region between the Nishiyama and Konpirayama craters, and presumably resulted in the activity of the Konpirayama craters.

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Airphotograph analysis was carried out on the west slope of Usu volcano. Hokkaido, Japan during the eruption, which was started on March 31, 2000. The set of four stages airphotographs which were taken between August 1993 and May 2000 were used for analysis. The comparison of the airphotographs which were taken on August 22, 1993 and April 3, 2000 showed the deformation caused by the deeper expansion source around 1.1 km below the sea level on the southwest foot of Konpira-yama. The maximum horizontal and vertical movement vectors were 10 meters and 19 meters respectively. The comparison of the airphotographs which were taken on April 3 and April 26, 2000 showed the deformation caused by the shallower expansion source around 0.3 km below the sea level on the northwest foot of Nishi-yama. The maximum horizontal and vertical movement vectors were 15 meters and 25 meters respectively. No large movement was detected after the April 26. The total maximum horizontal and vertical movement vector between August 1993 and May 25, 2000 were 24 meters and 40 meters, respectively. There was no direct evidence which indicated the shallow magma intrusion during the activity, especially as to showing the magmatic temperature. So the shallower expansion source during the April 2000 activity was not caused by the possible shallow magma intrusion. The up welling solid rock block might occur, which was formed by the group of earthquakes before the eruption, and it induced the surface deformation.

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Usu volcano started to erupt on March 31, 2000. The Geographical Survey Institute (GSI) repetitively monitored the surface displacement in and around Usu volcano by digital photogrammetric method using stereo aerial photos. The advantage of this method is that it can map the spatial distribution of deformation even in the area where instruments commonly used such as GPS cannot be applied. As a result of the analysis, it was found that; before the eruption of March 31, the uplifted area was concentrated around the cinder cone of Usu volcano. After the eruption began, the uplifted area shifted to Konpira-yama and Nishi-yama area. After April 3, the uplifted area was limited in the area around Nishi-yama.

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A familiar feature of Usu volcano, Hokkaido, is more than ten domes located on and around the volcano. The four domes formed in historical times are dacitic in composition. Ko-Usu, Oo-Usu and the 1944 Showa-shinzan dome are all clearly extrusive lava domes. In contrast, the 1910 Meiji-shinzan dome has been previously thought to be an intrusive cryptodome. Definitions of cryptodomes in recent textbooks define them in terms of shallow, subsurface intrusion of lava, which may or may not be related to surface upheaval. The presence of such features has rarely been confirmed by geophysics. In fact, volcanic domes reflect a huge range of structures and formation processes, and have not been satisfactorily defined worldwide. The 1944 eruption of Usu volcano and growth of the dome were monitored using standard geophysical techniques. In the present paper, pseudo growth curves for the development of the dome have been derived from precise leveling measurements traversing the eastern base of the dome. In the early stages, the surface was raised some 65 m into a conical mound by intrusion of viscous magma from a depth of approx. 100 m. This event was the formation of a cryptodome, which eventually ruptured, forming the 1944 lava dome.

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To monitor the crustal deformation associated with the Usu volcano unrest from 27th March, 2001, we started GPS observations from 28th March, 2001 using Ashtech Z-XII receivers. We succeeded installing 6 new continuous stations around the volcano before the beginning of the eruption at 13:07 (JST), 31st March, 2000. Data were collected via a cellular-phone telemetry system. We carried out automatic rapid data analysis every 3 hours using the Bernese GPS Software Version 4.2, Bernese Processing Engine and IGS predicted ephemeredes. Results were published immediately on the WWW server at Hokkaido University to monitor the volcanic activity. From our GPS observations, we find that: (1) cumulative horizontal displacements from 29th March to noon of 31st March, 2000 exceeded 1 m at the KON, OHD, UVO stations, (2) just before the beginning of the eruption, striking deformation was observed at KON located right above the new crater, (3) after the beginning of the eruption, large scale crustal deformation was generally terminated

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Real time monitors of the ground deformation around volcanoes clarify volcano eruption processes and contribute to the disaster prevention on the volcano eruptions. We made an experiment on the real time monitor of the ground deformation for 4.6 km baseline at the northern foothills of Usu volcano using RTKGPS (real time kinematic GPS) measurements in April 2000. Ground displacement toward north and uplift are detected with a resolution of 1 cm, and deceleration was detected precisely.

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Remarkable hydrological anomalies occurred in relation to the 2000 eruption of Usu volcano, Japan. The groundwater level at DT1 in Date City rose more than 4 m, and the groundwater discharged one day before the eruption. The groundwater level at DT2 rose 0.95 m before the eruption. Based on the well's response to tide, the rises of groundwater level are corresponded to the crustal strain changes caused by a spherical inflation source with a volumetric change of 0.3-1.7×10⁷ m³ (Matsumoto et al., 2002). The depth of the spherical source on March 29 is estimated at above 4.3 km by using the ratio of groundwater-level changes at DT1 to DT2. Assuming a vertical line source with the bottom depth of 10 km, the depth of the top on March 29 is estimated to be above 1.6 km. The ratio of groundwater-level changes gradually increased before the eruption, which might show the rising process of the magma.

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