Shallow subsurface structure of Shirane pyrocrastic cone, Kusatsu-Shirane Volcano is discussed with Pseudo Reflection Profiling. Pseudo Reflection Profiles are obtained from a controlled source seismic experiment in September-October 2003 as a part of the Fourth Joint Observation of Kusatsu-Shirane Volcano. A significant horizon is defined through correlation of the profile with borehole logging data. The horizon is compatible with a top face of Tertiary volcanic rocks as a substratum of Kusatsu-Shirane Volcano and is estimated at 1700 m a. s. l. in the craters. The horizon strikes NNE-SSW direction and dips eastward. Its dipping is not uniform and there is a bench zone beneath the craters which strikes NNE-SSW direction. Seismicity beneath Mizugama crater is concentrated and the active magnetization region locates beneath the horizon. Other clear reflection events within the substratum are also observed in the profiles but their geological correlation is not defined. One of deeper horizon at 1000m a. s. l. may correlate a sort of density contrast in Tertiary volcanics layer. Seismicity along the traverse line tends to concentrate under the deeper horizon beneath Yugama crater. The density contrast may constrain seismicity beneath the crater.

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The latest eruptive activity of Nakadake, Aso Volcano, occurred in the period 1988-1995. We observed the surface activity and the ash-fall deposits to determine the sequence and style of eruption during this period. The 1988-1995 Nakadake activity is divided into four stages : the early stage, the climactic stage, the post-climactic stage, and the final stage. The final stage means afterheat activity. In the early stage (March 1988-October 1989), the crater bottom, which had been occupied by water, gradually dried up. Thereafter, it emitted black ash from dried-up vents. The glass included in the ash fall deposits was composed mainly of blocky particles. In the climactic stage (October-November 1989), the activity alternated between strombolian and phreatomagmatic eruptions. During this stage, there was a rapid increase of brown and clear scoria glasses in the deposits, together with increased activity of isolated volcanic tremors. From the post-climactic stage (December 1989-February 1991) to the final stage (March 1991-October 1995), surface unrest at the crater continued due to a remarkable change in water level of the crater lake. Ash, strombolian and phreatomagmatic eruptions were observed. In the post-climactic stage, opaque blocky grains were predominant in the ash fall deposits. The largest explosive eruption of the 1988-1995 activity occurred at the crater lake on 20 April 1990, generating scoria deposits. Distinct fractures were identified on the surface of ash particles in the 20 April 1990 deposit, which strongly suggests that the ash grains were chilled rapidly by magma-water interaction (phreatomagmatic eruption). In the final stage, most of the blocky glass particles were surrounded by a brown skin, possibly a hydration skin. Thus, the surface activity and characteristics of the ash fall deposits probably varied between stages during the activity of 1988-1995. We believe that the presence of a large amount of water in and around the crater played an important role on the eruptive activity at the Nakadake crater. For hazard mitigation purposes, it is important to remember that phreatomagmatic eruptions can suddenly occur, not only in the climactic stage, but also in the post-climactic and final stages of an eruption.

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Deformation under the ground excites not only translational motion but also tilt changes, although seismologists usually analyze recorded waveforms under an assumption that seismogram only expresses translational motion. Since broadband seismometers have high enough sensitivity to tilt changes, the seismograms are sometimes contaminated by tilting motion at a seismic station. To check the effect of tilt change on seismogram recorded by a broadband velocity seismometer, we performed a simple experiment with a CMG40T seismometer and a tilt sensor of surface mount type. Tilt changes yield one-sided oscillation of the pendulums in a horizontal transducer. Such particular oscillation is clearly identified in the displacement trace calculated from the original velocity seismogram. Theoretical consideration predicts that slow temporal changes in tilt angle correlate with a time function of the apparent displacement of seismogram. Since results of our experiment well agree with the prediction, we estimate the conversion coefficient from the apparent displacement to tilt angle and confirm that the coefficient obtained experimentally well corresponds with the value from theoretical prediction. As a possible example in an active volcano, we consider slow isotropic volume change near a seismic station and calculate synthetic seismogram due to the volume change.

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