III. Geophysical Study

The works on volcano physics done in the last 4 years in Japan are summarized according to the following five categories. (1) volcano earthquake and tremor events, (2) structure of volcanoes, (3) monitoring of volcanic activity, (4) physical process of volcanoes, (5) new technology and instrumentation.

Volcano Earthquake and Tremor Events

Various types of vibration observed in volcanoes stimulated many works on volcanic earthquake and tremor events. Long period earthquakes (LP earthquakes), among them, are intensively studied in the last 4 years. Observations using broadband seismographs are made to search for unknown phenomena which cannot be detected with short-period sensors (Kaneshima et al., 1996; Ohminato and Ereditato, 19 97; Tsuruga et al., 1995; Tsuruga et al., 1997). Source process of LP earthquakes are studied using records of broadband seismograph (Uhira et al., 1995; Ohminato et al., 1998; Nishimura, 1998). Observation record of strainmeter, which can record much lower frequency signal than any broadband seismograph, are also used to know the volcanic process of Izu-Oshima volcano (Fukao et al., 1998). Studies using existing short period seismometers are also made. Moment tensor and/or single force inversion with short period records are frequently made to study source mechanism (Nishitomi and Takeo, 1996; Uhira et al., 1995; Nishimura, 1995a; Nishimura et al., 1995; Odaka et al., 1997). Source mechanisms of LP earthquakes are studied based on spectral characteristics using Fourier transform (Fujita et al., 1995) and newly developed spectral analysis (Nakano et al., 1998). Relationship betwee n the physical process of volcanoes and LP events and/or volcanic tremors are discussed based on more realistic physical model (Iguchi, 1995; Uhira et al., 1995; Nishimura 1995a). Nishimura (1995b) studied the statistical characteristics of volcanic tremor. Infrasonic wave generated associated with LP events also drew attention (Sakai et al. 1996; Yamasato 1997a, b).

In summary, broadband observations and analysis of LP earthquakes have shown the potential to reveal some basic process of volcano, such as mass movement, pressure buildup or release in relatively low frequency range. Source process of LP events in high frequency range that can be recorded with usual short period sensors still needs intensive study.

Structure of Volcanoes

Structure of volcanoes is most intensively studied in the field of volcano geophysics in Japan for the last four years. The results from the joint project for the exploration of volcano structure by explosion experiments are reported. Initial reports for the 1994 experiment at Kirishima volcano, Kyushu, Japan are made in the following two years (Kagiyama, 1997; Kagiyama et al, 1995; Kagiyama et al., 1997; Mikada et al., 1995; Maeda et al, 1997; Matsushima et al., 1995; Tsutsui et al., 1996). Details of the experiment are reported in another section in this national report on volcano science. The data sets obtained in these experiments stimulate new approach for volcano structure study (Mikada, 1996; Tomatsu et al., 1997; Nishi, 1997). Tomography studies are also made for velocity structure at Kuju volcano (Sudo and Matsumoto, 1998), Nikko-Shirane volcano (Hasegawa and Matsumoto, 1997) and Unzen volcano (Ohmi and Lees, 1995). Attemuation structures are extimanted with tomograpy at Kuju volcano (Sudo et al., 1996). Velocity and/or Q anomaly are interpreted with several kind of analysis (Yamamoto and Ida, 1997; Horiuchi et al., 1997; Yamazaki, 1996). Reflection or scattering of seismic wave are also used to reveal the heterogeneity of volcanic structure (Nishimura et al., 1997; Matsumoto and Hasegawa, 1997; Mikada et al., 1997; Hashimoto et al., 1995). Experiments using vibrator source as well as exploration one are made in Aso volcano (Tsutsui et al., 1997). Electromagnetic methods are also intensively used to survey magnetic or resistivity structure of volcanoes. Magnetotelluric survey for resistivity structure of volcanoes are made in various volcanoes in Japan; Usu volcano in Hokkaido (Ogawa et al., 1998; Nishida et al., 1996), Hokkaido-Komagatake volcano (Michiwaki et al., 1995), volcanoes in Nikko area (Ogawa et al., 1997), Aso volcano (Handa et al., 1998) and Kirishima volcano (Kagiyama et al., 1996; Munekane et al., 1997). After the cessation of the activity of Unzen volcano air-bone electromagnetic surveys are made to know the magnetic structure (Mogi et al., 1997; Honkura et al., 1995). Density distribution is also surveyed in Fuji volcano (Tanaka and Satomura, 1995), Sanbe caldera (Komuro et al., 1996) and Kirishima volcano (Kobayashi et al., 1995). Petrological inte rpretation are made for velocity and attenuation structure beneath volcanic area including mantle wedge by Sato (1995) and Sato and Hasegawa (1996).

In the last 4 years scattering of seismic wave are more frequently used than ever, though it shows slow development. More efforts are needed for the development of observation technology as well as analysis method.

Monitoring of Volcanic Activity

Geophysical study of volcanoes has been developed through monitoring of volcanic activity. In the last 4 years many efforts are made on the development of volcano monitoring although no major eruption took place in Japan. Results of the monitoring of seismic activity, which is most traditional technique, are reported for some active volcanic area; Nikko-Ashio and Eastern Izu (Rocco et al., 1997), Kuju volcano (Sudo et al., 1998), Eastern Izu (Uk awa and Tsukahara, 1996), Unzen volcano (Hashimoto et al., 1997) Guntur volcano in Java(Suantika et al., 1997) and a submarine seamount in Mariana trough (Sato et al., 1995). Volcanic tremors usually indicate eruption impending in near future. Ueki et al. (1996) and Ueki et al. (1997) reported the activity of volcanic tremor of Iwate volcano, which was in crisis in 1998. Monitoring the scattering nature of seismic wave is thought to be useful for detecting temporal variation of volcanic activity, but did not made with only one exception (Hashimoto, 1995). Crustal deformation are also used for monitoring the temporal variation of pressure sources at Unzen volcano (Nishi et al., 1998), Eastern Izu (Ukawa, 1997) and Sakurajima volcano (Tanaka et al., 1996). Theoretical study of crustal deformation due to a source with directivity was studied by Tsukakoshi and Odaka (1998). GPS measurements are also used not direc tly for crustal deformation but used for estimate the change of geoid height which reflect the mass movement beneath the Sakurajima volcano (Tanaka, 1997a, b; Tanaka et al., 1995). Precise measurement of gravity also revealed the movement of magma after the 1986 eruption of Izu-Oshima volcano (Watanabe et al., 1998; Watanabe et al., 1998). Trial for monitoring volcanic activity using electromagnetic technique continued from the previous 4-year period. Geomagnetic change is monitored in the Eastern Izu, where large crustal deformation associated with earthquake swarms repeated for about 20 years (Oshiman et al., 1997), and Miyakejima volcano (Sasai et al., 1997). Electric self-potential, which reflects the hydrothermal circulation, is monitored in many volcanic are (Ishido et al., 1997; Hashimoto, 1997; Hashimoto and Tanaka, 1995; Oyamada et al., 1996; Nishida et al., 1996; Nishi et al., 1998; Nishi et al., 1998) . Direct observation of temperature is also reported for monitoring of volcanic activity (Fukui, 1995; Kagiyama, 1997; Sudo and Hurst, 1998). Monitoring and detection of eruption using satellite image (Sawada, 1996a,b; Sawada, 1998; Tokumo, 1997), radar data (Arao et al., 1996) and infrasonic acoustic wave (Tahira et al., 1997) are reported.

In summary, continuous efforts have been made during the last 4 years despite of general dormancy of Japanese volcanoes. Application of new technology such as SAR interferometry or monitoring of propagation characteristics of seismic wave is needed.

Physical Process of Volcano

Theoretical and experimental studies are made on the physical process of volcanic eruption. Ida (1996) simulates the cyclic nature, which is generally seen in many aspects of volcanic activity, with a simple visco-elastic conduit system. Toramaru (1995) intensively studied the bubble grow th in the upwelling magma. Koyaguchi and Woods (1996) also studied the role of bubbles in the explosive eruption. Goto et al. (1997) estimated the viscosity of peraluminous silicate melts at high temperature. Takada (1997) developed the experimental study of dike intrusion.

New Technology and Instruments

Application of new technology and instruments is very important for new findings. A small-sized data-logger developed for seismic prospecting made an innovation in volcanic seismology in Japan. The detail of the data-logger is reported in Morita and Hamaguchi (1996) with a software (Tsutsui et al., 1995). Satellite telemetry is also used for volcano monitoring where wired telemetry is difficult (Neki et al., 1998).

Miscellaneous

Shimozuru (1996) summarize the case history of two eruption which occurred during his term of office as a chair of coor dinating committee for prediction of volcanic eruption.

(Koshun Yamaoka)

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Sudo, Y. and Hurst, A.W. (1998) Temperature changes at depths to 150 meters near the active crater of Aso Volcano: preliminary analysis of seasonal and volcanic effects. Jour. Volcanol. Geotherm. Res., 81, 159-172.

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(* In Japanese with English abstract. ** In Japanese)