Japan Meteorological Agency (JMA) has installed and is operating GPS networks around major active volcanoes since 2000 in order to monitor volcanic ground deformation. At present, 22 volcanoes are under continuous observation using about 100 GPS stations. At most observation points single-frequency receivers are adopted in consideration of power saving and mobility in rugged environments. GPS baseline solutions include errors due to tropospheric delays which are inhomogeneous in the actual atmosphere. Generally, computation of baseline solutions is done by using a simple atmospheric model assuming horizontal homogeneity. But if the adopted model is not consistent with the actual atmosphere, tropospheric delays cannot be accurately estimated, resulting in poor position estimates. Especially with regard to the volcano observation, the errors in the vertical component of baselines become large according to the large troposphere errors. In the case of baseline solutions between receivers with large vertical difference, the time variation of the vertical length is superposed by seasonal noise caused by spatial and temporal variations of refractive index of the atmosphere. For accurate monitoring of volcanic activities, more precise positioning in the vertical component is desirable, which should be realized by incorporating more accurate atmospheric model into the analysis procedure. For this purpose, an improved analysis process was developed, based on the JMA’s operational meso-scale numerical weather analysis (MANAL). The MANAL is applied to daily meso-scale numerical weather prediction as initial field. Generally in the differential analysis process of positioning, zenith tropospheric delay (ZTD) is estimated by least-squares method together with the positioning. In this case, initial value of ZTD is given from a simple atmosphere model. In our approach, ZTD between both receiver sites is calculated from MANAL, and then the conventional analysis process is done fixing ZTD between sites. In calculation, analysis software package Bernese Ver. 5.0 was used, while a part of the program was personally modified. This correction strategy using MANAL was applied to the baseline solutions at Asamayama volcano, where ground deformation has been observed associated with the eruption activity from 2008 to 2009. Consequently we could approximately eliminate the vertical seasonal noise at a baseline whose vertical difference reaches to 1.5km. This approach is quite convenient and effective for GPS observation at local and steep areas such as volcanoes.

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More than 500 volcanic ash fall units in Japan were summarized with the data base in the program of the “Research on volcanic ash fall hazard assessment and risk management for industrial location” and the “Impact analysis on the volcanic ash fall in the metropolitan area”. The digital data, including the thickness of the ash fall deposit, for around each one kilometer mesh, which is authorized by the Third Digital National Land Information System in Japan, was used for analysis. The degree of flatness, which is shown as the ratio of the short axis and long axis of the distribution pattern, for each unit were from 0.05 to 1.0, and the average was 0.5. There was a minor difference of the degree of flatness depending on their volume. The larger the magnitude the smaller the degree of flatness, excepts the case of caldera forming gigantic eruption. The distribution direction for each unit was determined by the straight line, passing through the crater, which divides the volume of the deposit into halves. Major of the Japanese air fall ash tend to distribute to the east by the strong west wind. Almost 57% and 77% of the distribution direction are in the east plus or minus 20 degrees, and 40 degrees, respectively. The probability of the ash fall deposition was calculated using the data of the degree of flatness and the direction of ash fall units for each classified volume. For example, the probability of the deposition of 1mm and 1cm ash fall in central Tokyo by the same magnitude of the Hoei (1707) eruption of Fuji volcano, which volume was measured to be as 1.3km³, were estimated as around 33% and 28%, respectively. And the probability maps of the volcanic ash fall deposit for all over Japan in the next ten thousand years were also shown using the same distribution model assumed that there should be eruptions as same size and frequency as the last ten thousand years from each volcano. This kind of probability map of the volcanic ash fall had not been published, and it is useful for the volcanic disaster mitigation staffs in each municipal office and people living in Japan.

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We describe reactivation processes of the Showa volcanic vent at Sakura jima volcano, Kagoshima, Japan. The Showa volcanic vent is located on the east side hill of Minami-dake summit vent at Sakura jima volcano. While Minami-dake summit vent has been active intermittently for the past five decades, the Showa volcanic vent was dormant for an interval of about 58 years until 4 June 2005. Ash samples analyzed were obtained from volcanic explosions of both Showa and Minamidake summit vents from 1981 to 2009. The analyses and observations on the volcanic ash samples include (1) amount of water soluble chlorine and sulfur adsorbed on volcanic ash particles, (2) mineral identification using X-ray diffraction patterns, (3) color measurement, (4) simplified particle size analysis of volcanic ash, (5) micrographic observations of ash particles using an optical binocular microscope and a SEM, and (6) chemical analysis of minerals and glasses in ash particles with an EPMA. (1) and (2) suggested that the proportion altered/non-altered blocky particles in volcanic ash from the Showa volcanic vent was decreased from 2008 to 2009. (2), (3), and (5) indicated a gradual increase in temperature of the vent with time. We regard (1)-(5) features as results of a gradual reactivation process of the Showa volcanic vent since 4 June 2005 by May 2008.

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Electro-optical distance measurement (EDM) and Global positioning System (GPS) observation are applied to monitor precise time variation of the ground deformation at active volcanoes. But observations using electromagnetic waves such as these are accompanied by errors associated with inhomogeneity of refractive index along the propagation path in atmosphere. In particular, the inhomogeneity in troposphere degrades the accuracy of positioning. An improved atmospheric correction method in EDM was developed, based on the Japan Meteorological Agency (JMA) operational mesoscale analysis (MANAL) for numerical weather prediction. In this method, the precise velocity and ray path of propagated lights are estimated from the adequate vertical profile of refractive index by MANAL. Consequently distance along the bowing ray path measured by EDM is corrected to be precise slope distance. Applying this procedure to EDM data at Asamayama volcano, the seasonal fluctuation caused by inhomogeneity of refractive index in atmosphere was removed entirely. At Asamayama volcano, very small eruptions occurred in August 2008 since the latest 2004 eruption, and then a small eruption occurred in February 2009. Based on the EDM observation by Meteorological Research Institute and Karuizawa Weather Station, we detected that the slope distance had been shortened since August 2008. Slope distances from the observation site to reflectors were corrected by using MANAL in this correction method. Though slope distances have increased in length at a rate of 1-7mm per year since the 2004 eruptions, ground deformation turned over to inflation in August 2008 and slope distances shortened to 5-28mm per five months by January 2009. In order to account for those observation data, we assumed a pressure source beneath the summit crater, whose depth and volume increase were estimated to be at a height of 2380m above sea level (200m under the summit) and 15,300m³, respectively. By developing this atmospheric correction method in EDM with the use of the JMA's numerical weather model, it became possible to precisely detect ground displacements and thus to reliably estimate their sources. Therefore, this method is very effective to monitor activity of volcanoes.

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A phreatic eruption occurred on August 22, 2008 from Shinmoedake Volcano, one of the members of Kirishima volcanic group, Kyushu, southwestern Japan. Some explosive craters and eruption fissures aligning in E-W direction for 800 meters were formed inside the summit crater and the western flank of Shinmoedake Volcano. These craters produced clay-rich tephra, consisting of non-juvenile lithic fragments with various degree of hydrothermal alteration. Ballistic blocks distribute in an area within 800 meters from the main crater. The total volume of the tephra produced this eruption is evaluated as 2×10⁸kg. Distribution of the tephra indicates that the main source of the tephra is S-17 crater, which is the largest crater located at the center of the crater chain. More than 70% of the tephra deposit inside the area within 1km from the craters, suggesting the low height of the eruption cloud. Absence of the juvenile materials suggests that this eruption was phreatic caused by a rapid release of steam from the hydrothermal system beneath Shinmoedake Volcano.

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