Hiromitsu Oshima

Effects of explosion energy and depth to the formation of blast wave and crater: Field Explosion Experiment for the understanding of volcanic explosion

We made field explosion experiments as an analogue of volcanic explosion to understand the relationship between the explosion condition and the resultant surface phenomena. The main parameters we employed were explosion depth and explosion energy. Through the experiments we confirmed that scaled depth, which is the depth divided by cube root of energy, is the main parameter determining the properties of explosive volcanism. The energy assigned to blast wave decreased exponentially against the scaled depth. The scaled crater diameter became maximum when the scaled depth was about 4 × 10−3 m/J¹/³. Scaled crater diameters by nuclear, chemical subsurface and some volcanic explosions were almost the same. From the scaling law, the overpressure at crater rim was estimated to be several MPa, which corresponded to typical rock strength. Probably the ground-forming materials were broken inside the area where overpressure exceeded their strength.

A resistivity cross-section of Usu volcano, Hokkaido, Japan, by audiomagnetotelluric soundings

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokkaido, Japan, which erupted in 1977 and is still active. We had a profile of 17 sites perpendicular to the regional tectonic strike, which crossed the 1977 cryptodome, Usu-Shinzan. Tensor-decomposed data were interpreted by a two-dimensional inversion. Outside the crater rim, the resistivity structure is simple. The resistive somma lava is underlain by a conductive substratum, implying altered Tertiary or Quaternary rocks. In the crater, there are two resistive bodies bisected by a vertical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AMT sounding in 1985. One of the possible causes of the development of the vertical conductor is a cold water supply from the surface into the vapor dominant fracture zone. One of the resistive bodies is located beneath Usu-Shinzan and implies an intrusive magma body which caused the 1977 uplift.

Effect of explosion energy and depth on the nature of explosion cloud: A field experimental study

Field explosion experiments using dynamite were carried out to investigate the effect of explosion energy and depth on the nature of explosion cloud. The scaling law of explosion was established among the energy, depth, and the nature of explosion cloud; shape, height and duration of flow out. The shape of explosion cloud changed systematically from funnel type via elongated to short types with increasing the scaled depth of explosion. At the shallow scaled depth, both scaled height and scaled duration increased with increasing scaled depth. Both values reached the maximum at 0.003–0.004 m/J1/3 of cube-root scaled depth, and then decreased with increasing scaled depth. If the cube-root scaled depth was over about 0.01 m/J1/3, we could not observe any explosion cloud on the ground surface. These experimental results should be applicable to the understanding of volcanic explosions if the buoyant effect of internal heat in the explosion cloud was negligible and ground medium is identical. We applied these results to the explosion clouds of Usu phreatic explosion at 14:53 h on 17 April 2000, and estimated that the explosion energy and the depth were 4×109 J and 6 m, respectively. Errors caused by uncertainty of scaling factors are less than one order of magnitude for the energy and depth.

Excitation process of infrasonic waves associated with Merapi‐type pyroclastic flow as revealed by a new recording system

We investigate excitation process of the infrasonic waves associated with Merapi-type pyroclastic flows at Unzen volcano, Japan, using data set obtained by a new system that permits recording of geophysical data with accompanying visual events on a single videotape. The accurate correlation of infrasonic waves with video images reveals that: (1) successive impulsive waves and the low-frequency wave (<∼0.8Hz) are excited during the collapse of lava blocks falling from the dome, which generates a pyroclastic flow; (2) an impulsive wave is excited by an explosive blowout of the pore-gases which are released by the breakdown of the falling blocks; (3) the low-frequency wave is excited by the volumetric change which results from expansion of the pore-gases released by pulverization of the falling blocks. The comparison between the observed and the synthetic low-frequency waves shows that the amount of volume change is about 2 times that of the initial falling blocks.

Tectonomagnetic study in the eastern part of Hokkaido, NE Japan: Discrepancy between observed and calculated results

Continuous and repeated geomagnetic observations have been performed at 8 stations in the eastern part of Hokkaido, NE Japan, to confirm a detailed picture of geomagnetic secular changes. The observation delineated anomalously large secular changes of about 1 nT/year that have lasted at least for about 3 to 30 years (depending on the period of observations) at 3 stations situated in the remarkable geomagnetic anomaly region. Contributions from the earth’s core or ionospheric origin are ruled out as source mechanisms because of the local distribution of the anomalous stations. Heat-triggered volcanomagnetic effect cannot be the origin of such secular changes because the stations are quite far from the volcanoes. Instead, we propose the changes originate from stress-induced tectonomagnetic effect (piezomagnetism). We performed piezomagnetic modeling under the condition that observed regional tectonic stress has been applied to the highly magnetized rock bodies inferred from the analysis of the observed geomagnetic anomalies. The modeling explained well the secular changes by assuming the stress sensitivity of the order of 10−2 MPa−1, which is one order larger than the ordinarily used value.

Integrated processing of muon radiography and gravity anomaly data toward the realization of high‐resolution 3‐D density structural analysis of volcanoes: Case study of Showa‐Shinzan lava dome, Usu, Japan

We have developed an integrated processing method for muon radiography and gravity anomaly data for determining the 3-D density structures of volcanoes with a higher spatial resolution than is possible by conventional gravity inversion. In the present paper, we demonstrate the performance of the proposed method by performing numerical tests using synthesized data, and we present the results obtained by applying the proposed method to a volcano, Showa-Shinzan lava dome, Hokkaido, Japan. We obtained the detailed shape of a vent beneath the dome and detected the presence of solidified dense lava near the top of the dome. The results demonstrate the advantage of a hybrid measurement based on both gravity and muon radiography for imaging small structures with sizes of a few hundreds of meters near the surface of a volcano.

Empirical method of calculating the viscosity of peraluminous silicate melts at high temperatures

Shaws method for the calculation of the viscosities of magmatic silicate liquids has been extended to multicomponent silicate melts in which Al2O3 exceeds the sum of alkali metal oxides and alkaline earth oxides. Al2O3 was converted to AlO2 and divided into the network-forming part, AlO20, and the excess part, AlO2*. AlO20 and AlO2* differed in their effect on viscosity. SAlO2*∘ value, which evaluates the effect of AlO2* on viscosity, was chosen to compensate for the difference in Arrhenius slope between the data obtained by viscosity measurement and the data calculated from the chemical compositions other than AlO2*. From the viscosity measurement data of synthesized multicomponent melts, SAlO2*∘ value was 5.6. This is smaller than the SAlO20∘ value of 6.7. Using the SAlO2*∘ value with other Si∘ values from Shaw (1972), the viscosities of peraluminous glassy rocks produced by the 1988–1989 eruptions of Tokachi-dake volcano (Hokkaido, Japan) were calculated. The calculated viscosities agreed well with the measured ones.

Precursory tilt changes of small phreatic eruptions of Meakan-dake volcano, Hokkaido, Japan, in November 2008

Although forecasting an occurrence of phreatic eruption is very difficult, it has been reported that some precursory activities often precede these eruptions at several volcanoes. In this study, we observed seismic activities before and during the 2008 phreatic eruption at Meakan-dake volcano, eastern Hokkaido, Japan, by using broadband seismometers and surface mount-type tiltmeters. The precursory increase in seismicity began in late September about 2 months before the first eruption on November 18. After several rises and falls in seismicity in October and in early November, a small volcanic tremor was observed early on November 16. Although the original velocity seismogram of the tremor generally appeared to be spindle shaped, an outstanding ramp function appeared in the displacement seismogram obtained by simple integration. Since the ramp function appeared only in the horizontal components and continued for about 3 min, which is sufficiently longer than the natural period of the seismometer, we regarded the ramp function as an expression of the tilting motions of seismic stations that was quantitatively confirmed by the strong similarity between horizontal displacement seismograms and tilt data from co-located biaxial tiltmeter. Azimuthal distribution of three tilting vectors obtained from broadband seismograms was not consistent with a simple spherical source but rather strongly suggested a vertical dike under the crater. In this study, we confirmed that an almost vertical single dike effectively explains the observed tilting vectors. The estimated volume increase in the dike was 4–5 × 104 m3. The strike direction of the dike is highly consistent with the alignment of the hydrothermal area on and around the volcano. Our dike model also partially explains the changes in global navigation satellite system (GNSS) measurement and in groundwater levels reported in previous research. Since a similar deformation coincided with a volcanic tremor preceding the 2006 eruption, we interpret that this must be an important preparatory process of phreatic eruptions at Meakan-dake volcano.

Gravity variation around Shinmoe-dake volcano from February 2011 through March 2012—Results of continuous absolute gravity observation and repeated hybrid gravity measurements

We report here on continuous absolute gravity measurements made between February 2011 and March 2012 and repeated relative gravity measurements in the vicinity of Shinmoe-dake volcano, which commenced erupting in late January 2011. We find that 20 of 24 eruptive events are associated with precursory short-term gravity decreases occurring over 5–6 hours followed by quick recoveries lasting 1–2 hours. Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm. To isolate the gravity signal associated with volcanic processes, we compared gravity measurements made at 15 sites in March 2011 and again in March 2012. The gravity changes and crustal deformation observed during the one year period are well explained by 6×106 m3 inflation of a magma reservoir at a depth of 9 km and intrusion at shallower depths of a dike with dimensions of 10 km × 0.5 km × 0.5 m.

Ice Box Calorimetry: A useful method for estimating heat discharge rates through steaming ground

We developed a new technique, Ice Box Calorimetry, for estimating heat discharge rates through steaming ground. Ice within an aluminum box provides a powerful tool for measuring the total heat transfer from the ground surface, including conductive heat, convective heat, and latent heat within vapor. Using this method, we are able to rapidly measure heat discharge rates without the need for specialized apparatus. Applying Ice Box Calorimetry to the Nishiyama steaming ground at Usu volcano, Japan in September 2006, we identified local variations in heat discharge rates. The total heat discharged from the entire geothermal area in September 2006 is estimated to be 27 MW, which represents just 1% of the rate immediately following an eruption from the volcano in 2000.