Shin Yoshikawa

Effect of UV scattering on SO2 emission rate measurements

We report the quantitative evaluation of the UV scattering effect on the SO 2 emission rate measurement by the compact UV spectrometer system. Plume spectra were obtained simultaneously at three measuring points with different distance to the volcanic plume. The apparent absorbance decreases with increasing distance to the plume and the attenuation becomes stronger at shorter wavelength bands. In addition, the attenuation intensity depends on the SO 2 column concentration. The underestimation of the measured absorbance caused by the UV scattering leads to the underestimation of the SO 2 emission rate. The attenuation was not significant with any wavelength band (<±10%) at 0.6 km but was 35-50% with shorter wavelength band at 2.6 km distance. The UV scattering effect on the SO 2 emission rate estimation can be evaluated by the comparison of the emission rates calculated with different wavelength bands.

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress (σ3), with its near north–south trend, is dominant throughout the entire region. However, the σ3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu–Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.

Degassing Activity of a Volcanic Crater Lake: Volcanic Plume Measurements at the Yudamari Crater Lake, Aso Volcano, Japan

Surface degassing is an important volatile discharge process for an active volcanic crater lake. The compositions of volcanic gases discharged from a lake surface (lake gas) were quantified by volcanic plume measurements using a Multi-GAS and alkaline-filter techniques at the Yudamari crater lake, Aso volcano, Japan. Compositions of the lake gases were quite variable and are clearly different from the gases from adjacent fumaroles. Differentiation processes of the lake gas, the lake water, and the fumarolic gases are evaluated based on their compositions. Concentrations of HCl in the lake gas and the lake water indicate that the lake gas composition is controlled by the equilibrium evaporation of the lake water at the lake temperature. Contrasting compositions of the lake gas and the lake water indicate that sulfate and elemental sulfur formation controls chemical differentiation in the lake. The composition of the hydrothermal fluids supplied to the lake is estimated based on mass balance of the lake gas and the lake water. The hydrothermal fluids have similar H2O/S and H2O/Cl ratios but lower CO2/S ratios than the fumarolic gases. This composition contrast indicates that the fumarolic gases are a mixture of magmatic gases and a vapor phase separated from the hydrothermal fluids supplied to the lake.

Seismic activity and ground deformation associated with 1995 phreatic eruption of Kuju Volcano, Kyushu, Japan

Abstract Kuju Volcano lies near Aso Caldera in central Kyushu. After a few hundred years of dormancy, a phreatic eruption began with the ejection of about 20,000 m 3 ash on 11 October 1995. A number of new vents have opened on a series of lines striking east–west on the eastern slope of Mt. Hossho, one of the domes of the Kuju complex, a few hundred meters from a pre-existing fumarolic area. After the eruption, there has been continuous steam emission from the new vents. There was the second ash eruption in December 1995. Before these eruptions, seismic events were rarely observed, either near the site of the new vents, or elsewhere under Kuju Volcano. In the nearly 2 years since the first eruption, several thousand earthquakes have been recorded. These events have been very horizontally concentrated just to the north of the new vents vertically between 800 m above sea level and 1000 m below sea level. Very few earthquakes have been located on the southern side of the new vents. There was clearly a strong high-frequency attenuation affecting the seismic waves which passed through the region beneath the new vents to the seismometers south of Mt. Hossho. This evidence possibly indicates a thermal fluid content beneath the new vents, suggesting that there is a seismic attenuating zone in the feeding area of the new vents. Nearly all the earthquake spectra were of dominantly high-frequency, but the percentage of earthquakes with predominantly low-frequency spectra increased at times of enhanced volcanic activity. Volcanic tremors were also observed around the times of peak activity. Slope distance measurements have been made since the eruption. The main results of these measurements are a contraction of more than 200 ppm in distances between Mt. Hossho and points further north. The significant distance changes occurred during seismic swarms. This indicated that the seismic activities influenced ground deformation, even though some of these swarms were 3 or 5 km from Mt. Hossho. The slope distance changes indicate that an area near the top of Mt. Hossho has been moving to the northeast.

Color change of lake water at the active crater lake of Aso volcano, Yudamari, Japan: is it in response to change in water quality induced by volcanic activity?

One feature of volcanic lakes influenced by subaqueous fumaroles existing at lake bottoms (called active crater lakes) is the remarkable color of their waters: turquoise or emerald green. The active crater lake named Yudamari at Mt. Nakadake of Aso volcano, Japan, takes on a milky pale blue-green. The particular blue component of the lake water color results from Rayleigh scattering of sunlight by very fine aqueous colloidal sulfur particles; the green component is attributable to absorption of sunlight by dissolved ferrous ions. An objective color observation conducted during 2000–2007 revealed that the lake water color changed from blue-green to solid green. The disappearance of the blue ingredient of the water color will result in diminution of aqueous colloidal sulfur from chemical analyses of lake waters sampled simultaneously. The aqueous sulfur is produced by the reaction of sulfur dioxide and hydrogen sulfide supplied from subaqueous fumaroles. However, its production efficiency decreases by domination of sulfur dioxide in the subaqueous fumarolic sulfur gas species with increasing subaqueous fumarolic temperature. The disappearance of blue ingredients from the blue-green color of the lake water may be attributed to activation of subaqueous fumarole activity.

Low‐velocity zones in the crust beneath Aso caldera, Kyushu, Japan, derived from receiver function analyses

Aso volcano, in central Kyushu Island in southwest Japan, has a large caldera (18 × 25 km) that formed by the ejection of more than 600 km3 of deposits 89 thousand years ago. We calculated receiver functions from teleseismic waveform data obtained from densely distributed stations in and around the caldera. We estimated the crustal S wave velocity structure from the receiver functions by using genetic algorithm inversion. We detected a low-velocity zone (Vs > 2.2 km/s) at a depth of 8–15 km beneath the eastern flank of the central cones. A sill-like deformation source has been detected at a depth of 15.5 km by analyses of GPS data, and a swarm of low-frequency earthquakes exists at depths of 15–25 km just beneath this low-velocity zone. Magma may be newly generated and accumulated in this low-velocity zone as a result of hot intrusions coming from beneath it. Except for the region beneath the eastern flank of the central cones, a second low-velocity zone (Vs > 1.9 km/s) extends in and around the caldera at a depth of 15–23 km, although phenomena representing intrusions have not been detected below it. From the estimated velocity structure, these low-velocity zones are interpreted to contain a maximum of 15% melt or 30% water.

Temporal variation in source location of continuous tremors before ash–gas emissions in January 2014 at Aso volcano, Japan

Volcanic tremor is often observed to be associated with an increase in volcanic activity and during periods approaching eruptions. It is therefore of crucial importance to study this phenomenon. The opening of a new vent and subsequent ash–gas emissions was observed in the active crater (Nakadake crater) of Aso volcano, Japan, in January 2014. These events were considered to be associated with phreatomagmatic activity similar to the small events of 2003–2005. During the period from December 2013 to January 2014, a significant variation in the amplitude of continuous seismic tremors was observed corresponding to surficial volcanic activity. We estimated the tremor source locations for this two-month period by a three-dimensional grid search using the tremor amplitude ratio of 5–10 Hz band-pass filtered waveforms. The estimated source locations were distributed in a roughly cylindrical region (100–150 m in diameter) ranging from the ground surface to a depth of 400 m. Migration of the estimated source location was also identified and was associated with changes in volcanic activity. We assumed that the source locations coincided with a conduit system of the volcano, consisting of networks of fractures. This area is likely situated above the crack-like conduit proposed in previous studies. Before the 2014 event, an increase in gas-dominated volcanic fluid first caused an enlargement of the conduit zone, followed by the migration of further magmatic fluid through other pathways, which resulted in a subsequent ash–gas emission. Although we do not have sufficient information to discuss the causal relationship between these processes, it seems reasonable that continuous tremors might change the conduit conditions.

Depth estimation of fumarolic gas source deduced by fume pressure measurement

The origin of fumarolic gas is hydrothermal boiling in the active fumarolic area and magma degassing in the active volcano where high temperature gas is discharged. The determination of the fumarolic gas source is important for understanding the geothermal activity or the eruption process. However, it is difficult to determine the gas source by geophysical prospecting using seismic or geomagnetic techniques due to the resolution limitation of these techniques. New measurement tools are therefore necessary. The aim of this study was to determine the conduit length—i.e., the depth of the fumarolic gas source—by measuring the fume pressure, which in turn facilitates the determination of the pressure oscillation of a conduit generated by acoustic resonance. We initially assembled the measuring device and measured the fume pressure at an active fumarole in Kusatsu-Shirane volcano, Japan. Our measuring device succeeded in measuring the pressure oscillation generated by acoustic resonance, revealing that the fumarolic gas source at Kusatsu-Shirane volcano has a depth of approximately 40 m. We propose that the use of an acoustic resonance is an effective approach for deducing the fumarolic gas source.