Yasuaki Sudo

Mechanism of Phreatic Eruptions at Aso Volcano Inferred from Near-Field Broadband Seismic Observations

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (approximately20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano.

Detection of a crack‐like conduit beneath the active crater at Aso Volcano Japan

To constrain the source of long period tremors (LPTs), we deployed a very dense broadband seismic network consisting of totally twenty-four stations around the active crater of Aso volcano in Kyushu, Japan. The spatial variation of the observed signal amplitudes reveals that the source of LPTs consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The detected crack has a dimension of 1 km and its center is located a few hundred meters southwest of the active crater, at a depth of about 1.8 km. The extension of the crack plane meets the crater chain including the active fumarole at the surface, suggesting that the crack has played an important role in transporting gasses and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.

Infrasonic precursors to a Vulcanian Eruption at Sakurajima Volcano, Japan

The May 1998 eruption sequence of Sakurajima Volcano was monitored by ten infrasonic stations, ten seismometers, and a video camera. During this seismo-acoustic experiment, we recorded hundreds of infrasonic tremor and long-period events associated with seismic signals, and observed a progression from relative quiescence to a Vulcanian eruption. The number of infrasonic events increased with escalating volcanic activity, and the dominant character of the infrasonic signals changed from impulsive to emergent. At 22:17 of May 19, Sakurajima released ash and gases to a height of 2 km above the vent, an event that was recorded continuously by one infrasonic and two seismic stations. We present the experimental setup as well as a procedure through which infrasonic signals may be incorporated into future eruption monitoring and forecasting algorithms for open-vent volcanic systems. In addition, our recordings suggest that infrasonic signals are more representative of processes occurring within the volcanic interior than are seismic signals, which are strongly altered by diffraction and scattering in the volcanic edifice.

Velocity structure and a seismic model for Nevado del Ruiz Volcano (Colombia)

Abstract A seismic tomographic study was performed for Nevado del Ruiz Volcano (NRV) using more than 1500 high-quality local and regional. Three low P wave velocity (low- V P ) and low S wave velocity (low- V S ) zones were found; one low- V S zone at depths 2–4 km located beneath the volcano; a second low- V P and low- V S zone at depths 5–10 km located beneath the crater, elongated and dipping to the E–SE; and a third low- V P and low- V S zone at 10 to ∼12 km farther to the east. These three low- V P and low- V S zones are believed to be the location of heat sources. A high-velocity zone for both P and S waves was found at shallow depths (0∼5 km) around the active crater. The upper part (0∼2 km depth) of the high- V P and high- V S zone was characterized by low- V P / V S ratios ( V P / V S ratios (>1.80). The low- V P / V S zone is correlated with a steam-dominated geothermal system. The high- V P / V S is interpreted as an intrusive body of magmatic origin which includes partial melting zones associated with low- V S anomalies. A small low- V P zone in which long-period (LP) earthquakes were clustered was found to the southwest of the volcano. Based on the data obtained with the tomography in combination with seismicity, geochemistry, geology and gravimetry, we suggest a model for the seismic activity of NRV. Volcano-tectonic (VT) earthquakes that occur very often in swarm-like patterns located in several clusters around the volcano seem to be due to changes in stress produced by the passing of fluids and/or gas through many small cracks. A fault and a caldera-like structure separate the VT swarms located to the west of the volcano from the source of LP earthquakes.

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.

An attenuating structure beneath the Aso Caldera determined from the propagation of seismic waves

The attenuation of amplitude is seen in seismic waves which pass through the central region of the Aso caldera, in Kyushu, Japan. It is also recognized from spectral analysis of seismic waves that the higher frequencies of the P-wave are reduced in the waves which pass through the central region of the caldera. It is shown that the relative attenuation increases remarkably for the frequency range of 5 to 10 Hz. The specific attenuation factor Q of the P-wave train is about 100. From the surface projection of the ray paths with low Q values through the Aso caldera to each station, the attenuating region is located beneath the center of the caldera, extending to the north of the central cones. In conjunction with the low Q value of the P-wave and the decreases of S-wave amplitudes, the relative P-wave residual times have comparatively large values for seismic waves passing through the central region beneath the caldera. In order to attempt to provide additional information on the depth configuration of the attenuating material, the ray paths of P-waves first arrivals are located in three-dimensional space. It indicates that the low-velocity material is located beneath the center of the caldera at depths of about 6 to 9 km. However, lowvelocity anomalies above the depth of 6 km and below the depth of 15 km were not able to be detected, because most of the available seismic ray paths had crossed the caldera at depths of about 6 to 15 km. Furthermore, the relative residual times have numerous errors resulting from incorrect hypocenter locations, origin times, inhomogeneities in the structure and uncertainty of the velocity structure. At shallow depths in the Aso caldera, refraction or reflection studies are required for an accurate estimate of the structure and more detailed properties of the attenuating material.

Seismic reflectors beneath the central cones of Aso Volcano, Kyushu, Japan

Abstract The subsurface structure of Aso Volcano, central Kyushu, Japan, has been imaged down to 10 km below sea level and notable features of subsurface structure in the central cones were revealed by a 3-D seismic reflection analysis. The 3-D reflection analysis included reflection enhancements and 3-D migration. The reflection enhancements and the NMO correction were applied on waveform data from the seismic experiment ASO98. The 3-D migration with a Kirchhoff integral was processed for 1397 NMO corrected traces to calculate the reflectivity at about 60 000 image points in an approximately 7 km by 7.5 km square region. The results of the 3-D migration show that the subsurface structure beneath the central cones can be divided into three parts, the western part, the eastern part, and the northwestern part. Bright reflectors appear in most of them and a significant discontinuity is inferred. The reflector horizon at 2 km below sea level spreads beneath the western and the eastern part and is probably the top surface of the Pre-Aso volcanic rocks or the basement rocks. Reflection voids appear in the western part and just beneath the active crater. The reflector void in the western part implies a hot region and is most probably the known seismic anomalies, such as low velocity region or attenuation region. Another reflector void beneath the active crater can also be associated with a hot region, apparently surrounding the conduit system of the current volcanic craters. The major discontinuity is located beneath the northwest flank of the central cones. This discontinuity divides the northwest region with weak reflectors from other parts. The discontinuity may be the trace of the Oita–Kumamoto Tectonic Line, which is the major tectonic line in central Kyushu, crossing the Aso caldera.

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.

Temperature changes at depths to 150 metres near the active crater of Aso Volcano: preliminary analysis of seasonal and volcanic effects

Abstract Two holes were drilled to depths of 150 m and 70 m from the surface about 200 m from the active crater of Aso Volcano and quartz thermometers were installed in the holes at depth intervals of 30 m and 35 m, respectively. This series of observations is one of the first measurements of temperature at depth so close to an active crater. The ground temperature at a depth of 2 m had an annual variation with a range of about 10°C, as expected, clearly corresponding to the atmospheric temperature variation, but delayed by about 1 month. Temperatures measured at depths from 30 m to 70 m had very small annual temperature variations. The range of temperature at 30 m depth was about 0.04°C. The temperature at a depth of 60 m, however, was particularly stable, probably because at this depth the hole is in the middle of a massive lava flow. At depths of 70 m or more, small (less than 0.2°C) annual temperature variations were again observed. These variations are probably due to the effects of surface water descending to these levels through cracks and fissures. At 120 m depth, the average temperature is about 17.5°C, over 5°C above the surface average temperature, and the annual temperature variation has a range of about 2°C, out of phase with the atmospheric changes. This is probably due to the interaction of rainfall descending from the surface with convecting hotter fluids from below. The temperature gradient below 100 m depth is very high, with the average temperature at a depth of 150 m being about 31°C. The temperature variations at this depth are dominated by long-period variations, with a steady decline after a peak in November–December 1989, overlain by a rather irregular seasonal variation, with a range of about 0.5°C. October 1989 was the time of most active volcanic activity, with Strombolian eruptions depositing ash to a distance of 50 km from the crater, accompanied by very high amplitude volcanic tremor. So the temperature changes at 150 m seem to be mainly the result of volcanic activity. The maximum temperature at this depth occurred 1 or 2 months after the peak of observed volcanic activity. So it is likely that the temperature variations show a delayed influence of the level of volcanic activity. These results show that in the unconsolidated materials often found by active volcanic craters, the effects of seasonal atmospheric variations are carried to substantial depths by groundwater flows, so that at Aso Volcano, only at the maximum depth of 150 m are the temperature variations clearly dominated by the level of volcanic activity.