Jun Oikawa

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.

Eigen oscillation of a fluid sphere and source mechanism of harmonic volcanic tremor

Abstract The eigen oscillation of a fluid sphere embedded in an infinite elastic medium is analyzed to understand the source mechanism of volcanic tremor that vibrates nearly monotonically and attenuates slowly. The dimensionless eigen frequencies of the sinusoidal oscillation are calculated in a complex form with the attenuation factor in its imaginary part for various combinations of the three parameters: the contrasts of P-wave velocity, density and rigidity between the fluid and the country rock. Eigen oscillations consist of a high attenuation mode with a rapidly decaying pulsive wave at a low frequency and infinite number of regular modes with slowly decaying vibrations. For regular modes, the frequency of oscillation obtained from the real part of an eigen value is distributed in approximately regular intervals while the attenuation factor from the imaginary part is almost constant independent of the mode. Each eigen frequency of regular and high attenuation modes is degenerated with two independent eigen functions describing different distributions of displacement, velocity and stress. The theory is applied to harmonic volcanic tremor observed at Kusatsu-Shirane Volcano, central Japan. Observed spectral peaks of the tremor are explained by the eigen frequencies and attenuation factors of several lowest regular modes if the spherical fluid oscillator has a radius of dozens of meters and a P-wave velocity of about several hundred meters per second.

Vulcanian eruptions with dominant single force components observed during the Asama 2004 volcanic activity in Japan

On September 1, 2004, Mt. Asama in central Japan erupted for the first time in 21 years. Between this moderate eruption and mid-November of the same year, 4 additional moderate eruptions occurred. We installed 8 broadband seismic stations in addition to the short period seismic network around the volcano and succeeded in recording the near-field seismic signals associated with the summit eruptions. The results of the waveform inversions clearly show that the force system exerted at the source region is dominated by vertical single force components. The source depths of the single force are shallower than 200 m from the bottom of the summit crater, and the order of magnitude of the single force is 1010–1011N. The source time history of each vertical single force component consists of two downward forces and one upward force. The initial downward force probably corresponds to the sudden removal of a lid capping the pressurized conduit. The drag force due to viscous magma moving upward in the conduit can explain the upward force. The correlation between the single force amplitudes and the amounts of volcanic deposits emitted from the summit crater are not necessarily positive, suggesting that the amount of deposits remaining within the summit crater may have played an important role in the excitation of the single force.

Radiographic imaging below a volcanic crater floor with cosmic-ray muons

We present a novel application of cosmic-ray muon radiography to image the shallow density structure beneath Asama Volcano, Japan. We use a single detector (emulsion cloud chamber) set up in an underground vault at an elevation of 2250 m on the eastern flank of Asama, 310 m below the summit of the edifice and 1 km away from the crater. The results point to two high-density anomalies located between the original pre-2004 eruption crater floor and post-2004 eruption crater profile. A third low-density anomaly is imaged immediately below the pre-2004 eruption crater floor. The spatial extent of each density anomaly is about 100 to 200 m. To know if this method, applied to other volcanoes, would produce contrasting results, we performed the measurement in 1944 Usu lava dome. We confirmed a bulbous shape measuring approximately 300 m in diameter and narrowing downwards. The diameter of the uppermost part of the conduit is estimated at 100 ± 15 m at an elevation of 260 m a.s.l. and 50 ± 15 m at an elevation of 217 m a.s.l., demonstrating a resolution that is significantly better than that typically achieved with seismic tomography based on picks of first arrival times from earthquakes or artificial sources.

Atmospheric pressure change associated with the 2003 Tokachi‐Oki earthquake

Clear atmospheric pressure changes associated with the 2003 Tokachi-Oki, Japan, earthquake with Mw 8.3 were recorded with the microbarographs distributed in Japan. The pressure change starts at the arrival of seismic waves and reaches its maximum amplitude at the arrival of Rayleigh waves, suggesting that the observed pressure change was driven by the ground motion of seismic waves passing by the site. We computed the seismic-to-pressure transfer function (i.e., the spectral ratio of the pressure change to the vertical ground motion velocity) for periods between 10 to 50 s from the co-located barograph and seismograph records. Comparison of the observed transfer function with the theoretical one including the finite frequency and wavelength effects for a gravitationally stratified isothermal atmosphere confirms that the observed amplitude and phase of the pressure change are explained by the acoustic coupling between the atmosphere and the ground just beneath the sensors.

Volume change of the magma reservoir relating to the 2011 Kirishima Shinmoe-dake eruption—Charging, discharging and recharging process inferred from GPS measurements

Using GPS data, we evaluate the volume change of the magma reservoir associated with the eruption of Kirishima Shinmoe-dake volcano, southern Kyushu, Japan, in 2011. Because ground deformation around Shinmoe-dake volcano is strongly affected not only by regional tectonic movement but also by inflation of Sakurajima volcano located approximately 30–40 km to the southwest, we first eliminate these unwanted contributions from the observed data to extract the signals from Shinmoe-dake volcano. Then, we estimate the source locations and volume change before, during, and after the highest eruptive activity occurring between January 26 and 31. Our model shows that the magma began to accumulate about one year prior to the sub-Plinian eruption, with approximately 65% of the accumulated magma being discharged during the peak of the eruptive activity, and that magma accumulation continued until the end of November 2011. An error analysis shows that the sources during the three periods indicated above are located in almost the same position: 5 km to the northwest of the summit at a depth of 8 km. The 95% confidence interval of the estimated source depth is from 7.5 to 13.7 km.

An isotropic source of volcanic tremor-observation with a dense seismic network at Izu-Oshima volcano, Japan

Abstract Volcanic tremor is frequently observed associated with activities of volcanoes. In order to study the nature of the volcanic tremor in detail we installed a dense seismic network covering the area of the summit crater of Izu-Oshima volcano. We succeeded in observing volcanic tremors preceding a small explosive eruption on November 16, 1987. The explosion was followed by a significant draining back of lava which had filled the summit crater of the 1986 eruption. We analyzed the low-frequency components of the tremor (near 0.7 Hz) and found that it was generated by an isotropic (volumetric) source located 350 m above sea level. Assuming a cyclic pressure change at the source the amplitude of the seismic moment is 3.4 × 10 10 Nm. This corresponds to a pressure fluctuation of 3 × 10 3 Pa (0.03 bar) in the system containing the cavity of 10 7 m 3 , which is comparable to the volume of subsidence at the 1987 eruption.

Ground deformation associated with volcanic tremor at Izu-Oshima Volcano

Izu-Oshima volcano had summit and fissure eruptions in November, 1986 after 12 years of dormancy, and three small eruptions followed these events within one year. Episodic and continuous volcanic tremors were observed for the period containing these eruptions. It is a remarkable discovery that the episodic volcanic tremor was accompanied by a small but sharp ground deformation, the polarity of which was variable. The distribution of tilt vectors reveals that the source of ground deformation was always located beneath the northwestern flank of the volcano, where a magma reservoir was predicted by other studies. On the other hand, the seismologically detected tremor source was determined to be at a shallow depth below the central pit crater, a few kilometers away from the predicted magma reservoir. It is thus inferred that the tremor source near the crater generated pressure increases or decreases that were simultaneously transmitted through the vent to the magma reservoir and lead to its expansion or contraction.

Source spectrum and source time function of volcanic tremor determined with a dense seismic network near the summit crater of Izu‐Oshima Volcano, Japan

Digital seismic records of episodic volcanic tremor, obtained with a dense seismic network near the summit crater of Izu-Oshima volcano, were analyzed to determine source spectrum and source time function. Source spectrum and transfer function could be separated because the seismic records showed a systematic change with distance from the source. The source spectrum of velocity amplitude had a different frequency, ƒ, dependence above and below a corner frequency of 8 to 10 Hz. At high ranges, the spectrum was proportional to ƒ−2, while at low ranges, it was proportional to ƒ2. Inversion of this frequency-dependent source spectrum yields a source time function that can be represented by an impulse that attenuates in about 0.1 s. Repeated impulses could explain observed volcanic tremor that persists for many minutes or longer and that have complicated phase spectra. The source spectrum gives an energy release rate of about 5.2×102 J/s, so that the total energy released is about 1.0×105 J during a tremor episode of about 3 min at Izu-Oshima. Such energy release is comparable to the seismic energy released by an earthquake of magnitude 0.1.

Free oscillations of a fluid sphere in an infinite elastic medium and long-period volcanic earthquakes

A source model of long-period volcanic earthquakes is presented. We consider that a fluid-filled spherical cavity surrounded by an infinite elastic medium is excited into resonance like the Earth’s free oscillations. The eigenequation of this system is derived in a general manner, making use of the spherical harmonic and spherical Bessel expansions. The solution is given as a complex number; its real part is the eigenfrequency and the imaginary part represents the attenuation coefficient of the oscillation. The eigenmodes are classified into five groups: (1) the compressional modes in a fluid sphere, (2) the compressional modes in a solid medium, (3) the shear modes in a solid medium, (4) the Stoneley modes, and (5) the torsional modes. We apply them to the long-period volcanic earthquake observed at Asama volcano, Japan. Estimating the characteristic frequencies and attenuation coefficients of the observed vibrations and assuming that the primary component (f = 1.73 Hz) corresponds to the fundamental translation mode of a fluid sphere as one of the compressional modes in fluid, we conclude that the resonator which is a spherical cavity of diameter 220 m filled with steam of temperature 500°C and pressure 170 atm is favorable.