Masato Iguchi

Deformation and seismic precursors to dome-collapse and fountain-collapse nuees ardentes at Merapi Volcano, Java, Indonesia, 1994-1998

Abstract Following the eruption of January 1992, episodes of lava dome growth accompanied by generation of dome-collapse nuees ardentes occurred in 1994–1998. In addition, nuees ardentes were generated by fountain-collapse in January 1997, and the 1998 events also suggest an explosive component. Significant tilt and seismic precursors on varying time scales preceded these events. Deformation about the summit has been detected by electronic tiltmeters since November 1992, with inflation corresponding generally to lava dome growth, and deflation (or decreased inflation) corresponding to loss of dome mass. Strong short-term (days to weeks) accelerations in tilt rate and seismicity occurred prior to the major nuees ardentes episodes, apart from those of 22 November 1994 which were preceded by steadily increasing tilt for over 200 days but lacked short-term precursors. Because of the combination of populated hazardous areas and the lack of an issued warning, about 100 casualties occurred in 1994. In contrast, the strong precursors in 1997 and 1998 provided advance warning to observatory scientists, enabled the stepped raising of alert levels, and aided hazard management. As a result of these factors, but also the fortunate fact that the large nuees ardentes did not quite descend into populated areas, no casualties occurred. The nuee ardente episode of 1994 is interpreted as purely due to gravitational collapse, whereas those of 1997 and 1998 were influenced by gas-pressurization of the lava dome.

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.

Influence of atmospheric structure and topography on infrasonic wave propagation

The effects of topography and atmospheric structures on infrasonic wave propagation from a volcanic source were investigated using observations and numerical modeling. This paper presents the first long-term observational data set showing spatiotemporal variations in patterns of infrasound propagation at distances of up to 60 km from a persistently active infrasound source (Sakurajima Volcano, Japan). The data show that the amplitudes of infrasonic waves received at distant stations relative to those received at a reference station close to the source can vary up to an order of magnitude over short time intervals and short distances and that they do not follow the theoretical geometric decay expected for homogeneous media. Moreover, waveforms also change significantly in both time and space. Numerical simulations were performed using a two-dimensional finite difference time domain (2-D FDTD) method. Effects of atmospheric structure and topography are included in a vertical section parallel to the wave propagation direction. The simulation successfully reproduced the variations of amplitudes and waveforms. Results are interpreted in terms of wave refraction due to sound and wind speed gradients and wave diffraction at topographic barriers. Our numerical results indicate that both atmospheric and topographic propagation effects are nonnegligible. To evaluate the propagation effects and determine source processes in spatially and temporally varying infrasound data, atmospheric data with a time resolution higher than is currently available are required. If the data are available, the present results suggest that the propagation effects could be evaluated using 2-D FDTD modeling at realistic calculation times.

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.

Multiparametric observation of volcanic lightning: Sakurajima Volcano, Japan

We recorded volcanic lightning generated by Vulcanian explosions at Sakurajima Volcano using a synchronized multiparametric array. Physical properties of lightning are related to plume dynamics, and associated electromagnetic field variations are revealed by video observations (high speed and normal speed) together with infrasound and high sampling rate magnetotelluric signals. Data show that volcanic lightning at Sakurajima mainly occurs in the plume gas thrust region at a few hundred meters above the crater rim, where the overpressure of the turbulent volcanic jets determines the electrification of particles generating a complex charge structure in the growing plume. Organization of charges may be achieved at later stages when the plume transitions from the jet phase to the convective phase. Comparison with atmospheric sounding and maximum plume height data show that the effect of hydrometeors on flash generation at Sakurajima is negligible and can be more prudently considered as an additional factor contributing to the electrification of volcanic plumes.

Thermomechanical controls on magma supply and volcanic deformation: application to Aira caldera, Japan.

Ground deformation often precedes volcanic eruptions, and results from complex interactions between source processes and the thermomechanical behaviour of surrounding rocks. Previous models aiming to constrain source processes were unable to include realistic mechanical and thermal rock properties, and the role of thermomechanical heterogeneity in magma accumulation was unclear. Here we show how spatio-temporal deformation and magma reservoir evolution are fundamentally controlled by three-dimensional thermomechanical heterogeneity. Using the example of continued inflation at Aira caldera, Japan, we demonstrate that magma is accumulating faster than it can be erupted, and the current uplift is approaching the level inferred prior to the violent 1914 Plinian eruption. Magma storage conditions coincide with estimates for the caldera-forming reservoir ~29,000 years ago, and the inferred magma supply rate indicates a ~130-year timeframe to amass enough magma to feed a future 1914-sized eruption. These new inferences are important for eruption forecasting and risk mitigation, and have significant implications for the interpretations of volcanic deformation worldwide.

A subsurface structure change associated with the eruptive activity at Sakurajima Volcano, Japan, inferred from an accurately controlled source

Temporal variations of Green functions associated with the eruptive activity at Sakurajima Volcano, Japan, were estimated using an accurately controlled routinely operated signal system (ACROSS). We deconvolved 400 s waveforms of the ACROSS signal at nearby stations by a known source time function and stacked the results based on the time relative to individual eruptions and the eruption intervals; the quantities obtained by this procedure are Green functions corresponding to various stages of the eruptive activity. We found an energy decrease in the later phase of the Green functions in active eruptive periods. This energy decrease, localized in the 2–6 s window of the Green functions, is difficult to explain by contamination from volcanic earthquakes and tremors. The decrease could be more reasonably attributed to a subsurface structure change caused by the volcanic activity.

Behavior of fluorine and chlorine in volcanic ash of Sakurajima volcano, Japan in the sequence of its eruptive activity

At Sakurajima volcano, strombolian eruptions forerun vulcanian explosions, and volcanian explosions are often followed by continuous ash eruptions. Change in the mode of its eruptive activity is drastic and release of volatiles from magma corresponding to its eruptive activity is examined by determination of fluorine and chlorine in volcanic ash. The F contents of all the ash samples are markedly higher than the arithmetic mean value of Japanese volcanic rocks (ca. 300 μg/g). Further, the water-soluble Cl contents of almost all the ash samples are also extremely higher than those of Japanese volcanic rocks (less than 50 μg/g). This is attributed to fixation of F and Cl in hot gas onto the volcanic ash particles in the crater and/or eruption plumes. The differences in those contents among the “strombolian eruption”-, “vulcanian explosion”- and “continuous ash eruption” groups are significant, which implies that hot gas rich in F and Cl are issued during strombolian eruptions, while those halogens are less concentrated in hot gas after strombolian eruptions. The water-insoluble Cl content of the “strombolian eruption” group is significantly higher than those of the other groups. This result implies that Cl and the other volatiles are intensely released from magma during the doming period before vulcanian eruptions.Yet, it is inferred that continuous ash eruption is the final phase of vulcanian one.

Evaluation of recent activity at Satsuma-Iwojima —Felt earthquake on June 8, 1996—

Emission of large amount of volcanic gas occurs at the summit crater of Satsuma-Iwojima volcano located south of Kyushu, Japan. Sakurajima Volcanological Observatory (SVO), DPRI, Kyoto University has conducted seismic observation at a permanent station 1.5 km WNW of the summit crater since June 1995. Volcanic earthquakes at the volcano are classified into A-type (high frequency), B-type (low frequency), C-type (monochromatic) and tremor. A-type earthquake generated by shear fracture and B-type earthquake with volumetric source are dominant at the volcano. Volcanic activity of Satsuma-Iwojima was evaluated from seismicity of the volcanic earthquakes. During the period of 1995–1998, the seismicity has remained at a constant level with seismic energy release rate of 7×1014 erg/year. The seismicity was as high as that during 1975–1977. The energy release rate of volcanic earthquakes at the volcano has been stable at least for recent 20 years. Within the stable activity, a minor increase in activity occurred in mid-1990’s, as shown by a felt earthquake (M2.9) on June 8, 1996. The earthquake on June 8, 1996 may be related to topographic changes and ground deformation around the summit crater, because a crack with length of 100 m was found at southeast rim of the summit crater in October 1996 and the benchmark near the crack moved eastward by 6 cm during the period from June 1995 to April 1997. The stable activity for 20 years and minor inflation of activity estimated from seismic observation coincides with those from SO2 flux, highest fumarolic temperatures and gas equilibrium temperatures. Although daily number of B-type earthquakes with smaller amplitude increased almost 10 times in September 1998, this increase in the seismicity did not accompany any change of eruptive activity. Unstable seismicity of B-type earthquake may reflect fluctuations of condition in the gas conduit.

Magnitude–frequency distribution of volcanic explosion earthquakes

Magnitude–frequency distributions of volcanic explosion earthquakes that are associated with occurrences of vulcanian and strombolian eruptions, or gas burst activity, are examined at six active volcanoes. The magnitude–frequency distribution at Suwanosejima volcano, Japan, shows a power-law distribution, which implies self-similarity in the system, as is often observed in statistical characteristics of tectonic and volcanic earthquakes. On the other hand, the magnitude–frequency distributions at five other volcanoes, Sakurajima and Tokachi-dake in Japan, Semeru and Lokon in Indonesia, and Stromboli in Italy, are well explained by exponential distributions. The statistical features are considered to reflect source size, as characterized by a volcanic conduit or chamber. Earthquake generation processes associated with vulcanian, strombolian and gas burst events are different from those of eruptions ejecting large amounts of pyroclasts, since the magnitude–frequency distribution of the volcanic explosivity index is generally explained by the power law.