Kazuhiro Ishihara

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.

Analysis of pressure waves observed in Sakurajima eruption movies

Four eruption movies of Sakurajima volcano taken in the 1980s are analyzed. Pressure waves associated with these eruptions are identified by changes in luminance using a new image processing method. Results of the processing determined the apparent speeds of these waves at approximately 342–574 m/s. Further, spatial characteristics of the pressure waves are quantitatively investigated to estimate the source conditions of volcanic explosions.

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.

Accurate evaluation of ocean tide loading effects for gravity in nearshore region: The FG5 measurements at Sakurajima Volcano in Kagoshima Bay, Japan

We detected 5–10 microgal diurnal/semidiurnal gravity signals of ocean tide origin by the FG5 absolute gravimeter at two stations in Sakurajima volcano located 300 m and 2500 m away from the shore. We computed the gravity originating both from the global ocean tides and from the local ones in Kagoshima Bay, considering the fine-scale sealand distribution around the stations. The observed FG5 records and the theoretical ocean tides agreed well with each other in both amplitude and phase, indicating that the oceanic loading effects can be predicted to an accuracy better than 2 microgal within several hundred meters of the shore in Sakurajima. The apparently puzzling errors that have so far arisen on nearshore LaCoste measurements were resolved by the correction of the effects.

Subtle changes in strain prior to sub-Plinian eruptions recorded by vault-housed extensometers during the 2011 activity at Shinmoe-dake, Kirishima volcano, Japan

This study focuses on strain change observations with a precision of 10−9 associated with the 2011 Shinmoe-dake eruptions in Japan, using vault-housed extensometers installed approximately 18 km northwest of the Shinmoe-dake crater. The extensometers recorded major strain changes of 10−7 during three sub-Plinian eruptions and subsequent magma effusion. Our analysis indicates that these extensometer records provide a reasonable estimation of the parameters of an isotropic point source that can explain eruption-related ground deformation. The extensometers also recorded subtle strain changes of 10−9 prior to the three sub-Plinian eruptions. Time series data indicate that changes in strain at these rates are generally only observed immediately before explosive eruptions, suggesting that these strain changes are precursors to sub-Plinian eruptions. The source of these subtle strain changes is likely to be shallower than the magma chamber associated with these eruptions. The precursory strain changes might have been caused by a pressure increase and a subsequent pressure decrease under the volcano. One possible scenario that can explain these pressure changes is the accumulation of volcanic gases at depth, causing an increase in pressure that was eventually released during gas emissions from the crater prior to the explosive eruptions.

Characteristic non‐down‐dip‐extensional intermediate‐depth earthquakes immediately beneath the volcanic front in South Kyushu, Japan

The hypocentral distribution and fault-plane solutions of intermediate-depth earthquakes in South Kyushu, Japan were investigated. The dipping seismic zone bends at a depth of 60∼70 km and continues to a depth of 180 km and the majority of the source mechanisms are down-dip-extension (DE). Non-DE earthquakes are concentrated at depths of 100–120 km beneath the volcanic region, that is, upward and westward of the dipping seismic zone, and have vertical P- and horizontal T- axes. The occurrence of Non-DE earthquakes may be related to dehydration processes at the base of the mantle wedge.

Reexamination of moment tensors for initial motion of explosion earthquakes using borehole seismograms at Sakurajima volcano, Japan

We estimated the seismic moment tensor for the initial phase of explosion earthquakes associated with Vulcanian explosions at the andesitic volcano Sakurajima. We applied a waveform inversion method to 24 events observed with borehole seismometers at 6 stations assuming a single triangular source time function. The three diagonal components representing volumetric change were dominant in the estimated moment tensors. The three diagonal components were positive and had almost the same values with the difference among them less than 10% for most of the explosion earthquakes, while non-diagonal components were negligibly small compared with the diagonal ones. Durations of source time functions had a range of 0.2–0.5 s and seismic moments were 1011 ∼ 1012 Nm. We conclude that the explosion earthquakes are initiated by an isotropic expansion.