Swelling of a lava plug associated with a Vulcanian eruption at Sakurajima Volcano, Japan, as revealed by infrasound record: case study of the eruption on January 2, 2007

Abstract

In order to clarify the time relation of the expansion of a gas pocket and failure of its overlying plug of lava during Vulcanian eruptions, infrasound records and video images of the Vulcanian eruption that occurred at Sakurajima volcano on January 2, 2007 were analyzed with respect to their origin times. Weak (≤3 Pa) and slowly increasing air pressure preceded the impulsive compression phase by 0.25-0.32 s, and a longer-period rarefaction phase of infrasound waves was recognized at all microphone stations. The velocity of the compression phase was assumed to be supersonic (ca. 400 m/s) up to 850 m above the crater bottom from other recent explosions. On the other hand, the propagation velocity of the preceding weak signal was regarded to be similar to the air sound velocity because the lack of impulsiveness is unlikely to be related to the main compression phase. Therefore, the estimated origin time of the main compression phase was delayed by 0.5-0.7 s from the preceding phase. The origin time of the preceding phase coincided with the onset of the isotropic expansion process of the pressurized gas pocket, which was obtained by the waveform inversion of the explosion earthquake. In contrast, the origin time of the main impulsive phase coincided with the time when the expansion rate reached its peak. This observation suggests that the volumetric increase of the gas pocket caused swelling of the surface of the crater bottom and its subsequent failure. When the expansion velocity exceeded a threshold level, the main impulsive compression phase radiated with a high velocity by the sudden releases of the pressurized gases. The volumetric change at the source was estimated to be 280-560 m 3 from the preceding phase of the infrasound. This volume change indicates that the vertical displacement of the swelling ground was on the order of 1.0 m, assuming the radius of the lava plug was ca. 10 m.

Continuous thermal monitoring of the 2008 eruptions at Showa crater of Sakurajima volcano, Japan

“…The peak amplitudes of infrasound waves associated with these eruptions were recorded as 80-150 Pa at a station 2.3 km from the crater. These values of infrasound amplitudes are similar to those of the recent Vulcanian eruptions that occurred at the summit crater (Yokoo et al, 2009). This episode lasted for only four days, with sudden onset and cessation.…”

“…During the 2008 eruption episodes, particularly for those after April, continuous thermal data were successfully recorded by the new monitoring system, even at night when video monitoring systems using high-sensitivity TV cameras (Yokoo et al, 2009) are ineffective. A total of 808 thermal events were recorded (Figs.…”

Introduction to an Open Community Infrasound Dataset from the Actively Erupting Sakurajima Volcano, Japan

“…After a period of vent sealing, the explosion begins with a slow rise in pressure lasting ∼0:75 s (Fig. 5b), which has been previously attributed to the swelling of the viscous lava plug (Yokoo, et al, 2009). A strong compression occurs next and indicates the explosive disruption of the plug.…”

“…This dataset was further analyzed by Morrissey et al (2008) using a joint seismo-acoustic approach. Yokoo et al (2009) used high-frame rate video and infrasound sensors to identify a precursory acoustic wave associated with the swelling of the viscous lava plug just prior to an explosion. This precursory wave was followed by a shock wave associated with the onset of the main explosion.…”

Multiparametric observation of volcanic lightning: Sakurajima Volcano, Japan

“…The time of the explosion onset was derived by infrasound signals recorded by a microphone array located at Kurokami Observatory ( Figure 1) and following the methodology of Yokoo et al [2009]. Volcanic lightning events were observed and recorded during both the jetting and the convective phases of plume dynamics.…”