Using infrasound waves from eruption video to explain ground deformation preceding the eruption of Suwanosejima volcano, Japan

Combined video, infrasound and seismic observations were conducted at Suwanosejima volcano in August 2005, and the processes producing infrasound radiation at the onset of an eruption were examined based on the results of the observations. Infrasound waveforms were found to exhibit a slow weak pressure increase (preceding phase) 0.7 s before the large rapid main compression phase. These pressure changes induced a subtle H2O phase change in steam discharged from the crater, which could not be easily recognized in the normal video images. However, propagation of the pressure waves could be identified by image processing as luminance variations above the crater. These variations coincided with the compression phase of the infrasound wave. The origin time of the preceding phase estimated from the arrival times of an infrasound wave at two different stations and the luminance data was almost identical to the time when, based on the seismic signals, a vertical expansion occurred at a depth of about 0.5 km beneath the crater bottom. Considering the eruption processes of Sakurajima volcano, it was concluded that the preceding phase of the infrasound wave was caused by swelling of the ground surface at the crater bottom due to this expansion beneath the crater. In the case of the Suwanosejima eruption, the volumetric change due this ground uplift was estimated to be 30-40 m(3). The origin time of the subsequent main phase of the infrasound wave coincided with the onset of volcanic material ejection. This suggests that the crater bottom, which acted as a lid to the pressurized magma and gases before the eruption, failed due to the expansion that occurred during the 0.7 s period prior to the eruption. (C) 2010 Elsevier B.V. All rights reserved.