Switching from seismic to seismo-acoustic harmonic tremor at a transition of eruptive activity during the Shinmoe-dake 2011 eruption

Abstract

This paper reports a sequence of harmonic tremor observed during the 2011 eruption of Shinmoe-dake volcano, Kyushu, Japan. The main eruptive activity started with subplinian eruptions, followed by lava effusion. Harmonic tremor was observed as seismic waves during the final stage of the effusive eruption. The tremor observed at this stage had unclear and fluctuating harmonic modes. In the atmosphere, however, many impulsive acoustic waves indicating small surface explosions were observed. When effusion stopped and explosive degassing began, harmonic tremor was observed as acoustic waves in the air and in the seismic data, and the harmonic modes became clearer and more stable. This transition in the character of the harmonic tremor coincided with rapid deflation of the lava that had accumulated in the crater. Based on these observations, and laboratory experiments reproducing the features of the wave fields, it is concluded that the harmonic tremor sequence at Shinmoe-dake was generated by gas flowing through channels in the gradually solidifying lava. Comparing our results with the few cases of similar transition observed at other volcanoes, we expect that the transition indicates changes in magma rheology and degassing conditions in the crater, and therefore of changes in eruptive activity.

Volcano dome dynamics at Mount St. Helens: Deformation and intermittent subsidence monitored by seismicity and camera imagery pixel offsets

“…Outgassing occurs when steam or gas that has separated from the magma rises and escapes to the surface or dissipates into the surrounding host rock, leading to compaction [ Ichihara et al , ; Matthews et al , ] or fracturing at the conduit margin [ Holland et al , ]. While this may generate larger‐magnitude earthquakes as described in section 5.2.1, it may also occur as a consequence of passing seismic waves triggering decompression and depressurization of deeper seated magma below the crater floor level.…”

“…However, these signals are mixed with many other processes associated with dome growth that may lead to deformation over a wide range of temporal and spatial scales. For example, internal pressurization of the lava dome may have large effects on the development of an eruption [ Sparks , ] but may also lead to deformation due to the repeated sealing of gas pathways on timescales of minutes [ Johnson et al , ] to days or weeks [ Ichihara et al , ; Matthews et al , ]. Experimental data show that on timescales of hours to years dome deformation may also be driven by densification due to the viscous reorganization of pores by surface tension [ Kennedy et al , ].…”

Relative Source Locations of Continuous Tremor Before and After the Subplinian Events at Shinmoe‐dake, in 2011

“…Lava effusion inside the crater was first recognized in the morning (Nakada et al, ) and increased its rate at night (N) (Ueda et al, ). Lava effusion ceased by the end of 31 January (Ichihara et al, ).…”

The outline of the 2011 eruption at Shinmoe-dake (Kirishima), Japan

“…Simultaneously, the camera record at Ohnami-ike showed the northward migration (or expansion) of the base of eruption column. Ichihara et al (2013) showed the temporal Snowcapped volcanic cone in the left, whose northern wall is collapsed, is Karakuni-dake, and the crater filled by water in front is Ohnami-ike. Taken by Kazuo Shimousuki at 17h14m, 26 January.…”