Earth Planets Space, Vol. 65 (No. 6), pp. 633-643, 2013
Mie Ichihara1, John J. Lyons2, and Akihiko Yokoo3
1Earthquake Research Institute, University of
Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, AK, U.S.A.
3Aso Volcanological Laboratory, Institute for Geothermal Sciences, Kyoto University, Minami-Aso, Kumamoto 869-1404, Japan
(Received October 31, 2012; Revised April 9, 2013; Accepted May 3, 2013; Online published July 8, 2013)
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.
Key words: Harmonic tremor, infrasound, lava deflation, degassing, lava viscosity, bubbles.