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

• Published in: Geophysical research letters Vol. 44; no. 21; pp. 10,871 - 10,877
• Main Authors: Ichihara, M., Matsumoto, S.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-11-2017
• Subjects: eruption; Eruptions; Lava; Magma; magma movement; Monitoring systems; Regional analysis; Seismic activity; seismic array; Seismic arrays; Seismic stability; source location; Stability analysis; tremor; Volcanic activity; Volcanic eruptions; Volcano monitoring; Volcanoes; Water depth; Windows (intervals); Japan
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL075293

Volcano monitoring systems are not always ready to resolve signals at the onset of eruptive activity. This study makes use of stations installed later to calibrate the performance of the stations that had been operated before the eruption. Seven stations recorded continuous volcanic tremor before and during the subplinian eruptions of Shinmoe‐dake, Japan, in 2011. We estimated the source locations of the tremor using the amplitude distribution. The stability of the analysis was obtained by careful selection of time windows in which signals from a single source are dominated. The site effects and the regional attenuation factor were evaluated using tremor recorded after the major eruptions by a dense seismic array and a good number of stations. A tremor source changed its depth beneath the crater for 1 week before the major eruption, rising from a depth of a few kilometer to the water layer 3 times, each of which occurred following shallow inflation and minor eruptions. It is interpreted as migration of gas probably with magma, which further transported heat to the water layer and triggered the subplinian eruptions.
Plain Language Summary
Shinmoe‐dake, Japan had a relatively large eruption in 2011 for the first time in 300 years. Although continuous seismic signal, which is called tremor, indicating magma or gas movement had been recorded before the main eruptive event, we did not have sufficient seismic stations to capture the magma movement. After the main eruption, more seismic stations were installed and recorded tremor that continued with less energetic eruptions. In this study, we make use of the data after the main eruption to calibrate the performance of the stations that had been operated before the eruption. We successfully revealed the movement of the tremor source before the eruption in correlation with shallow inflation and precursory eruptions. The result was interpreted as movement of magma and gas that supplied heat to the water layer in the ground and triggered the eruption. It is demonstrated that seismic stations even deployed after an eruption are useful in assessing processes preceding the eruption.
Key Points
A dense seismic array is used for calibration of seismic stations to improve accuracy of amplitude‐based source location
Upward source migration with shallow inflation and/or minor precursory eruption was clearly detected as an indication of fluid movement
Installing seismic stations including an array is important even after an eruption starts to understand processes before the eruption