Subtle changes in strain prior to sub-Plinian eruptions recorded by vault-housed extensometers during the 2011 activity at Shinmoe-dake, Kirishima volcano, Japan

Abstract

This study focuses on strain change observations with a precision of 10-9 associated with the 2011 Shinmoe-dake eruptions in Japan, using vault-housed extensometers installed approximately 18 km northwest of the Shinmoe-dake crater. The extensometers recorded major strain changes of 10-7 during three sub-Plinian eruptions and subsequent magma effusion. Our analysis indicates that these extensometer records provide a reasonable estimation of the parameters of an isotropic point source that can explain eruption-related ground deformation. The extensometers also recorded subtle strain changes of 10-9 prior to the three sub-Plinian eruptions. Time series data indicate that changes in strain at these rates are generally only observed immediately before explosive eruptions, suggesting that these strain changes are precursors to sub-Plinian eruptions. The source of these subtle strain changes is likely to be shallower than the magma chamber associated with these eruptions. The precursory strain changes might have been caused by a pressure increase and a subsequent pressure decrease under the volcano. One possible scenario that can explain these pressure changes is the accumulation of volcanic gases at depth, causing an increase in pressure that was eventually released during gas emissions from the crater prior to the explosive eruptions.