地球惑星物質科学科 第 1926 回 雑誌会 Earth and Planetary Materials Science Seminar (No. 1926) 日時：2018 年 4 月 19 日 Date & Time：April 19th 2018 13:10～15:30 場所：地学生物共通講義室 Room：Earth Science & Biology Lecture Room ------------------------------------------------ABSTRACT------------------------------------------------Presenter: Ekky Reno PRIYAMBODO (Group: Volcanology and Geofluids Research Group, E-mail: [email protected]) Title: Correlations of Volcanic Ash Texture with Explosion Earthquakes from Vulcanian Eruptions at Sakurajima Volcano, Japan Author: T. Miwa, A. Toramaru, M. Iguchi Journal: Journal of Volcanology and Geothermal Research 184; 2009 (A) Research background Studies based on geophysical observations, such as seismological, infrasonic and ground deformation observations, may help to elucidate the location and magnitude of overpressure sources and conduit geometry related to volcanic eruptions. However, there is no general tool that quantitatively relates observational data with the ascent rate or decompression rate of magma.there may be some magma movements that do not generate earthquakes or ground deformation. Therefore, the authors were tried to get direct information on the ascent rate or decompression rate experienced by erupted materials. (B) Method The authors have compared the texture of volcanic ash emitted by 17 vulcanian eruptions from Sakurajima volcano with the maximum amplitude of explosion earthquakes (Aeq). Glassy particles were classified into S-type particles (smooth surface) and NS-type (rough surface) based on stereoscopic observation. S-type particles were further classified into V-type particles (vesicles-bearing) and NV-type particles (no vesicles) using a polarized microscopic examination of thin sections. Plagioclase microlites analysis conducted using NV-type particles emitted by five explosive events without precursory BL-Swarms. (C) Results and Discussion The number fraction of S-type particles and the number ratio of NV-type particles have a positive correlation with Aeq. For explosions accompanied by BL-type earthquake swarms, the N/V number ratio has a negative correlation with the duration of BL-Swarms. The plagioclase microlite number density (MND) and the L/W (length/width) ratio have a positive correlation with Aeq. The positive correlation between the N/V number ratio and Aeq could indicate that a large amount of separated gas from fresh magma results in a large Aeq. A comparison between textural data and the result of a decompression-induced crystallization experiment (Couch et al., 2003) suggests that a plagioclase microlite texture of volcanic ash from eruptions without BL-Swarms generated from 100–160 MPa decompression. The positive correlation between the MND and Aeq suggest that Aeq becomes large when the effective decompression is large. The estimated magma ascent rate ranges from 0.11 to 0.35 m/s. This result suggests that the ascent rate of magma plays an essential role in the occurrence of vulcanian eruptions. The authors propose a simple model for vulcanian eruptions at Sakurajima volcano. This study shows that direct and quantitative information about obtained from volcanic eruptionthe erupted material will bring to the advanced method of volcanic eruption monitoring. Keywords: magma ascentVulcanian eruption, effective decompressionvolcanic earthquakes, geophysical correlationmicrolite, decompression-induced crystallizationtextural observation
