[S-VC31] 活動的火山

[SVC31-P19] Relationship of very long period events and tremor to eruption processes at Nevado del Ruiz volcano, Colombia

*服部 友貴¹、熊谷 博之¹、中野 誠之¹、前田 裕太¹、John Londoño² (1.名古屋大学大学院環境学研究科、2.コロンビア地質調査所)

Tremor has commonly occurred with eruptions (eruption tremor), and very long period (VLP) events have occurred with and without eruptions at various volcanoes. To investigate VLP events and eruption tremor contributes to better understanding of magma ascent and eruption processes. To clarify the relationship of VLP events and eruption tremor to eruptions, we analyzed VLP events and eruption tremor in association with eruption activity at Nevado del Ruiz volcano, Colombia. We used seismic velocity waveforms from the Nevado del Ruiz seismic network between March and December 2016. We performed waveform inversion to determine source locations and mechanisms of VLP events. We estimated that the source of the largest VLP event on 1 April, 2016 was located 1 km N and 2 km W of the Arenal crater at a depth of 1 km below the crater and tilted to NE. However, our waveform inversion results were not accurate enough to estimate temporal variations of VLP sources. Therefore, we estimated the spatial distributions of peak-to-peak amplitudes in a frequency band of 0.2–1 Hz for VLP events. The results suggest that VLP source locations migrated along an NS direction near the Arenal crater during this activity. Few eruptions occurred with VLP events. All VLP events between March and December 2016 were followed by tremor signals. We estimated the source amplitudes of these tremor signals following VLP events and eruption tremor in a frequency band of 5–10 Hz, and found that the source amplitudes of tremor following VLP events tended to be smaller than those of eruption tremor. These results can be interpreted as follows. It has been estimated that there is a magma chamber at a depth of 15 km below the crater by InSAR observations. CO₂ was degassed there, and the magma ascended and was provided to the shallow conduit tilted to NE along the NNE-striking Palestina fault. As the magma in the conduit cooled and crystallized, CO₂ and H₂O were enriched in the melt. Volume changes during rapid CO₂ degassing generated a VLP event, and associated tremor was generated by CO₂ ascent flows along the conduit. The H₂O solubility of the magma increased by releasing CO₂, and the density of the melt decreased. This caused the magma to become buoyant and erupt by magma fragmentation due to H₂O degassing, which generated eruption tremor. The occurrences of VLP events related to CO₂ degassing before eruptions may be useful to monitor eruption activity at Nevado del Ruiz.