Precursory ground deformation of the 2018 phreatic eruption on Iwo-Yama volcano, revealed by four-dimensional joint analysis of airborne and spaceborne InSAR

Abstract

We present detailed maps of local-scale 3D deformation preceding the 2018 phreatic eruption at Iwo-yama volcano (south of Kyushu Island, Japan), using a combination of airborne and spaceborne Interferometric Synthetic Aperture Radar (InSAR) data. The 3D and 2.5D deformation maps obtained at different periods allow us to successfully track their spatiotemporal evolution and to infer the transition of subsurface conditions responsible for the precursory deformation observed from 2014 to 2018. From 2014 to 2016, ground inflation depicted an axisymmetric pattern with the maximum displacement at the center of the deformed area. However, from 2016 to 2018, an inflation peak moved to the southern edge of the area deformed during 2014–2016 and became more localized, which was close to the newly generated vents in the 2018 eruption. Modeling of the inflations suggests that pressurization within a crack at a depth of 150 m beneath the Iwo-yama geothermal area caused the 2014–2016 deformation and had continued until the 2018 eruption. Modeling results highlight the persistence of the local ground inflation pattern just above the southern edge of the crack, which suggests the presence of a shallower inflation source contributing to the local inflation. Consequently, we interpret the sequence of these deformations as follows: from 2014, deeper-rooted fluid started to inject into a fluid-saturated crack at 150-m depth, which caused the 2014–2016 deformation. Then, after 2016, the crack inflation continued because of the continuous fluid injection and formed another pressurized part directly above the southern tip of the crack. Additionally, the results of the time-series analysis of the satellite InSAR data revealed that the local inflation started around April 2017 for which thermal activity including a mud emission became pronounced around the location of the local inflation. As a result of an episodic increase in supply rate of magmatic fluids from a deep magma reservoir from early 2018, a phreatic eruption finally occurred in the vicinity of the most deformed point, providing a clue for predicting future eruption sites, as was also observed in the Hakone 2015 eruption.

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: Eruption and intrusion of hydrothermal fluid observed by a borehole tiltmeter network

“…During the 2018 MPCG eruption we detected a progressive ground deformation by a borehole tiltmeter network. Although satellite synthetic aperture radar (SAR) is useful for observing spatial surface deformation associated with a phreatic eruption (e.g., Kobayashi et al 2018;Narita et al 2020), a tiltmeter network with high temporal resolution and dense spatial distribution provides details of progressive ground deformation during an eruption (e.g., Ueda et al 2013;Aloisi et al 2019;Zobin et al 2020). Our crack model implies that inflation of 5.1 × 10 5 m 3 beneath the main crater (MC) at Kagamike-kita PC for 2 min was followed by deflation of 3.6 × 10 5 m 3 .…”

Imaging the Hydrothermal System of Kirishima Volcanic Complex, Japan with L-band InSAR Time Series

“…1g). There is a minor peak location change during 2016-2018 around the Iwo-yama crater (Narita et al, 2020). Geochemical analysis in 2016 shows that the water is meteoric (Tajima et al, 2020), suggesting the upward migration of magmatic fluid is not the primary direct driver.…”

Imaging the Hydrothermal System of Kirishima Volcanic Complex With L‐Band InSAR Time Series

“…1gi). Localized pre-eruptive inflation suggests a very shallow pressure source at 150 m depth beneath the crater (Narita et al, 2020). Fumarolic activity and mud ejection continued from the southern and western vents as of December 2019 (JMA, 2019).…”

“…The unrest of Iwo‐yama started with an increase in seismicity in December 2013, followed by tremors in August 2014, thermal anomalies and weak fumarolic activity since December 2015, a steam blowout event on April 27, 2017, at the crater, and small phreatic eruptions on April 19 and 20, 2018, at two newly appeared vents on the southern and western side of the crater with plume heights of 100–200 m (Tajima et al., 2020; dashed blue lines in Figures 1g–1i). Localized pre‐eruptive inflation suggests a very shallow pressure source at 150 m depth beneath the crater (Narita et al., 2020). Fumarolic activity and mud ejection continued from the southern and western vents as of December 2019 (Japan Meteorological Agency (JMA), 2019).…”

“…Geophysical monitoring of phreatic eruptions has been challenging due to their localized deformation signals with small magnitude and sudden occurrence with few if any precursors (Kobayashi et al., 2018). Although not as frequently as magmatic processes, ground deformation associated with hydrothermal processes has been observed in all stages of volcanic eruption cycles, including the pre‐eruptive (Kobayashi et al., 2018; Narita et al., 2020), co‐eruptive (Himematsu et al., 2020), post‐eruptive period (Hamling et al., 2016; Nakaboh et al., 2003; Narita & Murakami, 2018), and non‐eruptive unrest period (Battaglia et al., 2006; Lu et al., 2002; Lundgren et al., 2001).…”