Ground deformation source model at Kuchinoerabu-jima volcano during 2006–2014 as revealed by campaign GPS observation

Abstract

We analyzed campaign Global Positioning System observation data in Kuchinoerabu-jima during 2006Kuchinoerabu-jima during -2014. Most benchmarks located around Shin-dake crater showed crater-centered radial horizontal displacements. Horizontal displacements at western rim of the Shin-dake crater were tended to be larger compared to those at eastern rim. In addition, benchmark KUC14 which locates near the cliff at Furu-dake showed westward horizontal displacement rather than crater-centered radial (southward) one. Meanwhile, small displacements were detected at the benchmarks located at the foot of Kuchinoerabu-jima. We modeled the observed displacements applying a finite element method. We set entire FE domain as 100 × 100 × 50 km 3 . We set top of the domain as a free surface, and sides and bottom to be fixed boundaries. Topography was introduced in the area within Kuchinoerabu-jima using digital elevation model data provided by Kagoshima prefecture and elevation information from Google earth, and elevation of the outside area was assumed to be sea level. We assumed a stratified structure based on a one-dimensional P-wave velocity structure. We applied a vertical spheroid source model and searched optimal values of horizontal location, depth, equatorial and polar radiuses, and internal pressure change of the source using the forward modeling method. A spherical source with a radius of 50 m was obtained beneath the Shin-dake crater at a depth of 400 m above sea level. The internal pressure increase of 361 MPa yields its volume increase of 31,700 m 3 . Taking effects of topography and heterogeneity of ground into account allowed reproduction of overall deformation in Kuchinoerabujima. The location of deformation source coincides with hypocenters of shallow volcano-tectonic (VT) earthquakes and the aquifer estimated from a two-dimensional resistivity model by audio-frequency magnetotellurics method. The obtained deformation source may be corresponding to the pressurized aquifer, and shallow VT earthquakes and demagnetization may be caused by pressure and strain accumulation in the rocks around the aquifer. Applying the obtained spherical source to the tilt change before August 3, 2014 eruption, we found that 520 m 3 of volcanic materials were supplied toward shallower in 1.5 h before the eruption. The depth and volume change of deformation source before May 2015 eruption detected by precise leveling surveys is deeper and two orders of magnitude greater compared to that before August 2014 eruption.

Two independent signals detected by ocean bottom electromagnetometers during a non-eruptive volcanic event: Ogasawara Island arc volcano, Nishinoshima

“…We attempted to investigate the physical processes behind the observed variations, although providing a thorough interpretation from the data at only four available observation sites was a limiting factor. There are many reports regarding the change in the geomagnetic field around and deformation of, a volcanic edifice, which are frequently interpreted by thermal demagnetization and inflation of the volcanic edifice, respectively, due to ascending hot magma (e.g., Hotta and Iguchi, 2017;Takahashi et al 2018). Our data show that both the total magnetic force and tilt variations during Period A were more significant at sites NS03 and NS04.…”

The Onset, Middle, and Climax of Precursory Hydrothermal Intrusion of the 2018 Phreatic Eruption at Kusatsu‐Shirane Volcano

“…As well as the 2018 eruption, observations near the vent have detected short‐term precursory ground inflation of phreatic eruptions at some volcanoes (Aizawa et al., 2022; Hotta & Iguchi, 2017; Maeda et al., 2017; Usu Volcano Observatory, 1997; Wiens et al., 2005). As far as we know, a phreatic eruption at Ontake volcano in 2014 is the only event we can compare our results (the tilt onset, VLP, and the total inflation volume) and eruption scales (Table S3 in Supporting Information S1).…”

“…Triggering hydrothermal fluid is expected to expand rapidly at shallow depths beneath the vent, which originates explosive surface phenomena of phreatic eruptions (e.g., Kilgour et al., 2019; Maeno et al., 2016; Narin et al., 1979). The recent geophysical examination has often captured the precursor of phreatic eruptions in various time scales (e.g., Ardid et al., 2022; de Moor et al., 2016; Hotta & Iguchi, 2017). In particular, short‐term precursory on a scale of minutes is a promising phenomenon to link the final hydrothermal intrusion as an eruption trigger (Aizawa et al., 2022; Jolly et al., 2010; Maeda et al., 2017).…”