Abstract
We analyzed campaign Global Positioning System observation data in Kuchinoerabu-jima during 2006–2014. 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 km3. 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 m3. Taking effects of topography and heterogeneity of ground into account allowed reproduction of overall deformation in Kuchinoerabu-jima. 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 m3 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.
Highlights
Kuchinoerabu-jima is an andesitic volcano, south off Kyushu Island, Japan (Fig. 1)
It is difficult to obtain size and shape of the spherical source which must be small enough compared to its depth, and homogeneous half space may not be realistic considering complicated surface topography near the crater
As we used benchmarks inside Kuchinoerabu-jima in the present study, topography was introduced in the area within Kuchinoerabu-jima using digital elevation model (DEM) data provided by Kagoshima prefecture, and elevation of the outside area was assumed to be sea level
Summary
Kuchinoerabu-jima is an andesitic volcano, south off Kyushu Island, Japan (Fig. 1). The volcano consists of three cones: No-ike, Shin-dake and Furu-dake; and the Shindake is presently active. Ground inflation detected by campaign GPS observations was significant. It is characterized by large displacements around Shindake crater in contrast to minimal changes at the foot of the volcano. This suggests a shallow source, and a source was obtained at a depth of < 300 m below sea level (bsl) under an assumption of Mogi-type source (Mogi 1958) in homogeneous half space (Iguchi et al 2007). It is difficult to obtain size and shape of the spherical source which must be small enough compared to its depth, and homogeneous half space may not be realistic considering complicated surface topography near the crater. A number of studies have applied a finite element method (FEM) to the modeling of ground deformation in order to examine how topographical effects or heterogeneity of ground affect and found that such factors affect to the results of deformation source analysis (e.g., Zhao et al 2004; Ronchin et al 2015)
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