An observation-based model for a vapor-driven crack growth leading up to the phreatic eruption of Mt. Ontake in 2014

Abstract:

At Mt. Ontake, central Japan, a phreatic eruption took place on 27 September 2014. The eruption was preceded for 25 s by a very long period (VLP) seismic event and for 450 s by an accelerated tilt change showing summit uplift. To deepen our understanding of the initiation of the phreatic eruption, we conducted waveform inversion and time series analyses of these preceding events.

Our waveform inversion of the VLP event pointed to an SSE-NNW strike subvertical tensile crack at around 600 m beneath the eruptive vent region. This crack orientation was subparallel to alignments of volcano-tectonic earthquake hypocenters (Kato et al., 2015) and eruptive vents (Geospatial Information Authority of Japan, 2014) as well as one of the maximum shear directions of the regional stress field (Terakawa et al., 2015). These observations suggest that the VLP source crack was one of the preexisting faults along the maximum shear direction that opened due to passage of ascending gas from depth to the surface.

Our waveform inversion of the tilt change pointed to an E-W to SE-NW strike subvertical tensile crack at around 1000 m below the surface. To investigate a background physics of the tilt change, we calculated the first and second order integrals of the tilt waveforms (I₁(t) and I₂(t), respectively, where t is time). The ratio I₂(t)/I₁(t) initially increased linearly with time and then reached an almost constant value. This time evolution is well modeled by a linear increase of the source crack volume

V(t)=V₀t/t₀ (t<t₀)

followed by an exponential growth

V(t)=V₀exp{(t-t₀)/τ} (t>t₀),

where V₀ is constant, τ≈84 s, and t₀/τ≈2. The initial linear growth of the crack volume may be explained by inflation of water vapor in the vertical crack under constant pressure and heat supply conditions. The latter exponential crack growth may be modeled by a force balance between an overpressure of 10⁴–10⁵ Pa and a viscous resistance of the water vapor near the crack tip (Lister and Kerr, 1991). These models, although tentative, would provide observation-based constraints on a heat-rock-fluid interaction leading up to the phreatic eruption of Mt. Ontake in 2014.