Chris H. Okubo
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Volcano Stability
Shaded relief map of Kīlauea volcano, Hawai‘i, showing the location of the Hilina Pali fault system and the Hilina slump. The location of the modeled profile is shown along line A-A'.
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The Hilina Pali fault system, located along the southeast coast of Kīlauea volcano, Hawai‘i, has been interpreted as heads carps of an active landslide [Swanson et al., 1976] with an volume of ~10,000–12,000 km3 [Smith et al., 1999]. Such giant landslides have been identified elsewhere in Hawai‘i, and on other volcanic islands [Duffield et al, 1982; Elsworth and Day, 1999], based on observations of ocean-facing fault scarps and large chaotic seafloor debris fans [Moore et al., 1994; Moore et al., 1995]. Active slides have also been identified based on coseismic displacements [Swanson et al., 1976; Lipman et al., 1985; Denlinger and Okubo, 1995].
I conduct limit equilibrium analyses [Janbu, 1954; Bishop, 1955] in order to gain insight into the stability and subsurface geometry of volcano landslides such as the Hilina slump. Limit equilibrium analyses compare the shear strength of the basal detachment against any prescribed driving stresses (e.g. weight of the slump, fluid or seismic pressure). The ratio of the basal shear strength resisting slip to the prescribed driving stresses is referred to as the Factor of safety (Fs) for the slide. Slip along the detachment surface is predicted when Fs<1.
The overall shear strength of a fractured rock mass, such as a volcanic edifice, is largely controlled by the shear strength of preexisting fractures. This is because faulting occurs at lower stresses along preexisting fracture surfaces than through intact rock. Therefore assessment of fracture shear strength can lead to practical estimations of the overall rock mass shear strength. Relations between fracture shear strength and rock mass shear strength have been developed for some time for use in the mining and geotechnical community. Common rock mass rating systems, such as the Rock Mass Rating system [Bieniawski, 1976; 1989] or Geologic Strength Index [Hoek, 1994; Hoek et al., 1995] rely upon field–based determinations of:
- Fracture aperture
- Fracture roughness
- Strength of the fracture wall rock
- Strength of fracture fill (e.g. gouge)
- Fracture density
- Groundwater conditions within fractures
- Strength of the intact rock
Note that the majority of the rock mass strength is controlled by fracture properties. Once a rating or strength index is determined in the field, mechanical strength parameters, such as friction coefficient and shear modulus, are calculated from sets of empirical equations [e.g. Hoek, 1983; Hoek and Brown 1997] that are based on back analyses of failed mines, foundations, and natural and engineered slopes.
Model-predicted least-stable basal detachment geometries and the attendant ranges of critical ground accelerations along line A-A'.
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Using such measurements of rock mass strength and deformability, as well as limit equilibrium methods in the software package XSTABL, I have analyzed the stability of the Hilina slump for circular and non-circular basal detachment geometries. During nucleation and incipient slip of a basal detachment, the pre-failure strength of the rock mass resists movement of the slump. As a well-defined failure surface (fault) develops with increasing displacement, the post-failure strength of the rock mass provides resistance to continued slip. Using the pre- and post-failure strengths to bracket my models, I have determined that the Hilina slump is predicted to be stable under gravitational loads. Failure (sliding) of this slump is predicted under ground accelerations greater than 0.2g for the pre-failure strength models, and 0.1g for post-failure strength models. These results are significant, given that current earthquake hazard maps of the area predict a 10% probability of exceedance in 50 years for seismic ground accelerations greater than 2g.
Stability curves for the modeled section of the Hilina slump. Ground accelerations greater than ~0.45-0.65 g are predicted to cause slip along the least stable failure surfaces.
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Detailed information on my volcano stability research is available in a paper, "Rock mass strength and slope stability of the Hilina slump, Kīlauea volcano, Hawai‘i", published in the Journal of Volcanology and Geothermal Research (available for download through my Publications page).
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