Geomechanics of volcano instability and the effects of internally elevated pore fluid (gas) pressures

Volcano flank collapse events affect volcanic edifices where a range of different processes are at work. However, there is at least one mechanism of generating instability that may be present at all volcano collapse locations: increases of edifice pore fluid pressures from internal sources. The use of geomechanical classification schemes such as the Rock Mass Rating (RMR) show that a volcano can be considered mechanically weak and little more than a pile of granular material. Initial field work and laboratory testing demonstrated that it is possible to estimate volcanic rock-mass compressive ([sigma][sub]cm), tensile ([sigma][sub]tm) and cohesive (c) strength from the RMR using the power law relationships ([sigma][sub]cm = O.5161e[sup](0.0581*RMR), ([sigma][sub]tm = 0.0055e[sup](0.0744*RMR) and c = 0.0349e[sup](0.0649*RMR) . Simple analogue models using sand piles as scaled proxies for a volcano edifice demonstrate that internal gas pressure is a viable mechanism for promoting structural instability. Complementary two-dimensional limit equilibrium methods (LEM) confirm this effect, showing a clear reduction in edifice stability with increasing degrees of internal pressurisation. However, internal pressures in excess of 25 MPa are needed to reduce the Factor of Safety below unity, indicating this mechanism is unlikely to be the solitary contributor to sector collapse. Three-dimensional numerical modelling of edifice stability using FLAc[sup]3D provides a sophisticated means of undertaking a complex analysis of volcano instability caused by internal pressurisation. Five model geometries were examined over a pressure range of 0 to 20 MPa that allowed the sensitivity of gas pressure on structural stability to be assessed quantitatively. Significant reductions in stability were observed in all cases, with the most unstable modelled edifice possessing a combination of 'weak' foundations and shallow regional gradient. A key finding is that the instability observed in both the analogue and LEM results are replicated in the 3D numerical models, confirming for the first time the significance of internal (gas) pressurisation as a potential trigger mechanism for volcano flank failure.