FACTORS INFLUENCING SUBSURFACE FAILURE GEOMETRY IN LARGE-SCALE VOLCANO COLLAPSE

Large-scale volcano collapse was first recognized in Javanese volcanoes by Van Bemmelen in 1949. However, it was not until the 1980 rockslide avalanche on Mt. St Helens, that catastrophic sector collapse was recognized as being common to volcanoes worldwide. Volcano collapse has unique characteristics distinctive to the volcano environment when compared to non-volcanic landslides, leaving distinctive horseshoe-shaped craters open at one end, reminiscent of glacial cirques characteristic of past glaciations. The largest of the catastrophic collapses are one to two orders of magnitude greater in volume than for other landslides in non-volcano terrain with bottom failure surfaces generally far flatter than that for non-volcanic failures and reminiscent of decollment surfaces. What produces this distinctive failure morphology? Rock strength anisotropy, internal structure, the in-situ stress field, fluid pressure distribution, lava injection and earthquake loading are possible factors. Volcanic edifices are inherently weak due to their composition of alternating strong (lava) and weak (ash) rock intermeshed with fractured and shattered domes and occasionally dikes. Overall the rock mass strength is low and volcanic materials subjected to the affects of hydrothermal alteration, further reduce the already low strength rock by the production of clay rich rocks.

Preliminary scoping investigations illustrate that some failure scenarios are more likely than others to produce the characteristic morphology associated with volcano collapse. Large-scale man-made earth structures are shown to fail along low permeable materials and within low strength rock material analogous to exposed shallow dipping failure surfaces observed after volcano collapse. Variations in pore pressure build up within units of different permeability in response to rapid stress increase can produce instability with the development of low lying failure surfaces.