Volcano instability induced by strike-slip faulting

Analogue sand cone experiments were conducted to study instability generate d on volcanic cones by basal strike-slip movement. The results of the analo gue models demonstrate that edifice instability may be generated when strik e-slip faults underlying a volcano move as a result of tectonic adjustment. This instability occurs on flanks of the volcano above the strike-slip she ar. On the surface of the volcano this appears as a pair of sigmoids compos ed of one reverse and one normal fault. In the interior of the cone the fau lts form a flower structure. Two destabilised regions are created on the co ne flanks between the traces of the sigmoidal faults. Bulging, intense frac turing and landsliding characterise these unstable flanks. Additional analo gue experiments conducted to model magmatic intrusion show that fractures a nd faults developed within the volcanic cone due to basal strike-slip motio ns strongly control the path of the intruding magma. Intrusion is diverted towards the areas where previous development of reverse and normal faults h ave occurred, thus causing further instability. We compare our model result s to two examples of volcanoes on strike-slip faults: Iriga volcano (Philip pines), which underwent non-magmatic collapse, and Mount St. Helens (USA), where a cryptodome was emplaced prior to failure. In the analogue and natur al examples, the direction of collapse takes place roughly parallel to the orientation of the underlying shear. The model presented proposes one mecha nism for strike-parallel breaching of volcanoes, recently recognised as a c ommon failure direction of volcanoes found in regions with transcurrent and transtensional deformation. The recognition of the effect of basal shearin g on volcano stability enables prediction of the likely direction of eventu al flank failure in volcanoes overlying strike-slip faults.