Edifice Instability and Basal Fault Friction

Edifice instability, which is a common feature of volcanoes, strongly affects eruptive phenomena and evolution of the volcano edifice. We investigate the interplay of several factors that determine sector failure of axi-symmetric volcanoes including slope of the volcano, the dip of the basal fault, sector angle, and effect of magmatic intrusion into flank rift zones. We employ a simple stability model of volcano sector failure in which failure is resisted by friction on the basal fault. In general, we find that axi-symmetric volcanoes are less stable, or require a higher coefficient of basal fault friction to resist failure, than Hawaiian-type linear rift volcanoes. Sector failures commonly define angles in the range 90°-120° - this is consistent with the model results, which show that larger sector angles to 180° are less favorable for failure. Also, negatively dipping basal faults are more stable than a horizontal or positively dipping fault. We apply this model to estimate the effective coefficient of friction of volcanoes built on a weak substrate such as Concepcion and Maderas in Nicaragua.