DIKE INTRUSION AS A TRIGGER FOR LARGE EARTHQUAKES AND THE FAILURE OF VOLCANO FLANKS

Pore fluid pressures induced through dike intrusion have the capability to trigger large earthquakes and to initiate and sustain massive and catastrophic failure of volcano flanks. Suprahydrostatic pore pressures are generated as a result of both mechanical and thermal straining of the rock-fluid medium. Mechanical strains and resulting pore pressures are described using an analogy to a moving volumetric dilation within a porous elastic medium. Thermal pore fluid pressures are simply represented by a static diffusive model subjected to uniform temperature rise at the dike interface. Resulting excess pore pressure distributions acting along the base of a wedge-shaped slide block are used to define the stability of the flanks when subjected to the magma pressures that accompany intrusion. The destabilizing influence of mechanically induced pore pressures is maximized as the intruded width, or corresponding overpressure, of the dike is increased. For realistic parameter magnitudes in volcaniclastic materials, induced pore pressures are capable of initiating failure; however, the mechanical influence is restricted to the proximity of the intrusion, and by itself may not be capable of sustaining sliding motion once it is initiated. The destabilizing influence of thermally induced pore pressures is conditioned by the severity of thermal forcing, ratios of thermal and hydraulic diffusivities, and the time available for the combined thermal and pressure disturbance to propagate outward from the dike. Thermal pressurization of pore fluid extends more slowly than mechanical straining but is shown to be capable of developing massive uplift forces that could initiate and possibly sustain failure along downslope-dipping failure planes. Deviatoric stress-induced generation of pore pressures and frictional heating of pore fluids may act following slide initiation to maintain the impetus of flank failure and enable long runout over compressible marine sediments. In addition to the development of shallow failure, Limited deep-seated sliding may also result from the mechanical or thermal pore pressure loading mechanisms. In addition, pore pressures generated through intrusion or frictional heating may trigger, or amplify, large earthquakes, and these in turn may aid shallower flank slip through seismic ground accelerations and vibration-induced pore pressure generation. These phenomenological models of pore pressure rise and stability control may aid comprehension of the cyclic growth, lateral expansion, and subsequent destruction of shield and stratovolcano flanks. Examples in this work refer particularly to oceanic volcanoes typified by the Hawaiian Islands and Reunion Island.