Tectonically-induced dike dilation as a model for long-duration episodes of precursory volcanic seismicity

Some explosive eruptions are preceded by episodes of volcano-seismic unrest lasting several months to a year or more. Volcanotectonic (VT) seismicity occurring during such episodes is often recorded at shallow depths many months before the eruption onset, and reflects a locally reoriented stress field suggestive of magma pressurization at shallow levels. However, the explosive nature of the ensuing eruption and other petrologic evidence often argue against prolonged shallow magma storage prior to these eruptions. Thus a new model for the generation of early precursory VT seismicity is required that accounts for all seismological and petrologic observations. It is hypothesized that a 'passive dike opening' model may account for long-duration precursory seismicity characterized by shallow and/or random VT hypocenters and reflecting a localized ~90° stress field reorientation, without requiring ascent of magma to seismogenic levels until a later stage of precursory unrest. Using a combination of gelatin analog and numerical models, it is shown that a localized increase in the magnitude of regional maximum compressive stress in a block of crust containing a mechanically weakened zone results in dilation of a dike-like conduit through which magma can subsequently rise. Crack dilation is accompanied by a local stress field perturbation that may induce early-stage VT seismicity throughout the brittle crust but focused at shallow levels. Model results are compared to patterns of precursory VT seismicity from the 1992 eruptions of Crater Peak, Alaska, and are found to be in good agreement with observations.