A model for the magmatic-hydrothermal system at Mount Rainier, Washington,from seismic and geochemical observations

Mount Rainier is one of the most seismically active volcanoes in the Cascad e Range, with an average of one to two high-frequency volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Des pite this level of seismicity, little is known about its cause. The VT eart hquakes occur at a steady rate in several clusters below the inferred base of the Quaternary volcanic edifice. More than half of 18 focal mechanisms d etermined for these events are normal, and most stress axes deviate signifi cantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes in response to processes associated wi th circulation of fluids and magmatic gases within and below the base of th e edifice. Circulation of these fluids and gases has weakened rock and redu ced effective stress to the point that gravity-induced brittle fracture, du e to the weight of the overlying edifice, can occur. Results from seismic t omography and rock, water, and gas geochemistry studies support this interp retation. We combine constraints from these studies into a model for the ma gmatic system that includes a large volume of hot rock (temperatures greate r than the brittle-ductile transition) with small pockets of melt and/or ho t fluids at depths of 8-18 km below the summit. We infer that fluids and he at from this volume reach the edifice via a narrow conduit, resulting in fu marolic activity at the summit, hydrothermal alteration of the edifice, and seismicity.