Sc. Moran et al., A model for the magmatic-hydrothermal system at Mount Rainier, Washington,from seismic and geochemical observations, B VOLCANOL, 61(7), 2000, pp. 425-436
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.