LARGE-SCALE PERMEABILITY MODELS FOR THE CASCADE ARC, INFERRED FROM HYDROLOGIC, THERMAL, AND SEISMIC DATA

The Cascades Arc is a volcanic range located along an active convergent plate boundary. In such a region, interactions between groundwater and both heat and earthquakes can be common and can provide valuable insight into the large-scale regional hydrogeology that is otherwise inaccessible to standard near-surface measurements. We use coupled heat and groundwater flow modeling techniques, borehole temperature measurements, seismicity data, estimates of magma intrusion rates, and spring discharge models to determine subsurface hydrogeologic properties. Because permeability largely determines groundwater flow rates, we investigate in particular the decrease in permeability with depth. Typical vertical permeabilities range from about log(k)=-13 to log(k)=-18.3 square meters for values near the surface to a depth of approximately z=12 km, respectively. These results are comparable to data compiled by Manning and Ingebritsen [Rev. Geophys, 1999] who suggest a permeability-depth curve of log(k)=-14 - 3.2log(z) for the continental crust in general. In addition, we suggest that elevated seismicity during summer in the Mt. Hood region is probably due to seasonal groundwater recharge. The time lag between groundwater recharge and seismicity, as well as the amplitude of the pore fluid pressure perturbation, further constrain the hydrogeologic model and provide insight into the state of stress in the crust in this region.