Fluid flow and the state of stress at Mt. Hood, Oregon, inferred from microseismicity induced by groundwater recharge

Pore pressure diffusion related time delays between seasonal groundwater recharge and seismicity can be used to estimate large-scale hydraulic diffusivities. Groundwater recharge rates in Volcanic Arcs such as the Oregon Cascades can be large. Here, the annual precipitation rate is about 2 m of which over 50 % infiltrates the ground mostly during snowmelt in spring. Therefore, infiltration rates of > 1 m per year can occur. Due to near-surface porosities of 5 to 10 % groundwater levels may fluctuate annually by 10 to 20 m resulting in pore fluid pressure variations of 1-2 × 10^5 Pa and more. We approximate the seasonal variations in groundwater recharge with discharge in runoff-dominated streams that show a peak discharge during snow melt in spring. Time series of daily number of earthquakes and seismic moment as well as stream discharge are interpolated by applying piecewise polynomial fits. Thus, the series can be cross correlated at equivalent frequency bands. Piecewise overlapping cross correlation coefficients are determined between stream discharge and both number of earthquakes and seismic moment. We find statistically significant correlation coefficients at a time lag of about 150 days. This time lag and a mean earthquake depth of 4.5 km are used in the solution to the pressure diffusion equation, under periodic (1 year) boundary conditions, to estimate the hydraulic diffusivity (0.3 m^2/s). Assuming a reasonable specific storage of about 5.5 × 10^-6 1/m implies a hydraulic conductivity of approximately 2 × 10^-6 m/s. The latter value is comparable with our results from coupled heat and groundwater flow studies that are based on bore hole temperature data at Mt. Hood. Moreover, the periodic boundary condition allows us to determine a critical fraction, P/P_0, of the applied zero-depth pore fluid pressure, P_0, that has to be reached to cause seismicity at the given mean earthquake depth (4.5 km). Here, P/P_0 ≈ 0.24 so that for the estimated 0.1 <= P_0 <= 0.2 MPa the critical pressure to cause hydroseismicity is approximately 0.02 <= P <= 0.05 MPa. The low value in P suggests that the state of stress in the crust near Mt. Hood is near critical for failure.