Seismicity induced by seasonal groundwater recharge at Mt. Hood, Oregon

Groundwater recharge at Mt. Hood, Oregon, is dominated by spring snow melt which provides a natural large-amplitude and narrow-width pore-fluid pressure signal. Time delays between this seasonal groundwater recharge and seismicity triggered by groundwater recharge can thus be used to estimate large-scale hydraulic diffusivities and the state of stress in the crust. We approximate seasonal variations in groundwater recharge with discharge in runoff-dominated streams at high elevations. We interpolate the time series of number of earthquakes, N, seismic moment, M-o, and stream discharge, Q, and determine cross-correlation coefficients at equivalent frequency bands between Q and both N and M-o. We find statistically significant correlation coefficients at a mean time lag of about 151 days. This time lag and a mean earthquake depth of about 4.5 km are used in the solution to the pressure diffusion equation, under periodic (I year) boundary conditions, to estimate a hydraulic diffusivity of kappa approximate to 10(-1) m(2)/s, a hydraulic conductivity of about K-h approximate to 10(-7) m/s, and a permeability of about k approximate to 10(-15) m(2). Periodic boundary conditions also allow us to determine a critical pore-fluid pressure fraction, P'/P-0 = 0.1, of the applied near-surface pore-fluid pressure perturbation, P-0 approximate to 0.1 MPa, that has to be reached at the mean earthquake depth to cause hydroseismicity. The low magnitude of P' approximate to 0.01 MPa is consistent with other studies that propose 0.01 less than or equal to P' less than or equal to 0.1 MPa and suggests that the state of stress in the crust near Mt. Hood could be near critical for failure. Therefore, we conclude that, while earthquakes occur throughout the year at Mt. Hood, elevated seismicity levels along pre-existing faults south of Mt. Hood during summer months are hydrologically induced by a reduction in effective stress. (C) 2003 Elsevier B.V. All rights reserved.