Effects of the Earthquake Cycle Observed in Pore Pressure Measurements

Abstract:

Well-aquifer systems respond to dynamic strains over a wide frequency band, making them sensitive to atmospheric pressure fluctuations, earth tides, and seismic waves. Dynamic pore pressures in damaged rock around active strike-slip fault systems are expected to have a reduced response to given dynamic strains: the rock will be in a drained state, with enhanced permeability and mechanical compliance, and cannot support elevated pressures over long times. Despite this expectation, spatial patterns of such a reduction in response are poorly understood. Here we use a set of strain and pressure measurements from Plate Boundary Observatory boreholes in southern California around the highly active San Jacinto fault to analyze variations in response to seismic waves. Stations near the major fault strand show a marked decrease in response compared to stations much farther away, and we show this reduction is controlled by secular shear-strain rates in the crust following an inverse power law. We propose that the spatial pattern in near-surface response is directly related to the earthquake cycle: recurring periods of high interseismic strain accumulation punctuated by large moment release reduce structural rigidity of the surrounding rock, and enhance its hydraulic transport characteristics.