Comparison of the stress-stimulated current of dry and fluid saturated gabbro samples

Abstract We investigate charge generation as a function of stress in fine-grained gabbro for both nominally dry samples and samples fully saturated with electrically conductive brine fluids similar to those observed in active earthquake fault zones. These experiments address a number of proposed and reported electrical precursory and coseismic phenomena associated with earthquakes. Compressive load was applied to one end of the sample in repetitive cycles using a pair of precision steel platens driven by a large hydraulic press. The samples were tested by cycling between constant low stress and constant high stress values with a 200-s periodicity. Net charge transport between the stressed and unstressed sample ends was monitored with a picoammeter. For the nominally dry samples, stress-stimulated current (SSC) transients on the order of 50–400 pA peak-to-peak were observed with a decay time constant ∼ 10 s during stress loading and unloading. Under constant compressive loads of ∼ 22 MPa, small negative polarity SSC of ∼ 15 pA magnitude was observed as an offset from the baseline current at low load (5 MPa) conditions. For the fluid-saturated samples, neither transients nor SSCs were observed as a function of stress when the load was cycled, an observation that is consistent with more rapid internal self-discharge due to higher electrical conductivity of the sample. Because the Earth’s crust is fluid saturated, observation of significant electrical charge buildup is not expected during the observed slow stress accumulation prior to earthquakes or during any slow precursory stress release that may occur in the region of earthquake nucleation. However, observation of coseismic charge generation due to electrokinetic, triboelectric, and other processes may occur during earthquake stress drops, surface rupture, and seismic-wave arrivals from dynamic rupture.