We are studying the how the uppermost part of the Earth's crust behaves when it also contains fluids such as water, using data from the borehole strainmeter (BSM) network operated by the Plate Boundary Observatory (PBO) component of Earthscope. This network includes measurements of ground deformation (strain) and fluid pressure at sites throughout the western United States, some of them in an active fault zone: the San Jacinto fault in southern California. We are examining how the strain and fluid pressure change because of changing air pressure acting on the ground, seismic waves from distant earthquakes, and the tidal motions of the Earth's crust. We are combining this information with data on the fractures around each borehole to estimate hydraulic properties of the surrounding material, and relate these to large-scale features (such as fault zones) in each region of the network. At one site we are also looking at how groundwater pumping (which we monitor) affects two strainmeters nearby. We are developing methods to correct these and other BSM data for nearby pumping or other groundwater changes.
