Repeating Earthquakes and Shear Wave Anisotropy Measurements from an Induced Seismicity Case Study, Wattenberg Disposal Zone, CO

Since June of 2014, a network of seismometers has been operating to monitor wastewater injection and the evolution of induced seismicity near Greeley, CO. A seismic catalog collected between 2014 and 2018 features more than 1000 manually detected local earthquakes within the upper 10 km of Earth's crust. We use a selection of these events as templates in a waveform cross-correlation procedure to expand the catalog, and to screen for clusters of repeating or near-repeating events. The seismograms often exhibit shear wave anisotropy, as indicated by shear wave splitting measurements collected at one station in the network. We expand on these measurements and analyze temporal changes in the splitting parameters within the context of path-dependent anisotropy, inter-event timing, regional stress, monthly injection rates and down-well pressure measurements.

Fluid diffusion and the resultant dilation of grain boundaries and microfractures is one potential mechanism for shear wave anisotropy in the Wattenberg Disposal Zone. In the presence of a changing pressure gradient, a succession of shear waves may exhibit varying degrees of anisotropy as they propagate through fluid-saturated, stress-aligned pore spaces. In an effort to discriminate between static path effects and pressure-induced anisotropy, we focus on clusters of repeating and near-repeating events with long inter-event times. Perturbations in pore fluid pressure from high-rate wastewater injection is a key mechanism in many documented cases of anthropogenic induced seismicity. Seismic observations of shear wave anisotropy could lead to an improved understanding of the link between earthquake triggering and pore pressure evolution during wastewater injection.