Small‐Magnitude Earthquakes in North‐Central Wyoming Recorded during the Bighorn Arch Seismic Experiment

Abstract

We took advantage of a high‐density seismic array deployed coincident with USArray to study small‐magnitude seismicity patterns in northern Wyoming. We detected and located a total of 89 earthquakes ranging in magnitude from −0.5 to 2.4 during the six‐month period of April to September 2010. Most events were located shallower than 25 km in the crust, with a few events as deep as 39 km and one very deep earthquake below the Moho, at 63 km. We also detected and recorded 1563 mining explosions in the area. We computed focal mechanism solutions for 14 earthquakes and observed dominantly normal faulting. Stress inversions obtained from these focal mechanisms show extension oriented ∼N20° E that matches borehole breakout data collected in the area. Though wastewater injection wells were active in the region, we see no evidence of induced seismicity in this catalog. The seismicity pattern shows earthquakes occurring beneath areas of high topography in the Bighorn and Owl Creek Mountain ranges. This may indicate a preference to slip on pre‐existing weaknesses created during mountain building. The absence of earthquakes between 39 and 63 km depths represents an aseismic zone in the lower crust that may indicate a rheological change between brittle and ductile behavior in the crust, and a strong mantle capable of brittle failure. Online Material: Maps of earthquake catalogs, figures of depth versus misfit, time of day histograms for earthquakes and mine explosions, earthquake density, and location of class II disposal wells relative to earthquakes.