USING A LARGE-N SEISMIC ARRAY TO EXPLORE INJECTION-INDUCED EARTHQUAKES IN NORTHERN OKLAHOMA (Invited Presentation)

The LArge-n Seismic Survey in Oklahoma (LASSO) array densely recorded a region of active induced seismicity in northern Oklahoma. The seismicity is associated with local disposal of saltwater, a byproduct of hydrocarbon production, into deep wells. Installed in 2016, the month-long deployment of 1,833 vertical-component nodes had a nominal station spacing of ~400 m and covered a 25-km-by-32-km area. The raw dataset of 500 sample-per-second continuous data is 8.7 Tb. Using an short term average over long term average method we developed a catalog of 1,375 earthquakes located within ~5 km of the array footprint. We used this catalog to explore the spatio-temporal evolution of seismicity, with a focus on fault segments that host larger (M2.5+) earthquakes during the deployment period. We find that these sequences often have similar behavior, where the majority of mainshocks are followed by short (~24 hr) aftershock sequences before activity returns to background levels. We also observe some sequences that have fairly consistent seismicity rates and migrate spatially through time, over the time span of the deployment. Using the array we also find evidence for delayed dynamic triggering following the teleseismic surface wave arrivals from the 16 April 2016 M7.8 Ecuador earthquake. Dynamic Coulomb stress changes associated with this earthquake are on the order of ~1 kPa, suggesting a high susceptibility to small stress changes. The delay in triggering (~8 hr) suggests aseismic processes or hydrological responses to transient stress perturbations may drive fault failure. Planned work includes estimation of focal mechanisms to explore variability of fault orientations in the region, as well as determination of stress drops and their associated uncertainties.