Understanding Saltwater Disposal-Induced Earthquakes: Insights from Seismic Observations and Modeling

In recent decades large volumes of saltwater, a byproduct of oil and gas production from unconventional reservoirs, were disposed into the subsurface resulting in increased earthquake rates. Here, we integrate seismicity observations and earthquake simulations to explore disposal-induced earthquake sequence behavior. Data recorded by dense seismic instrumentation in areas of saltwater disposal illuminate a range of spatiotemporal sequence evolutions. Disposal-induced earthquake sequences in Oklahoma and Kansas show strong variability in their magnitude distributions and clustering. Some sequences have recurrence times that are consistent with being driven by a steady stress increase, such as stresses due to long-term injection, while other sequences display more typical mainshock-aftershock clustering reflecting earthquake-earthquake triggering. We also find that clustered induced sequences tend to have more extended foreshock sequences than is typically observed for natural earthquakes. Pore-pressure modeling suggests that these behaviors correlate with pore-pressure changes from widespread injection. Seismic data can also map crustal stress orientations and reveal the types and orientations of activated faults. We find no evidence that fluid injection changes local stress orientations, but that faults that are both well and poorly oriented in the stress field can fail. Additionally, we explore the initial stress conditions and injection protocols that influence sequence behaviors using an earthquake simulator, RSQSim. We find that the maximum magnitude of the triggered events depends strongly on the pre-existing fault stress, such as initial stress level and heterogeneity. The specific injection schedule can influence sequence statistics, including the number of events and their distribution away from the injection well. Our observations and modeling provide insights into the first-order effects of how pore-pressure changes induce fault slip. By combining observation and modeling studies we can develop insights into the processes by which fluid injection may induce earthquakes, improving the potential for mitigating the resulting seismic risk.