Oil and gas activities in Oklahoma have led to unprecedented seismic hazard and risk levels
Iason Grigoratos
University of Texas at Austin
- Date & Time
- Location
- Online-only seminar via Microsoft Teams
- Summary
In the past decade, Oklahoma has experienced an unprecedented 100-fold increase in seismicity rates, including 4 events with Mw >= 5. These earthquakes affect infrastructure that was designed with little to no consideration of seismic loads, leading to some damages and widespread unrest in the public. Our goal was to assess the time-dependent nature of the seismic hazard and risk in Oklahoma and identify how much of the seismicity rate increase is related to wastewater injection, a widely used process in the oil and gas industry.
To that end, we develop a semi-empirical model to simulate the observed seismicity given the injection time-history. The proposed earthquake recurrence model is a modified version of the Gutenberg-Richter relation and builds upon the Seismogenic Index model [Shapiro et al., 2010] and the stressing-rate dependency of the time lag between injection and changes in the seismicity rates. First, we employ this model to test the statistical significance of the prevailing hypothesis that the seismicity rate changes are driven by pore-pressure changes related to wastewater injection. The results show that 84% of the recent felt seismicity in Oklahoma can be associated with wastewater injection at a 95% confidence level.
We also forecast seismicity rates given future injection scenarios and analyze the observed magnitude-frequency distributions, arguing that the reported elevated values of the Gutenberg-Richter b-value are an artifact of the finiteness of the pore-pressure perturbation areas around the injection wells. In terms of forecasting power, the model is able to predict the evolution of the seismicity burst starting in 2014, both in terms of timing and magnitude, even when only using seismicity data through 2011 for the calibration. Under a conservative scenario of injection rates, the seismicity is expected to reach pre-2009 levels after 2025, while the probability of a potentially damaging event remains at substantial levels.
Finally, we perform a probabilistic assessment of the time-dependent seismic hazard in Oklahoma and incorporate these results into an integrated seismic risk model to assess the evolution of the statewide potential for earthquake-induced losses. The resulting seismic hazard maps reach at times levels found across the San Andreas fault, illustrating the incompatibility of the seismic provisions in the state with the observed seismicity in recent years. Our direct-loss estimates are in reasonable agreement with the paid insurance claims, but show significant sensitivity to the ground motion model selection. During the peak of seismicity in 2015, the seismic risk was 275 times higher than the background level, with the majority of the losses originating from damages to non-structural elements.
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