Earthquakes Induced by Wastewater Injection, Part II: Statistical Evaluation of Causal Factors and Seismicity Rate Forecasting

Wastewater disposal has been reported as the main cause of the recent surge in seismicity rates in several parts of Central United States, including Oklahoma. In this paper, we employ the semi-empirical model of the companion paper (Grigoratos et al ., 2020) first to test the statistical significance of this prevailing hypothesis, and then to forecast seismicity rates in Oklahoma given future injection scenarios. We also 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 porepressure perturbation areas around the injection wells and a more appropriate value would be close to 1.0. The results show that the vast majority (76%) of the seismically active blocks in Oklahoma can be associated with wastewater disposal at a 95% confidence level. 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 to calibrate the model. Under the current disposal rates, the seismicity is expected to reach the pre-2009 levels after 2030, while the probability of a a potentially damaging M w ≥ 5.5 event between 2018 and 2026 remains substantial at around 45%.