Seismicity Declustering and Hazard for Oklahoma and Kansas

This project evaluates the suitability of several declustering algorithms for induced seismicity and their impacts on hazard analysis in Oklahoma and Kansas. We consider algorithms proposed by Gardner and Knopoff (1974), Reasenberg (1985), Zaliapin and Ben-Zion (2013) and the stochastic declustering method (Zhuang et al., 2002) based on the epidemic type aftershock sequence (ETAS) model (Ogata, 1988, 1998). Results show that the choice of declustering algorithm has a significant impact on the declustered catalog as well as the resulting hazard analysis in Oklahoma and Kansas. The Gardner and Knopoff algorithm, which is currently implemented in the U.S. Geological Survey one-year seismic hazard forecast for the central and eastern U.S., has unexpected features when used for this induced seismicity catalog. It removes 80% of earthquakes and fails to reflect the changes in background rates that have occurred in the past few years. This results in a slight increase in the hazard level from 2016 to 2017, despite a decrease in seismic activities in 2017. The Gardner and Knopoff algorithm also frequently identifies aftershocks with much stronger shaking intensities than their associated mainshocks. These features are mostly due to the window method implemented in the Gardner and Knopoff algorithm. Compared to the Gardner and Knopoff algorithm, the other three methods are able to capture the changing hazard level in the region. However, the ETAS model potentially overestimates the foreshock effect and generates negligible probabilities of large earthquakes being mainshocks. The Reasenberg and Zaliapin and Ben-Zion methods have similar performance on catalog declustering and hazard analysis. Compared to the ETAS method, these two methods are easier to implement and faster to generate the declustered catalog. Results from this study suggest that both Reasenberg and Zaliapin and Ben-Zion declustering algorithms are suitable for declustering and hazard analysis for induced seismicity in Oklahoma and Kansas.