AFTERSHOCK DENSITY DECAY IN SPACE AND TIME: OBSERVATIONS FOR OKLAHOMA AND IMPLICATIONS

Understanding the long-term seismic hazard in Oklahoma remains elusive due to decreasing yet historically elevated seismicity rates, targeted mitigation from the Oklahoma Corporation Commission for wastewater disposal rate reductions, and a combination of physical mechanisms at play in the near and far field from injection wells. Among these efforts lie assumptions regarding the physics of induced earthquake sequences in stable continental interiors. We endeavor to separate earthquake clusters from background seismicity to examine its role in the ongoing and future seismic hazard, since most hazard modeling is driven by background seismicity rates and hence declustered catalogs. We develop a framework using nearest-neighbor linear density measurements of stacked aftershock catalogs to systematically define mainshock-magnitude dependent space-time windows for aftershock identification in Oklahoma. We show that applying conventional windowing techniques (e.g. Gardner & Knopoff) can lead to skewed statistical interpretations of the data. Using refined space-time windows to identify earthquake clusters, we find the density decay of aftershocks in space to be more rapid in Oklahoma than in Southern California. The density decay of aftershocks in time is consistent with tectonically-driven aftershock sequences. These results show the necessity of well-founded, region-specific schema for aftershock identification in declustering procedures for probabilistic seismic hazard assessment. In addition, statistical seismology may elucidate unique properties of Oklahoma seismicity and provide first-order insight into the physics of induced earthquakes.