Hundreds of Earthquakes per Day: The 2014 Guthrie, Oklahoma, Earthquake Sequence

Abstract

Online Material: Gutenberg–Richter plot, b ‐values and associated p ‐values, magnitudes and number of detections per day, and earthquake catalog. A remarkable increase in seismic activity in Oklahoma since 2009 has been shown to correlate closely with enhanced hydrocarbon extraction and associated wastewater disposal; 99% of this recent Oklahoma earthquake activity has occurred within 15 km of a class II injection well (Ellsworth, 2013). In response to this increase in seismic activity, the U.S. Geological Survey (USGS) partnered with the Oklahoma Geological Survey (OGS) to exchange waveform data from permanent and temporary seismic stations to improve the cataloging of earthquake source parameters for a broad region of north–central Oklahoma. For a particularly persistent earthquake sequence near Guthrie, Oklahoma, a subspace detection method is applied to data from nearby seismic stations. This approach documents the occurrence of hundreds of readily detectable, highly similar, earthquakes per day, with rates occasionally exceeding 1000 earthquakes per day. Time‐varying changes in b ‐value appear episodic, suggesting a correlation with periods of reversible fault weakening and associated failure. Real‐time seismic monitoring typically uses automated transient detection‐based pickers to detect earthquakes and to define onset times of seismic phases. Such methods cannot reliably detect earthquake signals or time phases with low signal‐to‐noise, resulting in less complete earthquake catalogs than may be recovered by other means. Recognizing this limitation, recent studies of a suspected induced‐seismicity sequence near Youngstown, Ohio, used waveform template matching (Kim, 2013; Skoumal et al. , 2014) to significantly lower the detection threshold and to better characterize the spatiotemporal variability of seismicity. In this article, we build on these efforts using subspace detectors to objectively minimize the number of waveform templates required to fully characterize an earthquake sequence while increasing computational efficiency. Results demonstrate a new and scalable real‐time procedure to better detect and characterize the …