Tracking Thousands of Microearthquakes for a Month in Northern Oklahoma: What a Large-N Array Can Reveal About Induced Seismicity

The substantial increase in earthquake rate observed in Oklahoma during the last decade has largely been attributed to the disposal of wastewater from energy production activities. While existing sparse regional networks are able to capture the occurrence of most M≥3 earthquakes and some smaller magnitude events, micro to minor earthquakes typically cannot be detected. To enable the detection of microseismicity, we deployed a temporary array of 1,833 vertical-component nodal seismometers in northern Oklahoma. The LArge-n Seismic Survey in Oklahoma (LASSO) array operated for approximately one month in spring 2016, covering a 25-km-by-32-km region with a nominal station spacing of 400 m. We develop a local earthquake catalog from data recorded by the LASSO array, which allows us to investigate sequences of induced seismicity in this region of active wastewater injection with unprecedented clarity. A two-stage earthquake detection method utilizing standard short-term average/long-term average and waveform-correlation-based template matching techniques identifies more than 15,000 earthquakes recorded by the array, a factor of 160 increase over the regional earthquake catalog. Estimated duration magnitudes for the detected events are as small as -0.4. Initial hypocentral locations indicate some instances of very small-scale ( 0.5-1.5 km) lateral and vertical spatiotemporal migration of seismicity. Several dense clusters of seismicity that were active throughout the deployment are also observed. We use the catalog of local earthquakes recorded by the LASSO array to explore frequency-magnitude relationships and the spatiotemporal evolution of seismicity along individual fault segments. We also determine focal mechanisms for M≥2 earthquakes to constrain the sense of motion along these faults and potentially make inferences regarding the stress state of the shallow crust. The locations and orientations of active faults may provide insights into interactions between small-scale fault structures as well as potential locations of future induced earthquakes.