Seismic Swarm Analysis: Luther, Oklahoma, April 2013

An earthquake swarm designated S20130417.1 occurred 6 km east of Luther, Oklahoma, beginning at 06:45 on 16 April 2013 and concluding at 16:51 on 17 April 2013. Over 34 hours and 5 minutes, the sequence produced 29 events, with magnitudes ranging from 2.5 to 4.4 and focal depths between 1 km and 7 km. This activity exemplifies the shallow, clustered seismicity characteristic of the region during the early 2010s.

The swarm initiated with a magnitude 2.9 event at 7 km depth, rapidly followed by the largest shock—a magnitude 4.4 at 6 km depth—within eleven minutes. Subsequent events included multiple magnitude 3+ earthquakes, such as a 4.2 at 7 km depth roughly three hours later and a 3.7 at only 2 km depth in the late afternoon. Depths clustered predominantly between 2 km and 7 km, with several events at 2 km suggesting very shallow rupture consistent with fluid-related triggering. Later activity on 17 April featured three magnitude 3.2 events at depths of 6–7 km before the sequence tapered off.

Central Oklahoma sits within the stable continental interior of the North American craton, underlain by Precambrian basement rocks dissected by ancient fault systems including elements of the Nemaha Uplift. Although historically aseismic, the area experienced a dramatic rise in earthquake rates after 2009. This increase correlates with expanded wastewater injection from oil and gas operations into deep disposal wells, which can elevate pore pressures along pre-existing faults and induce slip. Depths of 1–7 km align with the typical range for injection-induced events in the Arbuckle Group and underlying basement.

The Luther swarm contributed to the broader 2013 surge in Oklahoma seismicity, which prompted regulatory reviews of injection volumes by the Oklahoma Corporation Commission. Post-2016 reductions in disposal rates led to a measurable decline in event frequency, yet the 2013 sequence remains a key example of how fluid injection can produce intense, short-lived swarms rather than isolated mainshock-aftershock sequences.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Oklahoma Geological Survey induced seismicity reports
Ellsworth, W. L. (2013). Injection-induced earthquakes. Science.