Prague, Oklahoma Earthquake Swarm of November 2011

A notable earthquake swarm, internally classified as S20111107.1, occurred near Prague, Oklahoma, beginning at 03:53 on 6 November 2011 and concluding at 03:05 on 8 November 2011. Over this 47-hour period, 31 events were recorded, with the sequence centered approximately 9 km west-northwest of Prague. The swarm featured a mainshock of magnitude 5.7 at a depth of 5 km, followed by numerous aftershocks ranging from magnitude 2.5 to 4.8, predominantly at depths between 3 km and 5 km.

The events unfolded rapidly in the initial hours. The magnitude 5.7 shock at 03:53 on 6 November was succeeded within minutes by a magnitude 4.0 event, with subsequent activity including multiple shocks of magnitude 3.0–3.9 throughout the morning and afternoon. Depths remained shallow, clustering around 5 km for most events, though several later shocks occurred at 3 km. Activity persisted into 7 November with events of magnitude 2.5–3.3, before culminating in a magnitude 4.8 shock at 02:46 on 8 November. The final recorded event reached magnitude 3.1 at 03:05 that same day.

This swarm took place in central Oklahoma, a region underlain by Precambrian basement rocks and Paleozoic sedimentary layers. The area lies within the stable North American craton, where natural seismicity is typically low. However, the Prague vicinity is transected by ancient fault systems, including segments of the Wilzetta Fault Zone, which can be reactivated under altered stress conditions. Historical records indicate sparse seismic activity prior to the late twentieth century, with the 2011 swarm representing one of the first well-documented sequences to draw attention to human-induced triggers in the region.

Geological studies attribute such swarms to changes in pore pressure from subsurface fluid injection associated with oil and gas operations. In the Prague case, nearby wastewater disposal wells likely contributed to fault slip along pre-existing structures. Depths of 3–5 km align with the sedimentary-basement interface where injection effects are most pronounced. Post-2011 research has refined understanding of these mechanisms, confirming that even moderate injection volumes can elevate seismicity rates in susceptible areas.

The Prague swarm contributed to broader recognition of induced seismicity patterns across Oklahoma. It highlighted the importance of monitoring injection rates and fault proximity, informing subsequent regulatory adjustments by state agencies. Updated assessments through the 2020s continue to emphasize the interplay between basement faults and fluid migration, with seismic catalogs showing similar swarm behaviors in analogous settings.

• Event sequence summary: 31 total earthquakes, magnitudes 2.5–5.7, depths 3–5 km.
• Temporal distribution: Peak activity on 6 November, tapering by early 8 November.
• Regional context: Shallow basement faults in a low natural-seismicity craton.

References
US Geological Survey Earthquake Catalog (events 2011)
Oklahoma Geological Survey annual reports on central Oklahoma seismicity
Peer-reviewed analyses in Geology and Journal of Geophysical Research (2013–2022 updates on induced seismicity mechanisms)