Seismic Swarm S20250928.1: Analysis of Activity Near Prague, Oklahoma

Seismic swarm S20250928.1 occurred 9 km northwest of Prague, Oklahoma, registering 263 earthquakes over 211 hours and 17 minutes. The sequence began at 09:49 on 27 September 2025 and concluded at 05:07 on 6 October 2025. Magnitudes ranged from -2.1 to 3.7, with the majority of events below magnitude 1.0 and focal depths clustered between 5 and 6 km.

Analysis of the first 100 events reveals a rapid onset of activity on 28 September 2025, including multiple events within minutes of one another. The largest shocks reached 3.7 and 3.5, both at 6 km depth, followed by numerous aftershocks of lower magnitude. Depths remained consistent at 5–6 km throughout the initial phase, with only isolated shallower detections near 1 km. Negative magnitudes indicate microseismic events typical of swarm sequences, reflecting brittle failure along small fault segments under elevated pore pressure.

Oklahoma’s central region lies within the stable craton of the North American interior, where natural seismicity is low. The Prague area sits near the intersection of ancient basement structures, including faults associated with the Nemaha Uplift and the Wilzetta Fault zone. These features can be reactivated when fluid pressures increase in the crystalline basement.

Since 2000, thirteen swarms have been documented in the vicinity, with one each in 2011 and 2023, ten in 2024, and one in 2025. The 2011 Prague sequence included a magnitude 5.7 mainshock that caused damage and was linked to nearby wastewater injection. Subsequent monitoring by the Oklahoma Geological Survey and USGS has shown that swarm activity often correlates with disposal well operations, which elevate pore pressure along critically stressed faults.

The 2025 swarm’s characteristics—shallow depths, rapid event clustering, and predominance of small-magnitude quakes—align with patterns observed in prior injection-related sequences. No surface rupture was reported, consistent with the modest energy release.

Continued seismic monitoring remains essential in this area because fluid-injection practices can sustain elevated seismicity rates for months to years. Integration of real-time catalogs with well-operation data helps distinguish natural from induced events and supports mitigation strategies such as reduced injection volumes.

References
Oklahoma Geological Survey, Earthquake Catalog and Swarm Reports (2000–2025).
U.S. Geological Survey, Earthquake Hazards Program, Regional Seismicity Summaries.
National Earthquake Information Center, Event Data Archive.