A seismic swarm designated S20231127.2 occurred approximately 17 km northeast of Milford in Beaver County, Utah. The sequence began at 21:31 on 26 November 2023 and concluded at 18:07 on 27 November 2023, spanning 20 hours and 36 minutes during which 31 earthquakes were recorded.
The events were predominantly microearthquakes with magnitudes ranging from -0.7 to 2.2. Depths were extremely shallow, with the majority occurring at or near the surface (0 km) and one event at 1 km. The initial pair of events at 21:31:54 on 26 November registered magnitudes of 2.2 and 1.4, marking the strongest activity. Subsequent events remained below magnitude 1.0, reflecting a rapid decay in energy release typical of swarm behavior rather than a mainshock-aftershock sequence.
This swarm aligns with the region's established pattern of clustered seismicity. Since 2000, seven swarms have been documented in the area, occurring in 2019 (one swarm), 2020 (four swarms), 2021 (one swarm), and 2022 (one swarm). Such recurrent swarms suggest ongoing crustal adjustments along local fault structures.
The Milford area lies within the Basin and Range Province, where extensional tectonics dominate. Normal faulting associated with the Intermountain Seismic Belt produces frequent small-magnitude events. The shallow focal depths observed in swarm S20231127.2 are consistent with activity in this province, where brittle deformation occurs in the upper crust. Historical records indicate that Utah experiences moderate seismicity, with the broader region influenced by both tectonic extension and possible geothermal influences near volcanic features such as the Mineral Mountains.
No damage or felt reports were associated with this swarm, underscoring its low-energy character. Continued monitoring remains essential given the area's history of swarm activity, which may signal episodic stress release without leading to larger events.
USGS Earthquake Catalog (earthquake.usgs.gov)
Utah Geological Survey seismic reports (geology.utah.gov)
Intermountain Seismic Belt historical summaries