The earthquake swarm designated S20250829.1 occurred 16 km northeast of Milford in Beaver County, Utah. It began at 05:21 on 28 August 2025 and concluded at 11:55 on 6 September 2025, spanning 222 hours and 34 minutes. During this period, 245 earthquakes were recorded.
Milford lies in the eastern Great Basin within the Basin and Range Province. This tectonically active region experiences crustal extension along north-south trending normal faults. The local geology features Quaternary alluvium overlying Paleozoic and Mesozoic sedimentary rocks, with nearby Cenozoic volcanic and intrusive units in the Mineral Mountains. Shallow seismicity is common due to these fault systems and possible geothermal influences.
Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.1 to 1.8, with most values falling between 0.5 and 1.5. Depths remained consistently shallow at 1–2 km, consistent with near-surface fault slip. The sequence began with events clustered in the initial hours of 28 August, showing a gradual increase in frequency before tapering. No single mainshock dominated; instead, the pattern reflected typical swarm behavior driven by fluid migration or stress triggering along minor faults.
Historical records indicate 16 swarms in the region since 1 January 2000. Yearly distribution includes one swarm in 2019, four in 2020, one in 2021, one in 2022, two in 2023, and seven in 2024. This frequency underscores recurrent seismic unrest linked to the area's extensional regime.
The swarm's characteristics align with documented patterns of low-magnitude, shallow events in the Intermountain Seismic Belt. Such activity poses minimal surface hazard but provides data for refining local fault models and monitoring networks.
SeismoSight internal swarm classification S20250829.1
USGS Earthquake Catalog (regional historical data)
Utah Geological Survey – Basin and Range Province reports