A seismic swarm designated S20240216.1 occurred 15 km northeast of Milford in Beaver County, Utah. The sequence began at 15:12 on 15 February 2024 and concluded at 19:39 on 23 February 2024, spanning 196 hours and 26 minutes. During this period, 308 earthquakes were recorded. The events clustered tightly in both space and time, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.
Analysis of the first 100 events reveals predominantly microearthquakes with magnitudes between -1.0 and 1.9. Depths remained shallow, mostly 0–3 km, with a few reaching 6–10 km. The initial hours featured numerous negative-magnitude events at depths of 0–2 km, followed by a gradual increase in larger events up to magnitude 1.9 on 17 February. This pattern indicates fluid migration or aseismic slip triggering successive failures along small fault segments.
The location lies within the Basin and Range Province, where east-west extension produces normal faulting. Milford sits near the western margin of the Mineral Mountains, a metamorphic core complex with Miocene volcanic and intrusive rocks. Geothermal activity in the area, including hot springs and the Roosevelt Hot Springs field, suggests hydrothermal fluids may influence seismicity by reducing effective stress on faults.
Utah lies along the Intermountain Seismic Belt, an active zone of intraplate earthquakes extending from Montana through Idaho, Utah, and into Arizona. Historical records document recurrent swarm activity in this region, often linked to volcanic or geothermal processes. Since 1 January 2000, eight swarms have been identified near Milford, occurring in 2020 (four swarms), 2021 (one), 2022 (one), and 2023 (two). These episodes demonstrate that low-magnitude, clustered seismicity is a persistent feature of the local crust.
The February 2024 swarm fits this established pattern. All recorded events remained below magnitude 2.0, producing no reported damage or felt shaking beyond instrumental detection. Continued monitoring by regional networks helps distinguish such swarms from potential foreshock sequences preceding larger events.
SeismoSight internal swarm classification S20240216.1
USGS Earthquake Catalog (Utah region, 2000–2024)
Utah Geological Survey – Seismic activity reports
USGS Professional Paper on Basin and Range tectonics