A seismic swarm designated S20240404.1 occurred 16 km northeast of Milford, Utah, from 18:15 on 3 April 2024 to 16:35 on 8 April 2024. Over 118 hours and 19 minutes, the sequence produced 172 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from -1.3 to 1.1 and focal depths mostly between 0 and 3 km. The sequence began with microearthquakes clustered near 1 km depth on the evening of 3 April, followed by a gradual increase in event rate through 4 April. Magnitudes remained below 0.5 for the initial 24 hours before isolated events reached 0.4–0.5. Activity continued at a steady but low level on 5 April, then intensified slightly on 6 April with several events of magnitude 0.7–1.1, all at shallow depths. The pattern shows no single dominant mainshock; instead, energy released through numerous small, closely spaced events typical of swarm behavior.
The Milford region lies within the Basin and Range Province of western Utah, an area characterized by extensional tectonics driven by normal faulting. This geologic setting produces frequent small earthquakes along range-bounding faults. The swarm epicenters fall near the western margin of the Mineral Mountains, a horst block of Precambrian and Paleozoic rocks intruded by Tertiary granites. Regional extension has created a network of north-south trending faults that accommodate crustal stretching at rates of several millimeters per year. Geothermal activity in the adjacent Roosevelt Hot Springs area indicates elevated heat flow, which can influence fluid migration and thereby modulate swarm occurrence.
Historical records document 12 swarms in the same locale since 2000. These episodes cluster in specific years: one in 2019, four in 2020, one in 2021, one in 2022, two in 2023, and three in 2024. Such recurrent swarms reflect ongoing strain accumulation and release along favorably oriented faults without producing damaging ground motion. Depths consistently below 3 km suggest that brittle failure occurs in the shallow crust, possibly facilitated by hydrothermal fluids circulating through fractured granite and volcanic rocks.
No events in the current swarm exceeded magnitude 2, indicating limited potential for structural damage. Continued monitoring remains important because the same fault system has hosted larger historical earthquakes elsewhere in the Intermountain Seismic Belt. The shallow, diffuse nature of the sequence aligns with fluid-driven swarm models observed in other geothermal regions of the Great Basin.
USGS Earthquake Catalog
Utah Geological Survey – Quaternary Fault Database
University of Utah Seismograph Stations Annual Reports