A seismic swarm designated S20231214.1 occurred 17 km east-northeast of Milford in Beaver County, Utah. The sequence began at 20:58 UTC on 13 December 2023 and concluded at 12:15 UTC on 14 December 2023, spanning 15 hours and 16 minutes. During this period, 24 earthquakes were recorded, with magnitudes ranging from -0.6 to 0.9 and focal depths between 0 and 2 km.
The events clustered tightly in both time and space, characteristic of swarm behavior rather than a mainshock-aftershock sequence. The largest event reached magnitude 0.9 at 08:00 UTC on 14 December, while multiple events registered negative magnitudes, reflecting the high sensitivity of modern seismic networks in detecting microseismicity. Depths remained consistently shallow, suggesting activity within the uppermost crust.
This swarm represents the seventh such episode recorded in the region since 1 January 2000. Earlier swarms occurred in 2020 (four events), 2021 (one), 2022 (one), and 2023 (one). The recurrence pattern indicates episodic release of strain in a localized volume.
The Milford area lies within the Basin and Range physiographic province, where active extensional tectonics produces distributed normal faulting. The local geology features Quaternary alluvium overlying Tertiary volcanic and intrusive rocks of the Mineral Mountains, which include rhyolitic domes and associated geothermal systems. The Cricket Mountains to the east expose Paleozoic carbonates and Mesozoic sediments deformed by Sevier-age thrusting. Regional heat flow is elevated due to thinned crust and recent volcanism, supporting known geothermal resources near Milford.
Shallow microseismicity in this setting commonly relates to fluid migration along pre-existing fractures, potentially influenced by both tectonic extension and hydrothermal circulation. The December 2023 swarm aligns with this framework, occurring at depths consistent with the brittle-ductile transition in a high-heat-flow environment.
Continued monitoring by regional networks remains essential for distinguishing between natural swarm activity and any anthropogenic contributions from geothermal development or other subsurface operations in the area.