On 6 April 2022, a seismic swarm designated S20220406.1 was recorded 17 km east-northeast of Milford in Beaver County, Utah. The sequence began at 16:59 UTC and concluded at 21:21 UTC, encompassing 29 earthquakes over 4 hours and 21 minutes. Magnitudes ranged from -0.9 to 2.1, with the majority of events occurring at depths of 0 km and one at -3 km. This shallow, low-magnitude activity is characteristic of swarm behavior in the region, where events cluster temporally without a dominant mainshock.
The Milford area lies within the eastern Basin and Range Province, a tectonically active extensional regime bounded by normal faults. Regional geology features Quaternary basaltic and rhyolitic volcanism associated with the Mineral Mountains and proximity to the Great Basin. These structures facilitate fluid migration along fault zones, a common trigger for earthquake swarms. Historical seismicity in western Utah reflects ongoing extension at rates of approximately 1–2 mm per year, with swarms often linked to geothermal or hydrothermal processes rather than large tectonic events.
Seismic records since 2000 indicate five prior swarms in the immediate vicinity. These occurred in 2020 (four swarms) and 2021 (one swarm), demonstrating recurrent clustered activity. Such patterns suggest persistent subsurface conditions conducive to episodic seismicity, potentially influenced by local heat flow or minor magmatic intrusions at depth.
Analysis of S20220406.1 reveals a rapid onset with peak activity between 18:28 and 19:45 UTC, including the largest events of magnitude 1.9 and 2.1. The predominance of negative and near-zero magnitudes underscores the microseismic nature of the swarm, typical of fluid-driven sequences. Depths clustered at or near the surface imply very shallow sources, consistent with geothermal or fault-zone processes observed elsewhere in the Intermountain West.
This event contributes to understanding swarm dynamics in Utah’s volcanic and geothermal corridors. Continued monitoring supports hazard assessment, as recurrent swarms may precede larger tectonic releases or indicate evolving subsurface conditions. No damage or felt reports were associated with the sequence.
SeismoSight internal classification data for swarm S20220406.1.
USGS Earthquake Hazards Program regional tectonic summaries.
Utah Geological Survey publications on Basin and Range seismicity.