Seismic swarm S20260312.1 was recorded 15 km northeast of Milford, Utah, beginning at 11:33 on 11 March 2026 and concluding at 12:43 on 13 March 2026. Over 49 hours and 9 minutes, the event comprised 47 earthquakes, with magnitudes ranging from -0.6 to 2.3 and focal depths primarily between 0 and 4 km.
The sequence displayed typical swarm characteristics, featuring a rapid onset followed by fluctuating rates of smaller events without a single dominant mainshock. The largest event measured 2.3 at 05:03 on 12 March at 4 km depth. Subsequent activity included multiple events above magnitude 1.5, concentrated in the first 36 hours before tapering. Depths remained shallow throughout, consistent with fluid-driven or fault-zone processes common in extensional settings.
Milford lies within the Basin and Range Province of western Utah, where crustal extension along normal faults produces frequent low-magnitude seismicity. The Mineral Mountains and adjacent valleys host geothermal systems and Quaternary fault scarps that accommodate regional strain. Historical records indicate that such swarms occur periodically in this tectonic environment, often linked to fluid migration or minor slip on pre-existing structures.
Since 2000, seventeen swarms have been documented in the broader area. Yearly counts show variability: four in 2020, one each in 2021 and 2022, two in 2023, seven in 2024, and two in 2025. Swarm S20260312.1 aligns with this pattern of episodic, clustered activity rather than isolated tectonic events.
Analysis of the provided event times and magnitudes reveals no clear migration of hypocenters, suggesting a localized source volume. Most events clustered between 1–2 km depth, with only one deeper shock at 4 km. Negative magnitudes indicate the network’s sensitivity to microseismicity, typical of modern monitoring in geothermal or mining-adjacent regions.
This swarm underscores the ongoing seismic hazard in rural western Utah, where even modest sequences can be felt locally and may influence geothermal operations or infrastructure planning. Continued monitoring remains essential to distinguish background swarm behavior from any potential escalation.
SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonic framework)
Utah Geological Survey (Basin and Range Province geology)