Seismic swarm S20201007.1 was recorded 17 km east-northeast of Milford, Utah, beginning at 12:40 on 6 October 2020 and concluding at 16:55 on 7 October 2020. Over this 28-hour period, 41 earthquakes were detected, providing a detailed record of clustered microseismicity in the region.
The events were predominantly of very low magnitude, ranging from -1.2 to 0.9. Depths remained shallow throughout, with the majority registered at 0 km and only two events reaching 3 km and 4 km respectively. Temporal clustering was evident, with an initial sequence of 16 events on 6 October concentrated between 12:40 and 13:42, followed by a quieter interval and a secondary burst of 25 events on 7 October, primarily between 04:12 and 16:55. This pattern illustrates the typical swarm behavior of numerous small earthquakes occurring in rapid succession without a distinct mainshock.
Such swarms are consistent with the tectonic setting of western Utah. Milford lies within the Basin and Range Province, where ongoing crustal extension produces normal faulting and distributed seismicity. The nearby Mineral Mountains host Quaternary volcanic features and active geothermal systems, conditions that can facilitate fluid migration and trigger episodic earthquake sequences. Historical records indicate that the area has experienced recurrent swarm activity, with four documented swarms since 1 January 2000. These include one swarm in 2019 and three in 2020, underscoring a recent increase in clustered seismicity.
Analysis of the magnitude-time distribution reveals no clear migration of hypocenters, suggesting a localized source volume. The predominance of negative magnitudes highlights the sensitivity of modern seismic networks in capturing events below the threshold of human perception. Depths near the surface align with known shallow brittle failure in the upper crust of this extensional regime.
This swarm contributes to the understanding of background seismicity levels in Beaver County. Continued monitoring remains important given the region's geothermal potential and proximity to infrastructure. Future sequences may exhibit similar characteristics, aiding in the refinement of local seismic hazard models.
SeismoSight internal swarm catalog S20201007.1
USGS Earthquake Catalog (historical context for Utah seismicity)
Utah Geological Survey publications on Basin and Range tectonics