On 30 July 2021, a seismic swarm designated S20210730.1 was recorded 16 km east-northeast of Milford, Utah. The sequence began at 01:49 UTC and concluded at 11:06 UTC, encompassing 44 earthquakes over a duration of 9 hours and 16 minutes. Magnitudes ranged from -0.8 to 1.1, with the majority of events registering below 0.5. Focal depths were predominantly at or near the surface, with only three events exceeding 0 km and the deepest reaching 2 km.
The temporal distribution showed peak activity in the initial hours, followed by a gradual decline. Early events clustered between 02:33 and 03:13, after which activity became more sporadic until the final recorded event at 11:06. Two events attained the maximum magnitude of 1.1, occurring at 03:57 and 07:45. Negative magnitudes, common in dense monitoring networks, indicate microseismicity detectable only by sensitive instruments.
This swarm represents the second documented sequence in the region since 2000, following an initial swarm in 2020. In total, four swarms have occurred in the area during this period, highlighting episodic rather than continuous seismic behavior.
The Milford region lies within the Basin and Range Province of western Utah, characterized by extensional tectonics driven by normal faulting. The surrounding terrain includes the Mineral Mountains and adjacent valleys, underlain by Quaternary sediments and Tertiary volcanic rocks. Geothermal resources in the area arise from elevated heat flow associated with crustal thinning and fault-controlled fluid circulation. Historical records indicate low to moderate background seismicity, punctuated by occasional swarms that may relate to fluid migration along pre-existing fractures.
Seismic swarms in this setting often reflect transient processes such as pore-pressure changes rather than mainshock-aftershock sequences. The shallow depths and low magnitudes observed in S20210730.1 are consistent with such mechanisms. No damage or felt reports were associated with the events.
Continued monitoring by regional networks remains essential for distinguishing natural tectonic signals from potential anthropogenic influences in this geothermally active corridor.
SeismoSight internal swarm classification S20210730.1
Utah Geological Survey regional tectonic summaries
USGS Earthquake Catalog historical data (2000–present)