A minor seismic swarm designated S20200502.1 was recorded 19 km east-northeast of Milford in Beaver County, Utah. The sequence began at 22:45 on 1 May 2020 and concluded at 07:27 on 3 May 2020, spanning 32 hours and 41 minutes. During this interval, 30 earthquakes were detected, with magnitudes ranging from -0.8 to 1.4 and focal depths between 0 and 4 km. All events remained below the threshold for felt shaking or damage.
The swarm initiated with a series of micro-earthquakes clustered tightly in both time and space. Early events on 1 May exhibited magnitudes near or below zero and depths around 3 km. Activity continued into 2 May with slightly higher magnitudes, including peaks of 1.1 at 00:22 and 1.4 at 01:51, both at 4 km depth. The final event, magnitude 0.3 at 1 km depth, marked the end of the sequence on 3 May. The pattern shows episodic bursts separated by quiet periods, typical of fluid-driven or stress-adjustment processes rather than mainshock-aftershock sequences.
Milford lies within the Basin and Range Province, where east-west extension produces north-south trending normal faults. The area forms part of the transition between the Colorado Plateau and the Great Basin, with Quaternary basaltic volcanism in the adjacent Black Rock Desert volcanic field. Geothermal systems and young volcanic features indicate elevated crustal temperatures that can facilitate small-magnitude earthquake swarms through hydrothermal fluid migration. Historical seismicity in the region remains low to moderate, consistent with the province-wide rate of extensional faulting.
Since 1 January 2000, two swarms have been identified in the catalog for this locale, with S20200502.1 representing the earliest recorded instance. The second swarm occurred after 2020. Both episodes share similar characteristics of low magnitudes and shallow depths, underscoring the persistent but low-energy seismic behavior of the local fault network.
No larger tectonic events were associated with this swarm. Continued monitoring by regional networks provides data for refining models of strain accumulation along the dominant north-south structures near Milford.
USGS Earthquake Catalog
Utah Geological Survey Seismicity Reports
SeismoSight Internal Swarm Classification Database