On 25 May 2003, a seismic swarm designated S20030525.2 was recorded 6 km south-southeast of Joseph in Sevier County, Utah. The sequence began at 00:02 UTC and concluded at 18:18 UTC, encompassing 24 earthquakes over 18 hours and 16 minutes. This event represents the sole swarm documented in the region since 1 January 2000, following an earlier episode in 2000.
The earthquakes exhibited magnitudes between 0.7 and 2.5, with the largest event registering 2.5 at 00:32 UTC. Depths ranged from 0 km to 9 km, clustering predominantly between 5 km and 9 km. Activity showed temporal grouping, with multiple events occurring within short intervals during the morning hours before tapering off by mid-afternoon. Such patterns are characteristic of swarm sequences, where numerous events of similar size occur without a dominant mainshock-aftershock progression.
Joseph lies within the Basin and Range Province of central Utah, an area defined by extensional tectonics and active normal faulting. The Sevier Valley occupies a structural basin bounded by range-front faults that accommodate ongoing crustal stretching. This setting forms part of the Intermountain Seismic Belt, a north-south trending zone of elevated seismicity extending from Arizona through Utah into Montana. Regional fault systems, including segments of the Sevier-Tushar fault zone, have produced historical earthquakes and host microseismicity consistent with shallow crustal deformation.
Swarm activity in this environment often reflects fluid migration or stress perturbations along favorably oriented faults. Depths observed in the 2003 sequence align with typical brittle failure zones in the upper crust of the Great Basin. No surface rupture or significant damage was associated with the events, consistent with their modest magnitudes.
Geological mapping indicates Quaternary fault scarps in the vicinity, underscoring the potential for recurrent seismic episodes. Monitoring by regional networks continues to track background rates, providing context for interpreting isolated swarms such as S20030525.2 within the broader tectonic framework.
SeismoSight internal swarm classification records.
U.S. Geological Survey Earthquake Hazards Program regional tectonic summaries.
Utah Geological Survey fault and seismicity databases.