Seismic swarm S20010224.1 was recorded 13 km southwest of Kanosh, Utah, beginning at 21:00 on 23 February 2001 and concluding at 09:05 on 25 February 2001. Over this 36-hour, 5-minute period, 38 earthquakes occurred. The event cluster provides a clear example of swarm behavior, where numerous small-magnitude shocks take place without a single dominant mainshock.
Magnitudes ranged from 0.8 to 3.9. The largest event, magnitude 3.9, occurred at 21:43:50 on 23 February at a depth of 2 km. Two other notable shocks reached magnitude 3.6 and 3.0 shortly after 10:54 on 24 February. Depths varied between -3 km and 6 km, consistent with shallow crustal seismicity typical of the area. Most events clustered between 0 km and 5 km depth, indicating activity within the uppermost brittle crust.
The swarm began with a magnitude-2.6 shock at 21:00 on 23 February, followed by a rapid sequence of smaller events. Activity remained elevated through the night, with another peak around 10:50 on 24 February. By the morning of 25 February, rates declined, ending with a magnitude-1.2 event at 09:05.
Since 1 January 2000, this was the sole swarm recorded in the SeismoSight classification for the region. The preceding swarm occurred in 2000, underscoring the episodic nature of such sequences near Kanosh.
Kanosh lies within the Basin and Range Province of western Utah, where east-west extension produces normal faulting along north-south trending structures. This tectonic setting generates frequent small earthquakes and occasional swarms. The Intermountain Seismic Belt, which traverses central Utah, encompasses the area and has hosted multiple historical earthquake sequences driven by the same extensional regime.
The region’s geology features Quaternary sediments overlying Paleozoic carbonates and Tertiary volcanics. Faults such as those associated with the Sevier Desert basin accommodate ongoing extension at rates of approximately 1–2 mm per year. Updated seismic monitoring by regional networks confirms persistent low-level activity consistent with this framework.
SeismoSight internal swarm classification S20010224.1
USGS Earthquake Hazards Program – Utah seismicity reports
University of Utah Seismograph Stations – Basin and Range tectonic summaries