What Is an Earthquake Swarm?
An earthquake swarm is a run of mostly small earthquakes in one area with no clear main shock. Instead of one large earthquake followed by smaller and smaller aftershocks, a swarm rises and falls on its own schedule: a few earthquakes, a busy spell, a pause, sometimes a second burst.
The line between the two isn't sharp, and even seismologists say there is no precise definition. The 2019 Bluffdale earthquakes are a good example. News reports called them a swarm, but the University of Utah Seismograph Stations classified them as a magnitude 3.7 main shock with 13 foreshocks and 177 aftershocks.
Swarms often return to the same places, and in central and southern Utah they are the most common pattern of all. A University of Utah study of about 2,300 earthquakes there, mostly magnitude 1 to 3, found that 80% of the sequences were swarms.
What Causes Earthquake Swarms?
Swarms happen where something keeps nudging a fault instead of releasing all its stress at once. The USGS points to three main causes:
- Fluids: most often, water moving through the rock raises the pressure on faults and lets them slip in many small steps.
- Slow slip: a fault creeps slowly, and small sticky patches along it break one after another.
- Magma: in volcanic areas, rising magma can crack the rock as it pushes upward.
Most swarms are linked to geothermal activity, where hot water circulates underground. That fits Utah's pattern: the University of Utah ties the swarms near Milford to hydrothermal activity, and the Utah Geological Survey says the 2026 Kanosh swarm may have been set off by fluids moving underground.
How Long Do Earthquake Swarms Last?
Anywhere from hours to months. Most swarms are short, but some keep a steady pace or even pick up over days or weeks, and a swarm can quiet down for a while and then flare up again.
The USGS gives one local yardstick from Southern California: past swarms in the Westmorland area lasted from 1 to 20 days, about a week on average. The area northeast of Milford is the opposite case in Utah: it is active every year, with more than 2,600 earthquakes in our count of the catalog from 2015 to 2024, and much busier in 2026.
Does a Swarm Mean a Big Earthquake Is Coming?
Usually not. The University of Utah Seismograph Stations says Utah's swarms are typically self-contained and fizzle out on their own. But no one can rule out a larger earthquake, and the answer comes in odds:
- Any earthquake has about a 5% chance of being followed by a larger one nearby within a week, according to the USGS. The University of Utah puts it as roughly one in twenty.
- A swarm doesn't use up a fault's stress either. It would take roughly 24 million magnitude 2 earthquakes to release the energy of one magnitude 7.
So a swarm is a reason to check your plan, not to panic. Review what to do during an earthquake and make sure your earthquake kit is ready.
Recent Earthquake Swarms in Utah
| Area | When | Earthquakes | Largest |
|---|---|---|---|
| Kanosh, Millard County | April to May 2026 | 122 (within 10 km) | M3.6 |
| Northeast of Milford, Beaver County | January to mid-November 2026 | 2,273 (within 8 km) | M3.1 |
| Circleville, Piute County | July to mid-August 2026 | 48 (within 15 km) | M2.6 |
| West of Lehi, Utah County | November 2025 to November 2026 | 34 (within 4 km) | M3.5 |
| Bluffdale, Salt Lake County | February to April 2019 | 191 (University of Utah count) | M3.7 |
Counts are our own, from the USGS catalog within the distance shown, except Bluffdale.
Kanosh, April 2026
A swarm began southwest of Kanosh at 5:45 a.m. on April 19, 2026, with a magnitude 3.2. The largest, a magnitude 3.6, followed at 8:43 a.m. In our count of the catalog, 84 earthquakes were located within 10 km that day and 122 by the end of May, against 7 in the same area in the year before. None was reported felt.
West of Lehi, Since Late 2025
A small cluster has formed about 4 miles west of Lehi, north of Saratoga Springs. In our count, 34 earthquakes were located within 4 km of it from November 2025 to mid-November 2026, five of them magnitude 2 or larger, compared with 0 or 1 a year from 2015 to 2024. The largest, a magnitude 3.5 at 12:53 p.m. on February 5, 2026, was widely felt across the Wasatch Front, with more than 2,500 people reporting it. A magnitude 2.9 followed on August 7. The University of Utah hasn't called it a swarm, so treat that label as ours.
Circleville and Milford, 2026
A swarm near Circleville in July 2026 had mostly earthquakes below magnitude 2; our count finds 48 within 15 km from July 1 to mid-August, the largest a magnitude 2.6. Northeast of Milford, swarms in 2021 brought at least 60 small earthquakes, and 2026 has been much busier, with 2,273 earthquakes within 8 km by mid-November, the largest a magnitude 3.1.
Bluffdale, 2019
From February 13 to April 20, 2019, the University of Utah located 191 earthquakes in the Bluffdale area. The largest, a magnitude 3.7 at 5:09 a.m. on February 15, came seven minutes after a magnitude 3.2.
How to Follow a Swarm on the Live Map
Every swarm earthquake shows up on the live Utah earthquake map as it's published. Pick the 7D or 30D view to see a cluster build, and check the weekly summary below the map, which names the busiest area of the past week. Near the Wasatch Front, the Salt Lake City earthquake map covers everything within 50 miles of downtown.