A new earthquake swarm, designated S20261105.1, has been detected in a seismically active region of southwestern Utah. The sequence began at 21:45 UTC on November 4, 2026, with an epicenter located approximately 15 km northeast of Milford. Over the subsequent 22 hours and 50 minutes, seismic networks registered 27 small-magnitude earthquakes, highlighting the dynamic geology of the area.
The events in this swarm have been minor, with magnitudes ranging from -0.1 to a peak of 2.0. The largest earthquake, a magnitude 2.0, occurred at 18:10 UTC on November 5. The vast majority of these tremors have been located at a very shallow depth of around 3 kilometers, with isolated events recorded at 2 and 4 kilometers. This shallow origin is a key characteristic often associated with specific geological processes, including the movement of fluids within the Earth's crust.
The timeline of the swarm shows an intermittent pattern. Following the initial two events on the evening of November 4, activity subsided for several hours before resuming with increased frequency on the morning of November 5. This clustering of small events over a short period, without a distinct mainshock, is the defining feature of an earthquake swarm.
The Milford region lies within the Basin and Range Province, a broad physiographic area covering much of the western United States that is defined by ongoing crustal extension. For millions of years, the Earth's crust here has been slowly stretching in an east-west direction. This process has created the region's distinct topography of alternating, north-south trending mountain ranges (horsts) and flat, sediment-filled valleys (grabens). The boundaries between these ranges and valleys are marked by large normal faults, which accommodate the crustal stretching and are the primary sources of seismic activity in the region.
The swarm's location is particularly significant as it sits near the eastern margin of the Basin and Range, adjacent to the transition with the more stable Colorado Plateau. This transitional zone is structurally complex and hosts numerous fault systems. More importantly, the area northeast of Milford is home to the Roosevelt Hot Springs geothermal area, one of the most significant geothermal resources in the United States. This system is driven by a deep heat source that superheats groundwater, causing it to circulate through a network of fractures and faults in the crust.
The presence of a major geothermal system is critical to understanding the seismicity near Milford. Earthquake swarms are a common phenomenon in geothermal fields worldwide. They can be triggered by natural processes, such as changes in pressure caused by the movement of hot water and steam through underground fractures. This fluid movement can alter the stress on pre-existing faults, making them more likely to slip and generate small earthquakes.
Furthermore, the area is the site of the Utah Frontier Observatory for Research in Geothermal Energy (FORGE), a U.S. Department of Energy-sponsored laboratory dedicated to advancing Enhanced Geothermal Systems (EGS). EGS technology involves injecting high-pressure fluids deep underground to create or enhance a fracture network in hot, impermeable rock, thereby creating a man-made geothermal reservoir. This process, known as hydraulic stimulation, is known to induce microseismicity. These small, induced earthquakes are an expected part of EGS development and are used by scientists to map the growth of the underground fracture network. The shallow depth of the current swarm is highly consistent with seismicity observed in active geothermal fields, both natural and engineered.
The region has a documented history of earthquake swarms. Since the beginning of 2000, a total of 27 distinct swarms have been classified in this specific area. An analysis of their occurrence reveals a notable increase in frequency in recent years.
The accelerated rate of swarm activity, particularly in 2024 and 2026, coincides with a period of intensified research and operational activities at the Utah FORGE site and the surrounding geothermal field. This correlation strongly suggests that the increased seismicity is linked to the dynamic interplay between natural tectonic stresses and fluid pressure changes within the geothermal reservoir, likely influenced by human activity related to geothermal energy development.
In conclusion, the S20261105.1 swarm is another manifestation of the complex geological environment near Milford. While the earthquakes are too small to be felt or pose any hazard, they provide valuable data for scientists monitoring the region's fault systems and the behavior of its world-class geothermal resource. The ongoing seismic activity is consistent with the known characteristics of this tectonically and geothermally active area.