A new earthquake swarm, designated S20260910.2, has commenced in a seismically active region of southwestern Utah. The sequence began at 02:34 UTC on September 10, 2026, with its epicenter located approximately 16 km northeast of Milford. In its first 19 hours and 49 minutes, the University of Utah Seismograph Stations (UUSS) registered 41 earthquakes. The events have been minor, with magnitudes ranging from 0.8 to 2.1.
The swarm's activity has been characterized by a series of small, shallow tremors. The two largest events, both registering a magnitude of 2.1, occurred at 09:27 UTC and 18:38 UTC. A notable feature of this sequence is the very shallow depth of the hypocenters, with the vast majority occurring at just 1 to 2 kilometers below the surface. One event was located at 4 km depth. Such shallow earthquakes, while small, can sometimes be felt by residents in the immediate vicinity as sharp, brief jolts. This swarm is the latest in a pattern of increasing seismic activity for the area, which has experienced 25 distinct swarms since 2000, with a marked increase in frequency in recent years.
The Milford region lies within a zone of significant geological complexity, at the transition between the Basin and Range Province and the Colorado Plateau. This area is a part of the Intermountain Seismic Belt (ISB), a north-south trending zone of elevated seismicity that extends from Montana to Arizona. The fundamental geological process driving this activity is crustal extension, where the North American Plate is slowly being stretched in an east-west direction.
This stretching has created the classic Basin and Range topography: a series of parallel, north-south oriented mountain ranges (horsts) separated by broad, flat valleys (grabens). These are bounded by large normal faults, which accommodate the crustal extension. While the Wasatch Fault to the east is the most prominent active fault in Utah, the area around Milford has its own dense network of faults. The swarm's location places it near the Mineral Mountains, an uplifted block of granitic and metamorphic rock bounded by active normal faults.
What makes this specific location particularly prone to earthquake swarms is the presence of a significant geothermal system. The area is home to the Roosevelt Hot Springs Known Geothermal Resource Area (KGRA), one of the most potent geothermal fields in the United States. This system is fueled by heat from a relatively shallow magma body that has produced volcanic rocks in the region as recently as a few thousand years ago.
Earthquake swarms in such geothermal areas are often not caused by the simple buildup and release of tectonic stress on a single fault. Instead, they are frequently driven by the movement of superheated water and steam through networks of fractures and faults in the Earth's crust. This process, known as hydrothermal activity, can alter the pressure within the rock pores (pore fluid pressure), which can reduce the frictional resistance on pre-existing faults and trigger slip, resulting in numerous small earthquakes. The shallow depth of the current swarm is highly consistent with processes occurring within the upper part of such a geothermal reservoir.
The S20260910.2 swarm is not an anomaly but rather a continuation of a well-established pattern. The region has seen a notable uptick in swarm activity since 2020. Data shows a progression from one or two swarms per year in the early 2020s to seven swarms in 2024 and now seven recorded so far in 2026. This increasing frequency highlights the dynamic and constantly adjusting nature of the crust in this region.
While these swarms are dominated by microearthquakes that pose little to no hazard, they serve as a constant reminder of the powerful geological forces at work beneath the surface. They provide seismologists with valuable data to study fault behavior, fluid migration, and the intricate relationship between tectonic stress and geothermal processes. The ongoing activity near Milford is being closely monitored by scientists to better understand the mechanisms driving these events and to assess the long-term seismic hazard in this unique and active corner of Utah.