Yellowstone recorded 77 earthquakes in September, the largest a magnitude 3.4

Yellowstone recorded 77 earthquakes in September 2026, the largest a magnitude 3.4, according to the monthly update from the U.S. Geological Survey’s Yellowstone Volcano Observatory dated Oct. 1. The strongest shock struck on Sept. 6, about 20 miles northeast of Moran, Wyoming, and the observatory describes seismicity across the Yellowstone region as being at background levels.

Nothing in the update points to an eruption. The volcano’s alert level remains Normal and its aviation color code is Green, and the ground-deformation data show little net change across the caldera.

The September count and the Moran earthquake

The observatory’s monthly volcano update, posted at 9:10 a.m. MDT on Thursday, Oct. 1, credits the University of Utah Seismograph Stations with locating 77 earthquakes in the Yellowstone region in September. The update states plainly that “earthquake activity in Yellowstone is at background levels.” The Moran event, a magnitude 3.4 on Sept. 6, was the largest of the month, and the update lists no larger shock. Michael Poland, the observatory’s scientist-in-charge, is the listed contact for the report.

The second event of note was a swarm. On Sept. 8, a cluster of 24 earthquakes occurred about 7 miles north-northwest of West Yellowstone, Montana, with magnitudes ranging from -0.1 to 2.8. A swarm is a burst of many small quakes in one place over a short time, and the March update recorded a smaller one in the same area. The update does not describe this one as unusual.

How September compares with earlier months

Counts vary from month to month, and the observatory’s own archive shows the range. In the April 1 update, the University of Utah network had located 61 earthquakes in March 2026, the largest a magnitude 1.9 microearthquake about 5 miles north of West Yellowstone, and a swarm of 10 that day. That makes September’s 77 a modest increase over March, with a largest event more than a full magnitude unit above it, though both months were still described as background.

A year earlier the numbers were higher. The update for August 2025 reported 94 earthquakes, the largest a magnitude 3.7 near West Yellowstone on Aug. 19, which had four foreshocks and 31 aftershocks. The observatory still called activity background in that month, and a separate swarm of 10 quakes near Old Faithful on Aug. 30 and 31 peaked at magnitude 2.0. The comparison shows that a month with 77 located events and a 3.4 sits comfortably inside the range the region produces, and that the monthly count depends heavily on whether a single sequence occurs.

Magnitude alone does not make a quake a major event in the agency’s own terms. The USGS Earthquake Hazards Program explains on its significant earthquakes page that it designates an earthquake as significant using magnitude, felt-report responses and PAGER alert levels together, so a magnitude 3.4 is judged by more than its number.

The earthquakes are located by the University of Utah Seismograph Stations, whose home page lists the Yellowstone Volcano Observatory among its partners and carries outreach material on the park. The Utah network supplies the counts that the federal observatory reports each month.

Ground deformation and Steamboat Geyser

Seismology is only one of the monitoring streams. GPS measurements show little net deformation of the caldera or the north caldera rim over the past several months, and the update says small variations are likely seasonal and tied to groundwater changes. In March, the observatory had reported that uplift on the north caldera rim that began in July 2025 stopped by mid-January 2026, so the September language is consistent with a quiet, stable ground surface. The same March report logged about 40 eruptions of Echinus Geyser in February after several years of dormancy, and an eruption at Black Diamond Pool in Biscuit Basin on March 9, a reminder that the hydrothermal system can change quickly even when the ground and the seismograms are calm.

Steamboat Geyser had only sporadic minor eruptions in September, and just one major eruption so far in 2026. The August 2025 update said the geyser had two major eruptions that year and that the frequent-activity period that began in 2018 is probably ending, which fits a slower 2026.

Seasonal swings are part of the deformation record. The August 2025 update described an annual summertime pause in caldera subsidence that began in late May, with the caldera rising about 1 centimeter, under half an inch, on top of a subsidence trend dating to 2015 and 2016, and it noted seasonal uplift at Norris Geyser Basin with no overall change over recent years. The October update’s reference to groundwater is the same kind of explanation, and the October update likewise describes deformation over “the past several months” and not over days.

The practical reading is that Yellowstone’s September matched the pattern in the observatory’s archive: dozens of small earthquakes, one minor swarm and a Normal alert level.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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