The Yellowstone Volcano Observatory located 77 earthquakes in the Yellowstone region during September 2026, and its monthly update, issued on Oct. 1, called that activity background. The strongest was a magnitude 3.4 about 20 miles northeast of Moran, Wyoming, on Sept. 6. The same month brought a small hydrothermal explosion at Biscuit Basin on Sept. 27.
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Neither number is an eruption warning. A hydrothermal explosion is a burst of boiling water, steam, mud and rock from the shallow plumbing under a thermal feature, and it is a different process from the magma-driven eruptions that the word volcano suggests. What follows is a reading guide to the monthly notice, written to stay true long after the month ends.
What the 77-earthquake count in the September update actually measures
In its Oct. 1 monthly update covering September 2026, the observatory reported that the University of Utah Seismograph Stations located 77 earthquakes in the Yellowstone region. The notice lists the volcano alert level as normal and the aviation color code as green, the lowest rung on both scales, and says the seismicity sits at background levels. A monthly total is a tally of located events, most of them too small for anyone to feel.
The largest, magnitude 3.4, struck at 10:25 p.m. Mountain time on Sept. 6, roughly 20 miles northeast of Moran. A magnitude 3.4 is strong enough to be felt close to its source but is a size the monitoring network handles as part of its normal monthly tally. The update also describes a small swarm of 24 earthquakes on Sept. 8, about 7 miles north-northwest of West Yellowstone, Montana, with magnitudes from negative 0.1 to 2.8. The swarm is included in the monthly total of 77.
Reading a number like 77 well means comparing it with the observatory’s own earlier notices. The July 1 update for June 2026 counted 118 located earthquakes, with a largest event of magnitude 2.4 about 12 miles south-southeast of Lake. Swinging from 118 to 77 in three months, with no change in the alert level, is the sort of month-to-month variation the observatory treats as normal for a large, active hydrothermal and tectonic system.
The Biscuit Basin explosion of Sept. 27 and the June 13 blast behind it
According to the same update, just after 8:12 p.m. Mountain time on Sept. 27 a small pool in Biscuit Basin threw boiling water, steam, mud and rocks at least 100 feet, or 30 meters, into the air. The pool had formed by collapse after an explosion on June 13, 2026, and the observatory called the Sept. 27 event the first significant explosion from that feature. Seismic, acoustic and camera stations nearby recorded it. Black Diamond Pool, also in Biscuit Basin, had a large mud-and-water event on Sept. 23.
The June 13 explosion has its own record. The observatory’s July notice says a small hydrothermal explosion occurred in Biscuit Basin around 5:09 a.m. Mountain time that day, about 30 to 60 meters north of Black Diamond Pool, and the observatory also published a monthly update video on the second Biscuit Basin explosion for June. Two blasts three and a half months apart, in the same few dozen meters of ground, point to a feature that is still adjusting.
The location matters because Biscuit Basin has a history. On July 23, 2024, Black Diamond Pool exploded at about 9:53 a.m. Mountain time, roughly 2.1 miles northwest of Old Faithful. The observatory’s August 2024 monthly update said water had flashed to steam because of pressure changes in the shallow plumbing, that the event was not associated with any earthquakes, and that more than 1,400 rocks larger than 40 centimeters landed around the pool. It added that silica buildup likely sealed some conduits and raised the pressure, and that explosions of that size probably recur once a decade or every few decades.
Michael Poland and the monitoring network that produced the notice
The update is signed by Michael Poland, the observatory’s scientist-in-charge, a USGS geophysicist. The Yellowstone Volcano Observatory is a partnership that includes the USGS, the University of Utah, Yellowstone National Park and other institutions, and its chief seismologist is Jamie Farrell, an associate research professor at the University of Utah Seismograph Stations. The earthquake count comes from the university’s network, while the explosion was caught by instruments that the partners installed in the basins themselves.
Some of that equipment is new. A Sept. 28 observatory article describes a hydrothermal station called YGH near Geyser Hill in the Upper Geyser Basin, carrying weather, seismic and infrasound sensors, plus a renewed Yellowstone Lake camera that updates every 15 minutes. A second station had been placed at Biscuit Basin in summer 2025, and it recorded the small explosion just north of Black Diamond Pool in June 2026. On Sept. 10, infrasound from the Biscuit Basin and Geyser Hill stations together pinpointed Giant Geyser as the source of a signal.
Deformation, geysers and what the green status covers
Ground movement is the other half of any caldera check. The September notice reports no significant uplift or subsidence of the caldera or its north rim over the past several months, with GPS data showing little net deformation and small variations likely tied to seasonal groundwater. Giant Geyser erupted on Sept. 10 and Sept. 22, its fourth and fifth eruptions of 2026, and the update says it may be entering a new stage of frequent activity. Echinus Geyser erupted on Sept. 15, while Steamboat Geyser has had only sporadic minor eruptions and one major eruption so far this year.
The observatory keeps its running archive of notices on the USGS volcano updates page, which is the place to find the next month’s count. Because each notice covers one named month, a reader can check a claim like 77 against the original and see what the observatory said, not what a headline summarized. The Biscuit Basin explosions are best understood as hazards to people standing near hot water, which is why the 2024 explosion led to the area being closed to visitors, and not as a sign that magma is moving.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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