Remember the 1975 Yellowstone earthquake

Editor’s note: The Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week’s contribution is from Dan Dzuresin, Geologist Emeritus at the USGS.

“It was a quiet, calm Thursday. Birds quietly chirped their songs amid lush streams while a gentle breeze kissed the treetops of Yellowstone National Park.”

So a local news source in Gillette, Wyoming, set the scene for a recent story about an event that took place nearly 50 years ago that received only fleeting attention in that time. The story gets the date and day of the week wrong, but this seems oddly fitting for an event that, while important, has received comparatively little attention.

Image of damage to the Mammoth-Norris Highway, just south of Mammoth Hot Springs, by the magnitude 6 earthquake of June 30, 1975 / Photo by Haynes Inc. for the Deseret News.

The epicenter of the magnitude 6 Yellowstone National Park earthquake of June 30, 1975 (Monday!) was along the north-central boundary of the Yellowstone Caldera, a few kilometers (miles) southeast of the Norris Geyser Basin. Despite being the largest earthquake ever recorded within the boundaries of Yellowstone National Park, the earthquake has not made much news to this day. There were reports of a damaged chimney and boulder blocking the road between Norris and Madison Junction overnight, and phone service was temporarily disrupted in West Yellowstone, Old Faithful, and Madison.

No injuries were reported, and all campgrounds and facilities remained open. Shortly after the earthquake, changes in several thermal features were observed in the Norris Geyser Basin, and the average interval between eruptions of Old Faithful Geyser increased by about 3 minutes. Water levels decreased and turbidity temporarily increased at some thermal features in the Norris Geyser Basin and there were a few new hydrothermal fluid collapses, and some nearby rivers turned muddy for a few days due to increased sediment loads. Otherwise, Park’s normal routine wasn’t affected.

Muddy heat discharge (foreground) near Congress Pond (Middle Earth) in the Norris Geyser Basin after the June 30, 1975 Yellowstone National Park earthquake of magnitude 6/NPS, Rick Hutchinson.

Yellowstone is home to many well-known superlatives: America’s first National Park and UNESCO World Heritage Site, one of the largest active magma systems and longest running geysers in the world, and the most numerous and diverse hydrothermal features anywhere on the planet, just to name a few. Why didn’t the largest earthquake ever recorded in Yellowstone National Park get much attention? An event like this today would rattle news outlets around the world and undoubtedly raise unfounded fears of an impending volcanic catastrophe (spoiler alert: earthquakes do not lead to an eruption of Yellowstone!).

There are several possible explanations, none of which diminish the significance of the earthquake. First, human memory is short, and the earthquake happened nearly 50 years ago. Secondly, when it comes to large earthquakes in Yellowstone, the 7.3 magnitude Hebgen Lake earthquake in 1959 is the undisputed leader. The epicenter was just outside the western boundary of Yellowstone National Park and about 10 kilometers northwest of West Yellowstone, Montana. This earthquake killed 28 people, mostly as a result of a large landslide that occurred in Madison Canyon. The landslide carried about 38 million cubic meters (50 million cubic yards) of rock, mud and debris down the south side of the valley and halfway to the north side, partially burying Camp Rock Creek.

It also dammed the Madison River, causing water to accumulate behind it, creating Quake Lake. Fault displacements of up to 6 m (20 ft) can still be seen. Because the 1959 earthquake occurred outside the park boundaries, the smaller 1975 earthquake was given the title of “largest within the park”. But it is easy to understand why the much larger human impacts of the 1959 Lake Hebgen earthquake overshadowed the latter.

Top left: Uplift profiles that occurred in the Yellowstone Caldera between settlement surveys in 1923 and 1975-1977. Bottom right: locations of major criteria. The central part of the caldera floor has risen by more than 700 mm (28 in), at a rate of 14 mm per year (0.5 in per year). The largest uplift occurred between the domes of Mallard Lake and Wall Creek, including near Old Faithful (OF, standard F10) and LeHardys Rapids (LH, standard DA3).

There may be a third reason why you haven’t heard more about the 1975 Yellowstone Park earthquake, and ironically, it helps explain the significance of the earthquake. Fifty years ago, scientists had a very different understanding of the state of the magma system in Yellowstone.

Yellowstone was known to have experienced three caldera-forming eruptions in the past 2.1 million years, and the youngest caldera was mostly filled by subsequent rhyolitic lava flows. But whether the volcanic system below the caldera was dying or still active was an open question. In this context, I felt another earthquake in the West Intermountain that would not have aroused much interest. But things were about to change.

A paper published in 1975, one month before the earthquake, provided several lines of evidence for magma beneath Yellowstone, but the larger question of whether the system is still active or just remnants from earlier eruptions remains open. Until after the 1975 Yellowstone Park earthquake, that is.

In response to the earthquake, scientists from the USGS and the University of Utah conducted a settlement survey to see if there was any measurable uplift or subsidence of the Earth’s surface near Norris. Surprisingly, the survey showed that there had been very little net movement there in the five decades since the previous survey in 1923.

But the results elsewhere have been anything but amazing – they have been amazing. The central part of the Yellowstone caldera has been raised about 72 cm (28 in) — a change that cannot reasonably be attributed to the 1975 earthquake (or the larger 1959 earthquake, for that matter). The rising region extended to the entire bottom of the caldera, and combined with newly reported evidence that magma still existed beneath the caldera, the conclusion was inevitable.

Gibbon River at Gibbon Meadows immediately after the June 30, 1975 magnitude 6 earthquake in Yellowstone National Park. The muddy color is due to sediment overload. NPS image by Rick Hutchinson.

In a landmark 1979 paper, the authors (one of whom was Bob Smith of the University of Utah, later to help found the Yellowstone Volcano Observatory) concluded: “The most likely cause of this unusually large and rapid surface deformation is a recent outflow of partially magma to a site within The crust under Yellowstone National Park.

Yellowstone was an active magma system, a living, breathing volcano!

The findings of the research in the mid-1970s, spurred on by the 1975 Yellowstone National Park earthquake, revolutionized our understanding of Yellowstone as an active magma system. Subsequent research has produced a wealth of new information about the structure and functioning of one of the largest and most dynamic calderas in the world. Part of the impetus behind these discoveries was an almost forgotten earthquake nearly 50 years ago. The rest, as they say, is history.

(Acknowledgments: Special thanks to MA Bellingham for her assistance in researching the aftermath of the 1975 Yellowstone National Park earthquake.)

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