Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dan Dzurisin, emeritus geologist with the U.S. Geological Survey.

“It was a calm, peaceful Thursday afternoon. Birds softly chirped their songs amid babbling brooks as a gentle breeze kissed the treetops of Yellowstone National Park.”

That’s how a local news source in Gillette, Wyoming, set the scene for a recent story about an event that occurred almost 50 years ago and that received only passing interest at the time. The story got the date and day of the week wrong, but that seems oddly appropriate for an event that, despite its importance, has received relatively little attention.

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

The epicenter of the magnitude-6 Yellowstone National Park earthquake on June 30, 1975 (a Monday!), was located along the north-central boundary of Yellowstone Caldera, a few kilometers (miles) southeast of Norris Geyser Basin. Despite it being the largest recorded earthquake inside the boundary of Yellowstone National Park, even to this day, the temblor did not make much news. There were reports of a damaged chimney and a rockfall that closed the road between Norris and Madison Junction overnight, and telephone service was temporarily down in West Yellowstone, Old Faithful, and Madison.

No injuries were reported, and all park campgrounds and facilities remained open. Soon after the quake, changes were noted in several thermal features in Norris Geyser Basin, and the average interval between eruptions of Old Faithful Geyser increased by about 3 minutes. Water levels decreased and turbidity increased temporarily at a few thermal features in Norris Geyser Basin and there were a few new breakouts of thermal fluids, and some nearby rivers turned muddy for a few days due to increased sediment loads. Otherwise, the Park’s normal routine was unaffected.

Muddy thermal discharge (foreground) near Congress Pool (middle ground) in Norris Geyser Basin following the June 30, 1975, magnitude-6 Yellowstone National Park earthquake/NPS, Rick Hutchinson.

Yellowstone is home to many well-known superlatives: the first U.S. National Park and a UNESCO World Heritage Site, one of the largest active magmatic systems and the tallest geyser in the world, and the most numerous and most diverse hydrothermal features anywhere on the planet, to name a few. Why did Yellowstone National Park’s largest recorded earthquake receive so little attention? Such an event today would rattle news outlets around the world and undoubtedly raise unfounded fears of an impending volcanic catastrophe (spoiler alert: earthquakes don’t trigger Yellowstone eruptions!).

There are several likely explanations, none of which diminish the earthquake’s importance. First, human memory is short, and the quake occurred almost 50 years ago. Second, when it comes to large Yellowstone earthquakes, the 1959 magnitude 7.3 Hebgen Lake earthquake is the undisputed leader. Its epicenter was just outside the western boundary of Yellowstone National Park and about 10 km WNW of West Yellowstone, Montana. That earthquake caused 28 fatalities, most of them as a result of a large landslide that was triggered 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 canyon and halfway up the north side, partially burying Rock Creek campground.

It also dammed the Madison River, causing water to back up behind it, creating Earthquake Lake. Fault offsets of up to 6 meters (20 feet) can still be seen today. Because the 1959 quake occurred outside the park boundary, the smaller 1975 quake is accorded the “largest inside the park” title. But it’s easy to understand why the latter is overshadowed by the much greater human impacts of the 1959 Hebgen Lake quake.

Upper left: Contours of surface uplift that occurred in the Yellowstone caldera between leveling surveys in 1923 and 1975-77. Lower right: Locations of key benchmarks. The central part of the caldera floor rose more than 700 millimeters (28 inches), at an average rate of 14 millimeters per year (0.5 inch per year). The greatest uplift occurred between the Mallard Lake and Sour Creek resurgent domes, including near Old Faithful (OF, benchmark F10) and LeHardys Rapids (LH, benchmark DA3).

There might be a third reason why you haven’t heard more about the 1975 Yellowstone Park earthquake, and ironically it helps to explain the quake’s importance. Fifty years ago, scientists had a very different understanding of the state of Yellowstone’s magmatic system.

It was known that Yellowstone had experienced three caldera-forming eruptions in the past 2.1 million years, and that the youngest caldera had been mostly filled by subsequent rhyolite lava flows. But whether the magmatic system beneath the caldera was moribund or still active was an open question. In that context, another felt earthquake in the Intermountain West would not have stirred much interest. But things were about to change.

A research paper published in 1975, one month before the earthquake, presented several lines of evidence for magma beneath Yellowstone, but the bigger question of whether the system was still active or simply remnant from past eruptions was still open. Until the aftermath of the 1975 Yellowstone Park earthquake, that is.

In response to the quake, scientists from the U.S. Geological Survey and the University of Utah commissioned a leveling survey to learn whether there had been any measurable uplift or subsidence of the ground surface near Norris. Surprisingly, the survey showed that there had been very little net movement there during the five decades since the prior survey in 1923.

But the results elsewhere were beyond surprising—they were astounding. The central part of Yellowstone Caldera had been uplifted almost 72 cm (28 inches)—a change that could not reasonably be attributed to the 1975 earthquake (or the larger 1959 quake, for that matter). The uplifted area spanned the entire caldera floor and, combined with the newly reported evidence that magma still existed beneath the caldera, the conclusion was inescapable.

Gibbon River in Gibbon Meadows immediate following the June 30, 1975, magnitude-6 Yellowstone National Park earthquake. The muddy color is due to increased sediment load. NPS photo by Rick Hutchinson.

In a landmark research paper published in 1979, the authors (one of whom, Bob Smith of the University of Utah, later went on to help found the Yellowstone Volcano Observatory) concluded: “The most likely cause of this rapid and unusually large surface deformation is a recent influx of partially molten material to a location within the crust beneath Yellowstone National Park.”

Yellowstone was an active magmatic system, a living and breathing volcano!

The mid-1970s research findings, spurred in no small part by the 1975 Yellowstone National Park earthquake, revolutionized our understanding of Yellowstone as an active magmatic system. Subsequent research has produced a wealth of new information about the structure and workings of one of the world’s largest and most dynamic calderas. Part of the impetus for those discoveries was a nearly forgotten earthquake almost 50 years ago. The rest, as they say, is history.

(Acknowledgement: Special thanks to M.A. Bellingham for her help in researching the effects of the 1975 Yellowstone National Park earthquake.)