In that context, another felt earthquake in the Intermountain West would not have stirred much interest. But whether the magmatic system beneath the caldera was moribund or still active was an open question. 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. Fifty years ago, scientists had a very different understanding of the state of Yellowstone’s magmatic system. 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. 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). 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). Lower right: Locations of key benchmarks. Upper left: Contours of surface uplift that occurred in the Yellowstone caldera between leveling surveys in 19-77. But it’s easy to understand why the latter is overshadowed by the much greater human impacts of the 1959 Hebgen Lake quake. Because the 1959 quake occurred outside the park boundary, the smaller 1975 quake is accorded the “largest inside the park” title. Fault offsets of up to 6 meters (20 feet) can still be seen today. It also dammed the Madison River, causing water to back up behind it, creating Earthquake Lake. 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. That earthquake caused 28 fatalities, most of them as a result of a large landslide that was triggered in Madison Canyon. Its epicenter was just outside the western boundary of Yellowstone National Park and about 10 km WNW of West Yellowstone, Montana. Second, when it comes to large Yellowstone earthquakes, the 1959 magnitude 7.3 Hebgen Lake earthquake is the undisputed leader. First, human memory is short, and the quake occurred almost 50 years ago. There are several likely explanations, none of which diminish the earthquake’s importance. 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!). 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. Yellowstone is home to many well-known superlatives: the first U.S. 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 photo by Rick Hutchinson. Otherwise, the Park’s normal routine was unaffected. 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. 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. No injuries were reported, and all Park campgrounds and facilities remained open. 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. 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. The epicenter of the magnitude-6 Yellowstone National Park earthquake on J(a Monday!), was located along the north-central boundary of Yellowstone Caldera, a few kilometers (miles) southeast of Norris Geyser Basin. 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. 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. Birds softly chirped their songs amid babbling brooks as a gentle breeze kissed the treetops of Yellowstone National Park.” “It was a calm, peaceful Thursday afternoon. Photo of damage to the Mammoth-Norris highway, just south of Mammoth Hot Springs, caused by the June 30, 1975, magnitude-6 earthquake.
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