Mount Rainier’s Earthquake Swarms Are Reminding Scientists of a Bigger Hazard

Seismic, excuse me, activity going on under Mount Rainier this morning. Here's a look at a map of the mountain and the dots there are shown represent a recorded earthquake. On July 8th, 2025, Mount Rainier began to shake. What followed became the largest earthquake swarm ever recorded beneath the volcano, with more than 1,350 quakes. Yet the U.S. Geological Survey kept the alert level at normal. Why? Because the greatest danger isn't an eruption. Scientists are far more concerned about something faster and far more destructive. Something Rainier has unleashed before and could unleash again. So what is Mount Rainier really hiding? The mystery. Let's start with what actually happened in July because the numbers tell a strange story. A normal month at Mount Rainier produces about nine earthquakes, nine. You could count them on your fingers. The mountain twitches a little, the way any volcano with a pulse does, and that's that. The swarm that began on July 8th produced more earthquakes in its first morning than the mountain usually generates in four months. The Pacific Northwest Seismic Network runs about 20 seismic stations on and around Rainier and they watched the rate climb to 41 located events per hour. Then they watched it do something almost more interesting. It didn't explode upward into an eruption, it just kept going. Day after day. Hundreds of tiny tremors, most of them so small that no human standing directly on top would ever feel them. The biggest one? A magnitude 2.4 on July 11th. To put that in perspective, that's roughly the energy of a truck hitting a pothole. Far too weak to feel. Far too weak to crack a window. So a question that nobody could answer in those first few days. If it isn't magma forcing its way up and the instruments showed no sign that it was, then what was shaking a mountain 1,350 times? The scientists at the Cascades Volcano Observatory had a candidate and it's not the one most people expect. Their best explanation is water, not lava. Water. Superheated, pressurized fluid moving through cracks in the rock about a mile and a half to four miles beneath the summit. The Pacific Northwest is soaked. Rainier carries more glacial ice on its back than every other cascade volcano combined, close to a cubic mile of it. All that water seeps down, gets heated by the magma sitting deep below, and turns into something the geologists call hydrothermal fluid. Think of a kettle the size of a city buried under a mountain. Shift the pressure in that buried kettle even slightly and the rock around it slips. A tiny earthquake, then another, then 1,350. That's the leading theory. Notice the word theory because here's what one of the observatory's own geophysicists admitted on Oregon Public Broadcasting in the middle of the swarm. They do not have a good estimate for how long it will last or whether it will fade out or intensify. Read that again. The people with 20 stations and 70 years of records were watching the largest swarm ever recorded at one of America's most dangerous volcanoes and they could not tell you what it would do next. Hold on to that uncertainty. We're going to come back to it and it's going to matter more than you think. If you're the kind of person who needs to know how the story ends, subscribe now because the ending of this one hasn't been written yet and we're going to walk right up to the edge of it. The History To understand why the scientists shrugged at 1,350 earthquakes but lose sleep over something else, you have to leave the summit and drive down into the valleys. About 30 miles from the peak, in a fertile green basin between two rivers, sits a town called Orting. Population somewhere around 8,000. There's a school there, Potamigan Ridge. From the playground on a clear day, the children can see Mount Rainier filling the sky. What almost none of them think about, what you wouldn't think about either, is what they're standing on. Orting is not built on ordinary ground. The entire town sits on top of a graveyard, layer after layer of old mud, hardened into earth, that came roaring down off the mountain in the past. In some places near Orting, that ancient mud is 20 to 26 feet deep. People mow lawns and walk dogs on top of it without ever knowing it's there. Geologists have a word for what made it, a word that came from Indonesia, from people who lived with the same nightmare, lahar. A lahar is a volcanic mudflow, but mudflow undersells it the way weather undersells a hurricane. A lahar is what happens when rock, ice and water tear off a mountain at once and mix into something with the consistency of wet concrete. And then that concrete flows downhill at highway speed, and Rainier has produced them again and again and again. The US Geological Survey has counted more than 60 lahars off this single mountain in the last 10,000 years. 60. The largest tend to come roughly every 500 to 1,000 years. Now, meet the worst of them. About 5,600 years ago, an enormous chunk of Mount Rainier's northeast flank simply collapsed. The failure carved a horseshoe-shaped crater nearly a mile and a half wide, almost the same size as the scar Mount St. Helens left in 1980. What slid off the mudflow and its scale is genuinely hard to hold in your head. It moved 3.8 cubic kilometers of material. It buried an area of around 212 square miles. It traveled at roughly 43 miles per hour, and it kept going for tens of miles, all the way down to the Puget Sound lowland, to the very ground where Kent, Auburn and the Port of Tacoma now stand. North of Auburn, the deposit it left is buried as deep as 260 feet, two-thirds of it below today's sea level. The communities that now sit on top of Osceola mud read like a regional map. Orting, Buckley, Sumner, Puyallup, Enumclaw, Auburn. Towns full of people who go to work, raise kids, and have no daily reason to think about the fact that their foundations are the disaster. Here's the question that should be making the hair on your arms stand up. If a mountain has done this more than 60 times, if the big ones come every 500 years or so, then when was the last one? And how long has it been? We'll get there. But first, you need to understand how this mountain does it. Because the mechanism is one of the strangest things in earth science. And it's the reason Rainier doesn't even need to erupt to kill. The Strange Science Most people picture volcanic death as fire, lava, ash, an explosion you can see coming for days. Mount Rainier's signature catastrophe is the opposite. It's slow, invisible, and it's happening right now inside the mountain while it looks perfectly serene from the valley. The mountain is rotting. Remember that buried kettle of hydrothermal fluid? It does more than cause earthquakes. The gases rising from the magma below are rich in sulfur. When that sulfur meets the groundwater soaking through the volcano, it forms sulfuric acid. And that acid sits inside Mount Rainier year after year, century after century, quietly eating the rock. It doesn't blast the rock apart. It dissolves it. It chemically converts solid volcanic stone into soft, wet, slippery clay. The US Geological Survey calls this hydrothermal alteration. And the result is a mountain that looks like granite on the outside but, in places deep within, has the structural integrity of wet pottery. Now stack that on top everything else Rainier is carrying. A cubic mile of glacial ice, steep upper slopes, and, hidden inside, growing zones of acid-weakened clay that can no longer hold the weight above them. You can probably see where this is going. Eventually, a chunk of that weakened mountain lets go. Gravity does the rest. The collapsing rock, already half-turned to clay and saturated with water, doesn't tumble like a rock slide. It flows. It becomes a lahar before it's even left the mountain. The Osceola mudflow began exactly this way, as a sector of acid-rotted rock that turned to liquid within a mile of where it broke loose. And here is the single, most unsettling fact in this entire story. Because the trigger is the clay, not the magma. Mount Rainier does not have to erupt to produce a catastrophic lahar. Let that sink in. The early warning playbook everyone imagines—rumbling, venting, weeks of obvious volcanic buildup, time to evacuate—may not apply at all. A large enough section of weakened rock could simply fail, on a calm day, with no eruption, with almost no warning. How do we know that's not just a theory? Because it has already happened. And it happened far more recently than 5,600 years ago. If your stomach just dropped a little, hit subscribe. What's coming next is the part the people living in those valleys try not to think about. The Hidden Danger Roughly 500 years ago, geologically at the blink of an eye, a section of Mount Rainier's west flank collapsed. There was no major eruption, no fire in the sky, just hydrothermally weakened rock finally giving way. It's called the Electron Mudflow. It surged about 60 miles down the Puyallup River valley. When it reached the lowland near the community of Electron, it was still roughly 100 feet deep. Call it a 10-story building made of moving mud, it rolled across 13 square miles. And the ground it covered is the same ground where Orting, Sumner, and Puyallup sit today. The Electron Mudflow is the warning shot. It proved that this mountain can kill on a quiet day, with no eruption to announce it, in living geological memory. So what would it look like now? The U.S. Geological Survey ranks Mount Rainier as the third most dangerous volcano in the United States, and one of only 16 decade volcanoes on the entire planet, a list reserved for volcanoes that combine real hazard with large nearby populations. The hazard here isn't the eruption. The official greatest danger at Rainier, in the agency's own words, is the lahar. The numbers on a modern flow are brutal in their simplicity. A lahar coming off the west flank could reach the Nisqually Park entrance in about 10 minutes. It could reach Orting in roughly 30 to 60 minutes, a wall of churning rock and mud moving somewhere between 20 and 50 miles an hour, deep enough in places to swallow buildings whole. How many people live in the path? Tens of thousands directly in the modeled inundation zones. By some estimates, well over 100,000 people live on top of old lahar ground across the region. Orting, Sumner, and Puyallup alone account for more than 50,000. An older USGS estimate put the lifetime odds of a destructive lahar reaching Orting at roughly 1 in 7 for a resident living their full life there. One in seven. That's not a freak accident probability. That's the kind of odds you'd refuse to fly on. This is why the region has done something no other place in America has. In March of 2024, around 45,000 schoolchildren took part in the largest lahar evacuation drill on Earth, kids walking the routes to high ground, practicing for the day the sirens go off. There are siren towers. There are posted evacuation routes along the valley roads. The Cascades Volcano Observatory has been expanding a dedicated lahar detection network, with sensors built to catch the ground-shaking signature of a flow the instant it starts moving. Because the only defense against something this fast is to know about it before it reaches you. On paper, the region is as prepared as anywhere in the world. But a drill is a sunny Tuesday with advance notice. What happens when it's real? The Future Disaster Here's where the comfortable story breaks down. Every preparation assumes warning. The sirens assume the detection network catches the flow in time. The evacuation routes assume people are awake. The roads are clear and there's no panic. Now, picture the version nobody likes to talk about. A section of acid-weakened rock fails at three in the morning. No eruption preceded it. Remember, it doesn't need one. The detection system triggers. The sirens sound across the valley. And tens of thousands of sleeping people have, at best, a few dozen minutes to get to high ground in the dark, on rural roads, against traffic, in the cold. Not every siren works perfectly. Not every road stays open. Not every person hears the alarm. And there's a newer pressure on the mountain that the ancient lahars never had to reckon with. Rainier's glaciers are thinning. Long-term climate records—the kind tracked by federal climate and earth science agencies—show ice retreating across the Cascades. Less ice changes the loads and the meltwater on those steep, weakened slopes in ways scientists are still working to understand. Whether that makes the next collapse more likely is an open question. It is not a reassuring one. Even Seattle—too high to be physically buried—isn't safe from the consequences. The ports, the freeways, the rail lines that tie the whole Puget Sound economy together run straight through the hazard zones. A major lahar wouldn't just be a local tragedy. It would sever the supply chains of an entire region for weeks. The bill would be national. And the thing standing between all that and a normal Tuesday is a mountain that is, by every measurement we have, slowly weakening from within. On a clock we cannot read. The Unanswered Question So let's go back to the question from the beginning—the one the scientists couldn't answer. The big lahars come roughly every 500 to 1,000 years. The electron mudflow was about 500 years ago. Do the math and you arrive at a sentence that no geologist will say out loud as a prediction because the timing of these events scatters wildly and the next one could be a thousand years away. But that hangs in the air anyway. We are inside the window. That doesn't mean tomorrow. It doesn't mean next year. The honest answer—the one the U.S. Geological Survey will actually give you—is that they cannot predict the day. A swarm of 1,350 earthquakes can rattle the summit and mean nothing. Or a quiet afternoon with no swarm at all can be the one that matters. The mountain keeps its own counsel. That's the strange, quiet horror of Mount Rainier. It isn't a monster. It's the most beautiful thing on the horizon for millions of people. They wake up to it. They put it on postcards. Children draw it. And the whole time, deep inside, the acid is working. The clay is spreading. The clock is running. And nobody—not the observatory, not the seismologists, not the sirens waiting silently in the valley towns—nobody knows what time it is. So I'll leave you with the question they left me with. When the largest earthquake swarm ever recorded at this volcano can come and go without raising the alarm, how would we ever know the warning when it finally comes that it is the warning? If this is the kind of story you want more of—the real ones, the ones hiding in plain sight under places people call home, subscribe, and turn on notifications—there's another mountain, and it's been quiet for a very long time.