Inside Mt Rainier’s most powerful seismic swarm on record

Earlier this month, Mount Rainier experienced its most powerful seismic swarm ever recorded by the U.S. Geological Survey. More than 1,000 earthquakes have been detected at the mountain since July 8 — far above the volcano’s usual activity level — and the swarm is still continuing.

However, despite the levels of seismic activity, the USGS’s Cascades Volcano Observatory say there’s no sign of magma movement or volcanic unrest. Instead, they believe the swarm was triggered by underground fluids shifting through cracks deep below the mountain.

Research geophysicist Alexandra Iezzi joins us to explain what makes Rainier a unique geological phenomenon and what scientists are watching for.

Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.

Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. Earlier this month, Mount Rainier experienced its largest seismic swarm on record. More than 1,000 small earthquakes were detected at the mountain since July 8 – far above the volcano’s usual activity level. Scientists at the U.S. Geological Survey’s [USCG] Cascades Volcano Observatory are not sounding an alarm, but we did want to learn more about what’s happening. So we’ve invited Alex Iezzi on to help us understand. She is a research geophysicist at the observatory. Welcome to the show.

Alexandra Iezzi: Thank you for having me.

Miller: What is an earthquake swarm?

Iezzi: Yeah, so an earthquake swarm is a series of earthquakes that are happening close in time and usually close in location, underneath the volcano.

Miller: How many little quakes have there been since this swarm started on July 8?

Iezzi: Yeah, so there’s been – I checked this morning – over 1,100 located events and we’re still working on locating a few more.

Miller: How does that compare to baseline regular seismic activity on Mount Rainier?

Iezzi: Over the past couple of decades, Rainier has had about 10 located earthquakes per month, so this is significantly more. It’s been a couple of weeks now and we’re over 1,100.

Miller: The most powerful one of these was a 2.4 magnitude. That’s what I’ve read. Can you just put that in perspective?

Iezzi: Yeah, so 2.4, especially compared to what you’ve been hearing in the news lately, is quite small. It’s very small. It’s about the limit of what you could feel if it was really shallow and you were very still on the surface, but no one has actually felt these earthquakes at Mount Rainier.

Miller: Wow. So if you’re jumping around or doing something, you wouldn’t even feel a 2.4.

Iezzi: No, I felt a 2.5-ish before that was really shallow on one volcano and it was kind of just a couple of seconds of like, huh, what was that?

Miller: But your instruments pick up earthquakes that are way weaker than that, right?

Iezzi: Correct. Yeah, they’re very sensitive instruments.

Miller: I mean, how much can they actually pick up?

Iezzi: We pick up a lot and there’s actually probably hundreds of more earthquakes that have occurred that we actually can’t locate. They’re so tiny, but we have a ton of instruments on Mount Rainier and we’re able to see these very, very tiny earthquakes, including negative magnitude earthquakes.

Miller: What does that mean?

Iezzi: Just very, very tiny.

Miller: OK, so it’s still an earthquake, but just the way the scale works, you call it negative, but it is an actual tiny, tiny earthquake.

Iezzi: Yes, it is an existing earthquake, yeah.

Miller: Why have we gotten such plentiful seismic activity on Mount Rainier this month?

Iezzi: This happens from time to time. Mount Rainier, because there’s a lot of water in the Pacific Northwest [and] it has a lot of glaciers on it, there’s a ton of what we call hydrothermal fluids. So you can think of this as really like hot water or gas, so it’s not magma. Those are moving through the cracks in the system and sometimes if you have a small change in that system, you can have earthquakes that are slipping on a fault. So just really tiny earthquakes that are happening. This happens from time to time at volcanoes worldwide, as well as Mount Rainier.

Miller: Why is it that this swarm is not a cause for regional concern?

Iezzi: The main thing is right now it’s just seismicity. So we’re only seeing these earthquakes that are happening. They’re not necessarily moving within the volcano, they’re kind of staying in a general same location. In addition to seismometers, so those earthquake detectors, we also have a ton of other different types of instrumentation on Mount Rainier.

One of those are geodesy or deformation sensors. So these are really high precision GPS sensors that can see if the volcano is inflating – that would be an indication of maybe we have magma movement under the surface. We are not seeing anything aside from just those earthquakes. So it’s not necessarily a cause for concern and over the past couple of weeks the number has been declining over time.

Miller: So, this is totally different from plates rubbing against each other?

Iezzi: Yeah, very different. The pressurization comes from basically the fluids moving, or in other cases, magma moving under the surface, not from necessarily the plates moving. Though the volcano exists because we’re on a subduction zone, because there are plates moving offshore.

Miller: Are earthquakes like this related in any way to the likelihood of some kind of eruption? I mean, because we’re talking about a volcano, so we’re thinking about both earthquakes and eruptions of different kinds.

Iezzi: Earthquakes themselves can be a precursor to eruptions, but these particular ones are indicative of just those hot fluids moving underground, so basically just water and not magma moving. So at this point, it’s not an increase for concern for people at Mount Rainier or an increased likelihood for eruption.

Miller: What would it take for you and your colleagues to say, “Uh-oh”?

Iezzi: So there’s a few different things. Number one, I’m a seismologist. I study earthquakes and I look at those seismic waves that are traveling through the earth. You could have a different characteristic of those waves. Right now, they’re pretty similar and repeating, but we could have these lower frequency or different types of seismicity. Another cause for concern would be if we actually started to see that volcano deform. So we saw other types of disciplines or instrumentation showing other signs of unrest, but right now we’re not seeing any of that.

Miller: Mount Rainier is the largest Cascade peak in Washington and Oregon. Does it also present different seismic threats or activities than the other big mountains in our region?

Iezzi: Not necessarily different seismic threats. But a lot of the volcanoes in the region, because we have so many glaciers and because they can have eruptions, they have a high threat level, where if you have an eruption, you can melt a lot of that glacier and create these things called lahars or these muddy debris flows that can flow down the valleys. And Mount Rainier has this hazard as well. Most of the other ones do, but Mount Rainier is one of the ones that we watch more closely.

Miller: What are the other ones that we should think about in that high threat category, just by virtue of geology and ice?

Iezzi: We actually have, I think it’s eight very high threat volcanoes in Oregon, Washington. And Idaho is also our jurisdiction, but they’re mainly in Oregon and Washington. And this threat designation is, yes, it includes these muddy lahars, but it’s any type of volcanic activity. That threat level is coupled with, how hazardous it is to either infrastructure or people nearby? So those two together combined to get whether it’s a very high threat or a low threat volcano.

Miller: So we’ve been talking about this swarm of low-level earthquakes on Mount Rainier, but as you suggested, just a few days ago, there was a massive earthquake – among the top 10 on record, I’ve read – in Kamchatka in Russia. What stands out to you about this quake?

Iezzi: This was a very interesting sequence. It wasn’t just that magnitude 8.8 that we heard about the other day. For the past 10 days or so, it’s been having pretty large earthquakes. There were a couple of magnitude 6’s. There was a 7.4 last week. And then we thought the 7.4, I was like, oh, maybe that’s the end of it, but then the 8.8 hit and it was just really large.

Miller: So you and your colleagues, you’ve been paying attention to this well before the rest of the world was?

Iezzi: Yeah, so the earthquakes, the 6.6’s that happened last week, were showing up on our stations here in the Cascades, from Baker all the way down to Crater Lake. It was shaking our really high precision seismometers.

Miller: In a bigger way than the quakes from Rainier?

Iezzi: Yeah. In a bigger way and also a different way. They’re really low frequency, so they’re these really long period waves, as opposed to the ones at Mount Rainier that are kind of like really quick and they’re like this. So they’ll shake our sensors for tens of minutes per earthquake. In this past one, I think it was like six hours or so between the 8.8 and the aftershocks that were happening that we were seeing seismicity on our sensors here.

Miller: I’m curious, you’re a physical scientist, but the work you do is … we pay a lot of attention to it in a kind of social science way. That earthquake led to tsunami warnings in an enormous chunk of the globe, including the Oregon coast. And thankfully, there was a minimal increase in wave activity here. But I’m curious, what effect do you think it could have for there to be a warning and then for there not to be a really serious life altering event that happens in our neighborhood?

Iezzi: We work really closely with the tsunami folks, as well as earthquake folks, because it’s all this multi-hazard that we can face here. If I were to pick one side or the other, I would rather have a warning and it not be so bad, as opposed to not have a warning and it’d be a lot worse than we would expect. But hopefully, we have that sweet spot where we have the warning that is precisely where the actual hazard is. That’s what we strive for, but there is a really big human toll on what we do for natural hazards as well.

Miller: Alex, thanks very much.

Iezzi: You’re welcome. Thank you.

Miller: Alex Iezzi is a geophysicist at the U.S. Geological Survey’s Cascades Volcano Observatory.

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