Exploring Old Faithful’s geology

My name is Jamie Farrell, I’m a seismologist
at the University of Utah. We operate the Yellowstone seismic network. Over the last three years we’ve been doing
some work at the Upper Geyser Basin around Old Faithful using these smaller seismometers. This is one of them right here. This little thing right here has everything
in here that we need to record seismic data. So there’s what we call a geophone in there
that’s monitoring ground movement in all three directions, north, south, east, west and up
and down. It has a battery. We just basically spike these things into
the ground and we can put up to, we’ve put up to over 500 of these things in the area. They’re very sensitive to ground movement. One summer we put in just a few stations to
see the difference between summer activity and winter activity. We put a few stations in, and this one in
particular that was up on Geyser Hill, so this is away from Old Faithful. It’s very quiet at night, which is what we
would expect. And then during the day it slowly ratcheted
up to higher and higher noise levels during the day. But i noticed this sawtooth pattern that was
going up and down. And I plotted timing of Old Faithful eruptions. You could see this noise level from people
walking around the boardwalk; it would build up and once it got close to an Old Faithful
eruption it would die down because all those people moved away from the station over towards
Old Faithful. One of the more interesting ones we’ve found
is this pool up on Geyser Hill called Doublet Pool. And it doesn’t erupt or anything like that,
but in 2015 about every 40 minutes or so the pool would start to bubble and get agitated
and then the surface would start to thump up and down. And if you’re really quiet, if you’re sitting
there on the boardwalk you can literally feel and hear these thumps. And interestingly enough in 2017 we noticed
something different about Doublet Pool. So, not only was it happening more often,
it was happening about every 29 to 30 minutes instead of 40 minutes in 2017, but each episode
was lasting about a minute or so longer. But really the focus of these experiments
we’ve been doing has been Old Faithful. So we’ve done three different, what we call,
campaigns around Old Faithful. So, each one of these occur in early November
after the park closes. In the first year, 2015, we put in about 133
stations. In 2016 we came back and we did something
a little bit different. We did a much denser array around Old Faithful. And then in 2017 we came back and did more
of a general array around Old Faithful and we moved the dense array up to the Geyser
Hill area and then we did a bigger footprint across the whole Upper Geyser Basin. If you look at the seismic signals recorded
around Old Faithful you see this general buildup of what we call hydrothermal tremors. The seismic signal for the actual eruption
is much smaller than this pre-play. What we figured out that we’re seeing here
is that as Old Faithful is building up towards an eruption, the underground plumbing system
is filling up with this superheated water and you have these bubbles being formed. They reach the top of that water column where
it’s relatively cooler than where these bubbles were formed, and they implode. And when those bubbles implode, that creates
the seismic signal that we’re recording. If it’s the same every time, or there’s some
predictable decay on that, then there is potential for us to really be able to predict Old Faithful
eruptions very accurately, to within less than a minute. One interesting thing you can do with these
really dense seismic networks is you can look at how these seismic signals travel through
this area that’s covered by these seismometers. And you can really look and see what’s going
on as these seismic signals travel through the earth. We saw that there’s this one area just basically
to the west of Old Faithful where it was consistently delaying seismic signals as it traveled through. We interpreted that as this underground reservoir
of hydrothermal fluids that’s feeding Old Faithful. And what we found is when these seismic signals
travel through this body, this – it’s not just an open body of water underground, it’s
really just a series of probably fractures that are filled with hydrothermal fluids in
this area. But those fluids, the presence of those fluids,
changes the way these seismic signals travel through this area. And what we see is that once these seismic
signals travel through there, they kind of get trapped in this area and they just bounce
back and forth and reverberate. Keep in mind that these are very small signals. You can’t feel these yourself. These are very small, only these instruments,
these very sensitive instruments can see these. And, so this can really tell us about, if
there was a larger earthquake in the area, say like Hebgen Lake, 1959, those seismic
signals would travel through that same low-velocity body and get trapped. And that has implications for seismic hazards
in this area. So the Old Faithful Inn is sitting right on
top of this body. And in fact the Old Faithful Inn was damaged
by the Hebgen Lake earthquake. The, some of the, the large rock chimney collapsed. This coming fall, this November, we’re going
to do similar types of experiments, only we’re going to move to Norris Geyser Basin. They’re in different geology, these two geyser
basins. The other thing is, Norris is one of the hottest
areas in the park and it’s also very dynamic. There’s always new features popping up here
or there. Also, Norris Geyser Basin is home to one of,
the tallest active geyser in the world, Steamboat Geyser, which actually has just erupted. If we get lucky, maybe Steamboat will erupt
when we have instruments in the ground, that’d be very lucky. Yellowstone is the location of some of the
largest, what we call, hydrothermal explosions in the world. And these things happen every year. Most of the time they’re small and they’re
in the backcountry so they’re not a hazard to people, but they have happened in frontcountry
geyser basins before. And what happens is these hydrothermal systems,
these nutrient-rich waters are coming up from underground and they erupt and they form these
really cool pools and stuff like that, but they’re also depositing these minerals as
they come up through the plumbing system. And sometimes they clog themselves up. And that pressure builds up and builds up
until it releases it in what we call a hydrothermal explosion where you’re releasing hot mud,
hot water, hot gas, and then rocks that blow out. That can be very dangerous to people if it
happens close to the boardwalk. If there are any kind of seismic precursors
to one of these hydrothermal explosions, and if we can record those and identify those,
we can look for those in real-time. One thing I really like about working in Yellowstone
is it’s a very active, very dynamic system. We record anywhere from 1,500 to 3,000 earthquakes
a year. Sometimes there’s gaps, sometime we get these
earthquake swarms, that we call, that you’ll get a lot of earthquakes in one area over
a short period of time. Just last year we had what we call the Maple
Creek swarm where we had over 2,500 earthquakes located near the town of West Yellowstone. And many of them were felt by people. Honestly, every single year we learn something
new about the Yellowstone volcanic system. And that’s, you know, that’s the fun of it.