What geologic hazard is most likely to occur
in the Yellowstone region on human timescales?
Spoiler alert, it has nothing to do with a volcano.
We'll answer that question in this August, 2023 update,
from the Yellowstone Volcano Observatory.
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A common misconception is that a volcanic eruption
is the most likely thing to occur in the Yellowstone region
on the timescale of a human life.
There have been some huge eruptions.
631,000 years ago was the formation of Yellowstone Caldera.
That was a massive explosion,
but those only happen about once or twice per million years.
Slightly more common are lava flows.
There have been a few dozen of those
in the last million years,
but the last one was 72,000 years ago,
so even that isn't so common.
But strong earthquakes,
those happen once or twice per century.
We're standing next to the actual fault scarp
that formed on August 17th, 1959
in a magnitude 7.3 earthquake, the Hebgen Lake earthquake.
This is the largest earthquake still ever recorded
in the Intermountain United States.
At the time, before that earthquake occurred,
these two bits of ground were level,
and they split apart during that great event.
We can actually put our hand
directly on the fault that ruptured.
The quake struck just before midnight.
It caused a major landslide in the Madison River canyon.
50 million cubic yards of rock, mud,
and debris came down the south side of the canyon
and went halfway up the north side.
It inundated camping areas and caused over two dozen deaths.
It also dammed the Madison River,
resulting in the formation of what is now known
as Quake Lake. It drowned trees,
it floated cabins off their foundations.
It threatened additional flooding
if the debris dam were to collapse.
Now, the Army Corps of Engineers stabilized the dam
and the lake level stabilized,
but you can still see today the drowned trees
in the margin of the lake and cabins
that were dropped in a field
as the lake level dropped and stabilized.
The shaking also caused quite a bit of damage to roads
and infrastructure, and it impacted the hydrothermal system
in Yellowstone National Park.
Within 24 hours, at least 289 springs erupted as geysers.
160 of those had no recorded previous eruptions.
Hundreds of springs became cloudy with stirred up debris
and some new thermal areas formed
as cracks allowed new pathways for heat and water.
It caused Old Faithful's eruption interval
to become a bit erratic and ultimately the average interval
between Old Faithful eruptions increased by several minutes.
Yellowstone sits within a part of the Western US
that's very active tectonically.
There are large faults everywhere,
like the Teton fault at the base of the Teton range.
And these are all capable
of magnitude six to seven earthquakes.
There was even a magnitude six earthquake
within Yellowstone National Park in 1975.
These events will happen again in the future,
typically at a rate of one or two per century.
So strong earthquakes,
these are really the most important regional scale hazard
on human timescales in the Yellowstone area.
It was another quiet month of earthquakes
in the Yellowstone region.
The University of Utah Seismograph Stations
located just 88 events during July.
The largest was a 2.2
just north of Canyon Junction on July 25th.
There was also one swarm of 12 earthquakes
located just to the northeast of West Yellowstone
that occurred during July 10th to the 13th,
the largest, a magnitude 1.5,
and it was part of this band of seismicity
that extends from Hebgen Lake
into the north central area of the park.
So pretty average month as far as earthquakes go.
Turning now to ground deformation.
This is vertical deformation for the past two years
at the White Lake GPS station
on the east side of the caldera.
Each blue dot is one day of data.
Downward trends indicate subsidence
and upward indicate uplift,
and the overall trend is what we've seen since 2015,
and that is subsidence of a few centimeters,
about an inch or so every year.
It's interrupted during the summer months
by a pause in the subsidence or a little bit of uplift,
and that's caused as groundwater gets recharged
by snow melt and runoff, as that percolates into the ground,
sort of puffs the ground up like a bit of a sponge.
You can see that uplift here in 2022 and again in 2023.
So seasonal uplift, interrupting this overall trend
of subsidence in the caldera.
Finally, looking at the tallest geyser in the world,
Steamboat Geyser, this is the temperature measured
in the geyser's outflow channel,
and all of this sort of spiky behavior
is minor geyser activity
that's superimposed on daily temperature variations.
No major eruptions during the month of July for Steamboat.
So the total number of major eruptions
for the year remains at five.
Well, that does it for the August, 2023 update
of the Yellowstone Volcano Observatory.
Please remember to like and subscribe,
and if you have any questions at all,
feel free to email us at yvowebteam,
that's all one word, @usgs.gov.
We'll see you next month and until then,
stay safe and stay healthy.
Bye-bye.
(gentle music)