On January 17th, 2022, a swarm of earthquakes began underneath the Mount Saint Helens volcano
in Washington State.
Although minor earthquakes occur underneath the volcano on a weekly basis, these were
occurring at far higher than normal levels.
Over the next 28 days, earthquake activity only continued to increase, as the volcano
showed minor signs of unrest.
These earthquakes were caused directly or indirectly by the movement of magma, as the
magma chamber underlying the volcano is slowly recharging.
So, is an eruption likely?
What is the context of these earthquakes?
This video will answer these two questions, and discuss what might happen next at this
volcano.
For some context, Mount Saint Helens is located in southwest Washington State, where it lies
directly west of another volcano, Mount Adams.
Of the 39 volcanoes in the lower 48 US states, Mount Saint Helens is by far the most active.
In fact, just in the last 1,000 years, it has erupted 22 times.
The most famous of these eruptions occurred in 1980, when the north slope of the mountain
completely collapsed, producing a lateral blast that incinerated everything up to 19
miles or 31 kilometers distant.
Due to the effects of this eruption, the volcano was extensively studied, and now has the most
thorough monitoring network of any volcano in the lower 48 US states.
It is through this network that we were able to determine the location and depth of its
magma chambers, which is important in the context of the recent earthquakes.
Mount Saint Helens and the Indian Heaven volcanic field to the southeast share a massive underlying
magma chamber, which is located at a depth of between 8 and 25 miles in the crust.
Underlying each volcano is a secondary smaller magma chamber, which in the case of Mount
Saint Helens is located at a depth of 2.5 to 8 miles.
Overlying this is a third small magma chamber at a depth of 2 miles, which is essentially
a portable storage unit for molten rock when eruptions occur.
The earthquakes which have occurred in the last month have originated in or above the
middle magma chamber, which makes perfect sense given that it is slowly refilling.
These earthquakes are directly caused by heated water, gas, or magma intruding into the crust,
which expand due to the heat, cause pressure to build and trigger the movement of a fault.
The vast majority of volcanic earthquakes in this case are hydrothermal earthquakes
which occur above the magma chamber, as groundwater is heated by underlying rock.
Only a select few earthquakes were directly caused by magma itself, such as a magnitude
2.0 quake which occurred on February 10th.
This marked the largest earthquake in the swarm so far, as quakes have ranged in magnitude
from -0.6 to 2.0.
Negative earthquakes can occur as the overall rating system is logarithmic, so a magnitude
0 has 10 times less of an amplitude than a magnitude 1, and a magnitude -1 has 10 times
less of an amplitude than a magnitude 0.
Similar earthquake swarms have occurred 6 times since 2015 at Mount Saint Helens, although
the ongoing earthquake swarm contains more frequent quakes than its predecessors, as
a total of 115 have occurred since it began.
While such earthquake swarms are relatively common at large volcanic systems, what is
ongoing does truly represent above normal levels of activity.
However, an eruption is by no means imminent or even likely.
Fumarole and hot spring temperatures around the volcano remain stable, which is a more
important indication of activity then earthquakes.
In the context of the overall volcano, these earthquakes are nearly identical to those
witnessed between 1987 and 2004, when the underlying magma chamber was also recharging.
Although an eruption is highly unlikely, it is a near certainty that Mount Saint Helens
will erupt again in the next 50 years.
It is difficult to say how or when the next eruption will occur, but the most likely scenario
would involve the creation of a new lava dome near the site of the one built during the
2004 to 2008 eruption.
With all of this being said, the alert level at Mount Saint Helens remains at green.
Thanks for watching.
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