On March 18th, 2022, a swarm of earthquakes began underneath the Mount Hood volcano in
the state of Oregon.
Although minor earthquakes occur underneath the volcano on a monthly basis, these were
occurring at far higher than normal levels.
Over the next 12 hours, a series of 21 earthquakes rapidly occurred, representing what is typically
4 to 5 months of earthquake activity.
These earthquakes occurred at primarily 4 to 5 kilometers depth, which is consistent
with the known depth of Mount Hood's underlying magma chamber.
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 Mount
Hood.
For some context, Mount Hood is located in northwest Oregon where it is 48 miles east
of the city of Portland.
Of the 14 active volcanoes within the state, it has been the far most active during the
last 1,000 years.
Its last major eruptive episode lasted between 1781 and 1869, during which lengthy pyroclastic
flows were generated by partial lava dome collapses on its southern flank.
These collapses and associated steam explosions sent large volumes of ash and acidic material
in what are termed lahars to the southeast and west down river channels, one of which
reached the outskirts of Portland.
Due to the threat of a similarly large lahar occurring in the future, Mount Hood has been
extensively studied and has a thorough network of seismometers, temperature gauges which
measure fumarole produced gas, and GPS sensors.
It is through this network that the latest series of earthquakes were detected.
These quakes were centered on the very southern flank of Mount Hood where they were 9 tenths
of a mile west of Mount Hood Meadows.
The 21 measured earthquakes were all relatively small, ranging from magnitude -0.5 to 1.7.
To further indicate just how small these quakes were, none of them were large enough to be
detected by anyone on the volcano at the time.
Negative magnitude earthquakes can occur as the overall rating system which measures earthquake
magnitude 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.
Although these quakes did occur at the known depth of Hood's underlying magma chamber,
their location suggests that their occurrence is not related to volcanic activity as its
reservoir of molten rock is largely centered underneath its summit.
Instead, these quakes occurred due to the movement of a north-northeast trending fault
line in the area, which commonly ruptures in small amounts due to extensional forces
in the vicinity.
In other words, the quakes involved common active fault lines which are quite abundant
in the western United States.
Whenever an earthquake swarm occurs at Mount Hood, it originates almost universally at
1 of 4 locations, which are labeled A through D on screen.
90% of these swarms like the one which just occurred originated at site A. Two such earthquake
swarms occurred there in 2021, one on June 5th and one on October 18th.
While it could be argued that these quakes are related to pressure exerted by superheated
water, this is difficult to prove.
Site C earthquakes align with those seen in site A, while site B quakes are centered on
a northwest trending fault line.
The only area known to produce definitively volcanic earthquakes are just beneath the
summit of Mount Hood aka site D. Quakes that have occurred there since regional sensors
were emplaced were not related to the movement of magma but rather hot water heated by underlying
magma.
Luckily, no earthquakes have occurred in this region during the current swarm, although
one small volcanic earthquake did occur there on March 10th.
Thus, Mount Hood's alert level remains at green, indicating normal levels of activity.
As a final note, it is true that earthquake swarms have been slowly becoming more common
over the years at Mount Hood.
Thus, it is the second most likely volcano in the lower 48 states to erupt next only
after Mount Saint Helens.
Thanks for watching.
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