Just before midnight on July 15th, a major magnitude 7.2 earthquake occurred offshore
of a section of Alaska in the eastern Aleutian Island chain.
This quake was not caused by a so-called megathrust quake related to two tectonic plates colliding
and then snapping back.
Rather, a large reverse fault on the subducting slab slipped and moved by several meters,
generating widespread ground movement.
This ground movement was experienced across an area the size of Wisconsin ranging from
Unalaska in the southwest to Pilot Point in the northeast, awakening many.
While this quake had the potential to be devastating, the closest populated place aka Sand Point
Alaska only experienced shaking equivalent to the simulation shown on screen.
While this did not generate a damaging tsunami due in part to the quake’s depth, it might
have caused a change in activity at the Shishaldin volcano on the adjacent Unimak island.
As, only an hour after the large magnitude earthquake, Shishaldin produced a major eruption,
sending a sustained plume of ash to a height of 16,000 feet which then formed an ash cloud
125 kilometers in length.
This begs the question, did the earthquake cause this sustained ash plume or would it
have erupted at the same time anyways without the quake?
The short answer per the Alaska Volcano Observatory is that the two events are not related and
one did cause the other.
However, it is surprisingly plausible that the magnitude 7.2 might have caused Shishaldin’s
eruption to temporarily intensify in my opinion.
At some Indonesian volcanoes, it is generally accepted that an earthquake which generates
shaking of MMI 5 or higher can while highly unlikely, trigger a volcanic eruption at a
volcano already primed to erupt.
Shishaldin experienced shaking equivalent to an MMI of 4, which although a bit low means
the quake technically could have caused a temporary increase in eruptive intensity as
it is fairly close to the aforementioned threshold.
And, after the sustained vulcanian explosions ended, Shishaldin experienced a drop in eruptive
activity for the next 24 hours.
Shishaldin is rightfully considered to be one of the most if not the most symmetrical
volcano in Alaska.
Covered by glaciers and always having a white summit, it might come as a surprise that it
is also one of the state’s most active volcanoes with more than 20 historic eruptions.
Shishaldin’s latest eruption began without much fanfare on July 11th, 2023 when satellites
detected strong heat signatures originating from its summit region.
A webcam then picked up a faint orange glow, which was later confirmed to be fresh lava
emitted from a 15 foot high spatter cone.
This eruption remained almost completely effusive until a significant explosion occurred at
1:09 am local time on July 15th, sending a plume of ash to a height of 40,000 feet.
Weaker explosions subsequently continued before becoming intermittent, then the magnitude
7.2 earthquake struck.
And again, on July 18th, Shishaldin produced its 3rd major eruption in its current eruptive
sequence, where after a continuous increase in volcanic tremor in the preceding 12 hours,
a sustained 30,000 foot high plume was generated.
More explosions like this have the potential to disrupt some regional air traffic, primarily
in the eastern Aleutian Islands region.
While these explosions have melted snow to generate lahars on the flanks of the volcano
and created 1 mile long pyroclastic flows, I would only estimate its current eruption
to represent a VEI 3 on the 0 to 9 volcanic explosivity index.
The last week seemingly represents this volcano’s most powerful eruption since 1999.
Although we do not know how long this volcano will erupt for, some of Shishaldin’s eruptions
have continued for more than 6 months.
For now, more small volume explosions with infrequent large vulcanian explosions are
to be expected while Shishaldin is erupting.
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