For a little over a year, Iceland's most dangerous volcano known as Katla has been producing
a slightly elevated number of shallow earthquakes, including several weaker earthquake swarms.
Then, on May 4th 2023, beginning at 9:40 am local time, an unusually energetic earthquake
sequence began.
This sequence began with 2 earthquakes that were less than magnitude 2 before being followed
by the largest earthquake to hit this ice covered volcano in 64 months, a magnitude
4.5.
Two more magnitude 4 events followed in the next 5 minutes, including the largest earthquake
to have struck Katla since at least 2009, a magnitude 4.8.
More than 30 smaller earthquakes also occurred, and as a result, Katla's alert level was raised
from green to yellow and an uncertainty phase was declared around the volcano.
While this activity is unusual, it is not unprecedented.
Two similar earthquake swarms occurred during 2016, one on August 29th and one between September
29th and September 30th.
These 2016 swarms each involved more earthquakes than the current swarm, but the largest earthquake
was slightly lower in each case than the ongoing earthquake swarm.
With all of this taken into account, the earthquake swarm at Katla at least for now seems to be
a once in a 5 year event, with a similar event occurring about once every 5 years.
What made the latest swarm all the more suspicious is that the earthquakes almost universally
appeared to occur at a depth of less than 1 kilometer, marking them as incredibly shallow.
When earthquake swarms of this magnitude occur, there are generally 4 possibilities.
The first is that a fault line moved due to nonvolcanic activity, triggering movement
at other faults.
However, these earthquakes are seemingly centered on the northeastern edge of Katla's large
caldera, so this seems unlikely.
Possibility 2 is that a large influx of meltwater is moving through the volcano, either due
to increased fumarole temperatures beneath the ice or meltwater related to unusually
high temperatures.
Yet, there does not appear to be any signs that Katla's glacial ice is sinking which
would be expected during the beginnings of a glacial flood known as jokulhlaups and the
temperature on top of the volcano is still a cool 3 degrees Celsius.
Possibility 3 is that magma is intruding into shallow depths underneath the volcano.
This possibility cannot be immediately discounted, although if magma was at shallow depths you
would expect ground deformation above the intrusion center in addition to an increased
release of volcanic gasses and the detection of harmonic tremor.
Additionally, the quakes seem a bit too shallow to represent a magmatic intrusion if one was
occurring.
None of these 3 key features have been detected as of yet.
Magmatic quakes would probably start around 2.5 kilometers beneath the top of the volcano
and then migrate upwards.
Therefor, in my opinion the fourth possibility appears for now to be the most likely culprit;
that a small to moderate change occurred in Katla's hydrothermal system.
Due to the overlying glaciers in combination of ground water, hydrothermal fluids in the
ground are heated when they encounter rock overlying an active magma chamber which indeed
exists beneath Katla albeit at 2.5 kilometers depth.
This rock heats the water which carries volcanic gasses along with it, causing it to be pushed
upwards and reach the surface, but sometimes along the way encountering a fault line, causing
it to rupture and generate an earthquake.
The magnitude 4 earthquakes which occurred on May 4th 2023 were merely three orders of
magnitude larger than what is typical, with the aftershocks not representing additional
hydrothermal activity but rather stress release due to the aftershock itself.
This fact is best displayed via when the recorded aftershocks occurred in terms of the number
of minutes after the mainshock, as it somewhat closely follows the expected 1 over x integral
rate of aftershocks.
Thus, on my 0 to 10 opinion based volcanic unrest scale I am placing Katla at a 2, with
a possible case being made to raise it to a 3.
Due to all of this activity and the small but present chance a volcanic eruption could
still occur, several roads leading to the volcano are closed off, with many of them
being in the middle of pathways historic jokulhlaups have taken.
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