On March 19th, 2022, a swarm of earthquakes began underneath the island of Sao Jorge and
its volcano in the Portuguese Azores Island chain.
Although earthquakes are quite common in this island chain, it is rare for them to occur
in such great numbers.
Over the next 24 hours, more than 700 earthquakes occurred, dozens of which were large enough
to be felt by the local populace.
These quakes occurred on the west central portion of the island, which just happens
to be where the island's most recent eruption was in 1808.
As a result, some third party sites unofficially raised the volcano's alert level from green
to yellow.
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 Sao
Jorge.
For some context, the island of Sao Jorge is located 1600 kilometers west of mainland
Portugal.
This island has an unusual shape, measuring 53 kilometers long and 8 kilometers wide.
Its coastlines are defined by more than 300 meter or 1,000 foot tall sheer cliffs which
surround the island.
The weird shape of this island is owed to the nature of volcanic activity which formed
it.
Unlike the nearby Mount Pico volcano to the south, this island is not a stratovolcano.
Instead, it is a massive fissure system.
In fissure systems, eruptions occur when the fissure system widens, allowing for magma
to intrude into the crust and eventually erupt onto the surface.
This type of volcanism is most frequently seen at or near mid ocean ridges, and this
is no exception.
The island of Sao Jorge is located adjacent to the Atlantic mid ocean ridge, where like
many of its adjacent islands it lies on a lengthy transform fault.
This fault defines the shape of the island, as it marks the path of least resistance for
molten rock that intrudes underneath the island to erupt from.
The several hundred earthquakes which have occurred so far have been defined by a west-northwest
trending pattern.
Since the tectonic faults on Sao Jorge perfectly align with the shape any intrusion of magma
would take, it makes differentiating between the two quite difficult.
Although the quakes started across a broader area, they have since migrated increasingly
towards the area just north of the town of Velas.
Each historical volcanic eruption of the complex began with a similar series of earthquakes
to what is ongoing right now.
For example, a series of earthquakes which occurred in 1964 resulted in a 1 day long
eruption at a submarine vent 4 kilometers southwest of the island's coastline.
Although the ongoing earthquakes were just determined to be completely tectonic in origin,
this is a misleading statement.
A magma chamber exists underneath the island, and significant fault movement could widen
or change the overlying faults and fissures just enough that an eruption is triggered.
Thus, while the odds of an eruption are difficult to quantify, they are in the low double digit
percentage range at the present.
I even ran a satellite dataset to check for uplift indicating the intrusion of fresh magma,
but luckily found none, although this could change.
If an eruption was to occur, it would most likely follow a pattern of the island's 7
prior eruptions. 5 of these eruptions occurred at two separate submarine vents; one to the
southwest of the island, and one to the east.
Each of these eruptions lasted less than 24 hours.
As for the two eruptions which occurred on the island, each eruption was defined by the
opening of a several kilometer long fissure resulting in lengthy fields of lava which
flowed towards the coastline.
These eruptions primarily created cinder cones through strombolian style volcanic activity
which is defined by the intermittent ejection of ash and bombs of lava.
However, due to the abundant groundwater in the area the two historical eruptions in 1580
and 1808 were also quite deadly as they produced pyroclastic flows.
These pyroclastic flows were triggered by phreatomagmatic explosions, that is the interaction
of magma with groundwater.
The heated groundwater flashed to steam and exploded, releasing a 100 kilometer per hour
flow of lethal superheated gasses.
I have marked on screen where the pyroclastic flows travelled during those eruptions with
orange arrows.
If an eruption was to occur, it would not be safe to be within a 3 kilometer radius
of an erupting fissure or within a 5 kilometer radius downslope of a vent.
Although warning signs are not currently pointing towards an eruption, we are still monitoring
the ongoing situation.
Thanks for watching!
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