On August 9th, 2022, a swarm of earthquakes began striking the island of Ta’u in American
Samoa.
For several hours, the tremors continued intermittently, jolting the island repeatedly.
Then, residents on the island of Ofu watched the ocean began to mysteriously recede.
Reacting correctly, some people temporarily evacuated to higher ground and an alert was
sounded.
Luckily, a far reaching tsunami did not occur, but earthquakes continued on for several more
hours.
These earthquakes were difficult to locate due to a lack of thorough seismic stations
in the region, but were almost certainly volcanic in origin.
They were occurring somewhere between Ta'u to the west and 40 kilometers or 25 miles
to the east of this island at a tall submarine volcano known as Vailulu'u.
Although both volcanoes have produced eruptions in the last 10,000 years and are thus considered
active, the submarine volcano Vailulu'u is the most likely culprit.
While Ta'u has presumably not erupted in several thousand years, Vailulu'u has erupted 3 times
since 1970.
If the earthquakes did originate from this volcano, then it is quite possible that a
submarine volcanic eruption may have already occurred or is ongoing.
As for the apparent small scale tsunami, it suggests a rapid morphology change on the
volcano, potentially associated with an earthquake or eruption induced landslide.
So, what is likely to happen next?
How dangerous is this volcano?
This video will answer these two questions and discuss the ongoing seismic unrest.
For context, the islands within American Samoa are located south of the equator northeast
of Tonga, meaning that they are closer to New Zealand than they are to Hawaii.
Along with many islands in Samoa to the west, these exist due to an underlying hotspot in
the Earth's mantle which formed a chain of activity with eastward islands generally representing
a younger eruptive center.
At the present, the Samoa Hotspot is centered underneath the Vailulu'u seamount.
This seamount could best be described as a submarine shield volcano which contains a
central caldera.
Its central caldera measures 2 kilometers wide and 400 meters deep, likely having been
constructed due to ground subsidence after large volume flank eruptions.
Although the overall volcano may not look like much, it is unusually tall, rising 4,200
meters or 13,779 feet from the seafloor to within just less than 2,000 feet of the ocean
surface.
Due to the great depths this volcano is located at, it is unlikely to produce a highly explosive
eruption like witnessed at the Hunga Tonga Hunga Ha'apai volcano earlier this year.
The overlying pressure exerted by more than 1,000 feet of water is sufficient to minimize
most explosive activity.
Thus, each of its historic 3 eruptions were rated on the 0 to 8 volcanic explosivity index
as a 0, indicating an almost complete lack of explosive activity.
Within Vailulu'u's caldera is a large 900 meter wide and 293 meter tall basaltic volcanic
cone which was completely constructed by an eruption between 2001 and 2005.
Despite being a very young feature, this cone and its surrounding caldera rim represent
points where landslides could occur, as volcanoes are inherently unstable.
One such landslide may have occurred from this area on August 9th but this is purely
speculation.
The phenomena witnessed is equally as likely to have represented instead a rogue wave.
While highly unlikely, a larger scale landslide could occur at this volcano anytime it is
erupting with little or no warning, creating a destructive tsunami.
Otherwise, Vailulu'u isn't a particularly dangerous volcano, being on par with hazards
posed by the volcano formerly known as Loihi in Hawaii.
What could happen next at Vailulu'u is up for debate as it may not even be erupting.
The vast majority of times, an earthquake swarm at a volcano does not lead to a volcanic
eruption.
Thus, this volcano is likely to produce more small to moderate magnitude earthquakes over
the next several days, which has a low albeit statistically possible chance of triggering
a new eruption.
Although a tsunami is highly unlikely, be cautious around the shoreline in the immediate
future and if the ocean recedes be sure to flee to higher ground immediately.
Thanks for watching!
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