In the mid afternoon hours of May 14th of 2023, a magnitude 4.7 earthquake struck a
section of the ocean northwest of Hachijō-jima island.
Yet, an hour later, a magnitude 5.5 quake hit the same area.
And then 10 minutes later, a magnitude 5.8 quake.
Within the next 8 hours, a total of 11 earthquakes would occur, 7 of which were in the magnitude
5 range and 4 of which were in the magnitude 4 range.
Many of these quakes were felt across sections of the Japanese mainland, causing minor shaking
but no true damage there.
Because of the location of these quakes, some people thought that they were volcanic in
origin.
And to this I say you might be correct.
Typical tectonic quake sequences follow a 1 over x integral pattern in terms of the
number of aftershocks which occur such as seen in this example of the earthquake swarm
which occurred at the Salton Buttes volcano in California on April 30th 2023.
As you can see, the integral 1 over x function is a pretty good match to the actual total.
Yet, the series of earthquakes which occurred northwest of Hachijō-jima do not fit such
a pattern, suggesting that they are not tectonic.
Rather, in my opinion I think that they are volcanic in origin, possibly being triggered
by the movement of magma, as no other explanation is a better fit for the available data.
The closest known volcano to the series of events which just occurred is the little known
and little studied Kurose Hole volcano.
Kurose Hole is a purely submarine volcano that measures 14 kilometers wide and comes
within 110 meters of the ocean surface.
It does not have a single known eruption in recorded history, but a recent study of this
volcano notes that its most prominent feature, that is a 6.9 kilometer wide caldera looks
somewhat recent and has rhyolitic pumice on its walls.
This thick unit suggests that it originated in a powerful explosive eruption, which I
estimate ejected around 36.35 cubic kilometers of tephra when it erupted, probably a minimum
of 8,000 years ago.
Yet, these quakes are centered 32 kilometers west of the Kurose Hole, and when I checked
from satellite I did not see any volcanic plumes or discolored water which would imply
that a volcanic eruption occurred.
And, when you look at the last 52 years of magnitude 2.5 or higher earthquakes recorded
in the region, while almost the entirety of the seafloor and regional islands have seen
at least 1 earthquake, there is a mysterious dead zone around the Kurose Hole, suggesting
that it might not be active after all.
Possibility 2 is that these quakes are related to a submarine flank eruption far away from
one of two regional volcanoes.
Such a scenario occurred in 2000 at the Miyakejima volcano, where large volumes of magma seemingly
drained into a several dozen kilometer long fissure either on or beneath the ocean floor
to the west, causing a section of the overlying mountain on the island to collapse into a
1.6 kilometer wide caldera.
Some other regional volcanic islands also have ridges to the west of the main volcanic
cone, suggesting that such fissure flank eruptions are not exclusive to Miyakejima.
Yet, I also do not think this is what is going on, as if this were to be occurring, you would
expect earthquakes to start underneath a volcano's center and then migrate along a fairly straight
line to the eventual eruption site but this has not happened.
Instead, I suspect that an intrusion of magma is occurring underneath a section of the ocean
floor, perhaps being associated with a previously unknown volcano.
While Kurose Hole has a prominent caldera that helped it receive its name, it also seemingly
occurs alongside 3 other caldera to the south and southwest.
And, although seafloor resolution is very low so we could be making up something out
of nothing, it appears that there might be a fifth even larger caldera beneath where
6 of the 7 magnitude 5 quakes occurred.
In the last 51 years, a definite cluster of earthquakes has occurred around this possible
5th caldera at the same site as the current earthquake swarm, including 8 prior earthquake
swarms.
All of these occurred at fairly similar depths in the vicinity and occurred with similar
magnitudes, suggesting a similar origin.
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