Beginning on May 23rd at 23:00 local time, a swarm of earthquakes began underneath the
Yakedake volcanic complex in Japan.
Although earthquakes are relatively common at this volcano, these were occurring at far
higher than typical levels, indicating a change in the overall complex.
These quakes were almost all centered around the summit lava dome of Yakedake which has
been the site of 25 eruptions since 1905.
As a result, the alert level of the Yakedake volcano was raised from green to yellow the
following day due to an increased likelihood of a volcanic eruption.
This carried with it a 1 kilometer exclusion zone around its summit as previous eruptions
have occurred while at this alert level such as in 1962, which threw large boulders of
superheated rock in about the same radius.
However, I want to express an additional level of concern that this exclusion zone may not
be enough.
Although the vast majority of Yakedake's eruptions have occurred on or around its summit, a small
explosion could theoretically occur at any location around its edifice which has a fumarole,
hot spring, or significant hydrothermal alteration with little or no warning.
Although rare, this is exactly what occurred during the volcano's last eruption in 1995,
which ejected several thousand cubic meters of rock near a riverbed 3 kilometers southeast
of its summit.
While there is no indication that a repeat of this event will occur as the temperature
of the area's fumaroles are monitored, I would recommend against travelling in the area marked
on screen despite it technically not being closed off.
The Yakedake volcano is located in central Japan where it is 30 kilometers east northeast
of the city of Takayama.
Yakedake is a stratovolcano which is covered by a prominent andesite to dacite composition
lava dome that formed in 350 BC.
Carved into this dome are a series of more than a half dozen explosion craters which
formed in the last 2300 years.
These craters formed through largely phreatic eruptions, which often threw large boulders
of rock several hundred meters, flattened trees, and in exceptional cases even generated
lahars or landslides.
Over the last several thousand years, this complex generally only erupted about once
every several hundred years.
Then, in the 20th century, Yakedake erupted 26 times.
The vast majority of these eruptions occurred from three summit craters shown on screen,
inbetween two of which there is now a crater lake.
These craters are where the current seismicity is centered, suggesting that if an eruption
were to occur, one or more of these three vents would represent a likely eruption site.
Such an eruption would largely only affect the immediate summit area and be small in
size.
However, it is also possible that an eruption could occur several hundred meters outside
the summit much like seen in 1915 and 1962.
Those eruptions produced a chain of explosion craters that formed 2 fissure like valleys.
Due to the ferocity of their associated eruptive episodes, and the slope of the mountainside,
they generated voluminous lahars.
The large lahar in 1915 killed a large amount of vegetation but is most notable for damming
the Azusagawa river, forming the scenic Taishoike pond.
Although a raised alert level might make it seem like an eruption is inevitable, in all
likelihood one will not occur.
Magmatic intrusions or a raised volcanic alert level only rarely result in volcanic eruptions,
between 10 and 20% of the time.
In the context of this individual volcano, it has produced earthquake swarms 10 times
since 1953, and only twice did this activity eventually result in a volcanic eruption.
Although this gives us a figure of 20% for the odds of an eruption in the current situation,
I deem the odds to be slightly lower due to the number of quakes which have occurred.
But, lets suppose an eruption did occur.
There would be an 87% chance that lava would not be erupted and an event would be completely
phreatic.
Lava dome or lava flow forming eruptions from this volcano are quite rare, with the last
such magmatic event occurring 2350 years ago.
If a phreatic eruption originated at the peak of Yakedake's summit, the risk to surrounding
areas would be low.
However, if an eruption occurred at a flank vent the eruption would be larger and be quite
likely to generate damaging lahars and regional roads would need to be closed off.
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