On July 8th, 2022 at 2:47 am local time, a series of volcanic earthquakes began underneath
the Tsurumidake volcanic complex in Japan.
Although this volcano has been the site of a half dozen quake swarms in the last decade
this one was different.
Not only was it shallower than most previous swarms, but it was also centered to the east
of the Garandake lava dome.
Previous swarms had occurred at the other two main domes in the complex, so what is
ongoing truly is unusual.
As a result, the alert level of this complex was raised from 1 to 2 which translates to
a color code of green to yellow only a few hours after the swarm began.
This change in alert level carried with it a minimum 1 kilometer exclusion zone around
the summits of both Garandake and Tsurumidake.
Although the prospect of an eruption at the moment appears unlikely, the chance of one
occurring is heightened with its raised alert level.
This complex is very similar to Mount Unzen, as not only is it centered in a highly populated
area with similar threats but the volcano is referred to as a lava dome complex.
At lava dome complex or plug dome volcanoes, lava with a viscosity similar to peanut butter
is erupted out of a vent and is pushed upwards and outwards.
The high viscosity prevents the erupted material from collapsing into a thin river of material,
resulting in the creation of a typically gray colored mound of lava.
For context, the Tsurumidake or Yufu-Tsurumi volcanic complex is located on the northeast
section of the island of Kyushu in Japan where it is 6 kilometers west of the city of Beppu.
This volcano consists of three large lava domes, and at least one notable landslide
scar.
The landslide scar is located on the northern flank of Mount Yutu, and formed during a large
scale eruption in 200 BCE when an erupting lava dome collapsed, taking with it part of
the mountain.
This left behind an up to 40 meter deep ravine in the mountainside.
Although this is the only confirmed recent scar in the volcanic complex, I suspect that
there are two additional landslide scars on the two adjacent main domes.
My evidence is a combination of the horseshoe shaped scars in the mountain but more importantly
a series of small hills to the east of where the collapse would have sent debris.
When a significant volcanic collapse occurs, which is fairly common at volcanoes since
they are inherently unstable, the landslide often leaves behind a series of small to medium
sized mounds.
The scientific term for these mounds is Hummocky terrain, and an example of such can be found
at the site of Unzen's 1792 collapse and tsunami, which formed a series of small islands.
Although the suspected hummocky terrain in the city of Beppu is less pronounced, I suggest
this is due to the age of the associated landforms.
The Tsurumidake volcanic complex has been erupting at a low rate for approximately 60,000
years.
During this timespan it has been estimated to produce an eruption about once every 1,000
to 1,500 years.
This rate is seemingly matched by the 3 confirmed eruption at this complex, which have occurred
in 200 BCE, 771, and 867.
Although it has been more than 1,000 years since the last eruption at this complex, this
fact alone does not imply that an eruption is overdue.
In the event that the earthquakes suddenly got shallower and uplift was detected, it
would indicate the movement of magma at depth.
Currently, the earthquakes are at a depth of 1 to 4 kilometers which suggests that the
ongoing swarm is primarily hydrothermal.
Of course, this carries with it a low but ever-present threat that a phreatic eruption
could occur, hence the 1 kilometer exclusion zone.
Phreatic eruptions involve groundwater heated by magma flashing to steam and then exploding,
producing a volcanic crater.
Although it is unknown what will happen next, consider this.
In the last 12 months, the alert level has been raised from green to yellow at 4 other
Japanese volcanoes.
Yet, not a single one of them erupted, as in the vast majority of times a change in
alert level or magmatic intrusion does not indicate that an eruption will occur.
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