It is now day 23 of the eruption of the Mayon volcano in the Philippines, which is considered
to be one of the planet's most perfectly symmetrical volcanoes.
The last several days have seen not only an increase in pyroclastic flows from the volcano,
but also a dramatic spike in earthquakes.
During the first 17 days of Mayon's eruption, only 47 earthquakes occurred, largely due
to the movement of magma, occurring at an average rate of a little less than 3 a day.
Then, the number of volcano tectonic earthquakes dramatically increased jumping from 2 on June
24th to 24 on June 25th to 102 on June 26th.
June 27th had a similarly large number of earthquakes occur, totalling 107, with this
representing more than had previously occurred during the entire eruption's first 19 days.
At some volcanoes, such an increase in seismic activity while it is erupting often signals
the intrusion of a new batch of magma.
For example, during the 2021 explosive eruption of the La Soufriere volcano on Saint Vincent,
a fairly effusive eruption turned into a highly explosive one after dramatically increased
seismic activity occurred and the associated batch of magma reached the surface.
Since Mayon has also produced highly explosive eruptions where what I just described actually
occurred at this volano, you might wonder if the same thing is about to occur, and result
in a highly explosive eruption.
Luckily, in my opinion this rapid increase in volcanic earthquakes was not caused by
the intrusion of a new batch of magma at Mayon, but rather something else which is fairly
harmless and might seem somewhat unexpected; rain.
While groundwater exists essentially everywhere on the planet, rainfall can have a particular
effect on the hydrothermal systems of some volcanoes.
I am specifically referring to water heated at depth by magma or rock heated by this magma
with then heats the water, which eventually travels to the surface as either steam or
through fumaroles, hot springs, and sometimes geysers.
These hydrothermal systems utilize water, and sometimes can be filled to fairly high
levels via periods of heavy rainfall or intense snow melt during periods of unusually warm
weather.
If sections of ground in these hydrothermal systems become overloaded with groundwater,
the pressure the water places on overlying rocks when associated with occasional increases
in residual heat from at depth magma makes it far more likely to fracture overlying rock
and move upwards.
While this water is fracturing rock, it occasionally encounters a fault, reducing the friction
present on the fault line, allowing for shallow earthquakes to occur.
This series of events is why the Long Valley Caldera in California often sees a greatly
increased number of earthquakes during high periods of snowmelt on hot days in the Spring
or early Summer, or why Mount Rainier between April 23rd and April 27th of 2023 experienced
an earthquake swarm when temperatures were 20 degrees above normal.
In the case of Mayon, all 5 days so far in an eruption with the highest number of earthquakes
occurred when it rained, while all 5 days with the lowest earthquakes totals occurred
on days when it did not rain, as shown by color coding days where it did rain blue.
So, not only were these earthquakes caused by rainfall, but they were also incredibly
low energy unlike what would be expected if a new batch of magma was intruding underneath
Mayon.
As a quick correction, Phivolcs on June 23rd stated that the two pronged lava flow traveling
down the Mi-isi and Bonga Gullies had reached 2500 and 1800 meters from Mayon's summit,
respectively.
It turns out that this figure was an overestimate, as Phivolcs incorrectly interpreted some falling
lava blocks from parts of the lava flow which crumbled as part of the actual lava flow length.
As a result, Phivolcs came out with a revised figure, meaning that the lava flows by June
26th have only reached 1300 and 1200 meters along the two aforementioned drainages.
Yet, by the following day, the lava flow along the Mi-isi drainage had increased by approximately
300 meters in length.
What all of this means in the context of the eruption is that since the lava flow length
was overestimated, the revised figure means that lava is now less likely to travel outside
of the 6 kilometer exclusion zone, as it would take a longer period of time.
While the amount of sulfur dioxide emissions at Mayon has for the most part remained below
1,000 tons a day like would be expected in a VEI 0 eruption, over time the local minima
figures have seemingly continued to progressively increase as the eruption continues.
Thanks for watching!
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