In the morning hours on June 15th of 2023, a fairly sizable magnitude earthquake struck
offshore of a section of southwestern Luzon island in the Philippines.
At 10:19 am local time, this earthquake occurred, causing the ground to shake over a wide area,
which registered in as a magnitude 6.3.
This quake was centered 34 kilometers west of the city of Batangas, originating beneath
a section of Balayan Bay.
Yet, despite the magnitude of the earthquake, it only caused minor damage with no significant
damage thus far being reported.
The reason for this was the earthquake did not occur at shallow depths of less than 30
kilometers but instead at a fairly deep depth of 112 kilometers.
This depth mitigated the intensity of the earthquake, although it also caused it to
be felt over a wider region than it otherwise would have, allowing the majority of people
on the island of Luzon to feel some form of shaking.
Because this earthquake occurred in fairly close proximity to the Taal volcano, and the
fact that Taal has been displaying signs of unrest in recent weeks, I received an influx
of questions on whether this earthquake could trigger a volcanic eruption.
To this I can luckily answer in my opinion no, probably not.
Although large earthquakes can trigger activity or even eruptions under rare circumstances
at fairly close volcanoes, such as at Mount Ararat in Turkey after an 1840 earthquake,
such occurrences only seem to only be possible if the region experienced truly intense shaking.
Yet, the Taal volcano itself only experienced shaking equivalent to a bit less than 4% of
the peak intensity Ararat did in 1840.
And, more importantly, this Philippines earthquake did not occur on a fault line related to the
volcanic complex of Taal, with the earthquake being tectonic rather than volcanic in origin.
For context, the Taal volcano exists due to the offshore collision of two tectonic plates
to the west of Luzon.
There, the Sunda Plate is being subducted underneath the Philippine Mobile Belt which
happens to make up the majority of Luzon.
The melted material from this collision then migrates upwards, intrudes into the crust
as magma, and occasionally produces eruptions, forming volcanoes like Taal.
Yet, while the melting occurs at around what in my opinion is probably 130 kilometers depth
on the subducted slab of the Sunda Plate, this place also contains a number of large
fault lines.
Thus, the earthquake which occurred was caused by a fault within this slab suddenly moving.
This motion was consistent with what is known as a reverse fault where the hanging wall
moves upwards in relation to the footwall.
Such earthquakes occasionally occur at similar depths on other subducting slabs around the
planet such as the Juan de Fuca plate underneath Washington state which generated a magnitude
6.8 quake in 2001, and a magnitude 6.7 quake in 1965.
But, back to the Philippines, there is another reason why Taal will not erupt specifically
due to the earthquake which just occurred.
By the way if Taal does erupt in the near future, it will not be due to this earthquake
as it was already displaying some signs of increased unrest starting 6 days before the
quake occurred.
Also, an even larger earthquake occurred at essentially the same location 3.5 kilometers
to the southeast on July 24th 2021.
That older quake occurred at a depth of 110 kilometers and registered in as a magnitude
6.7.
It did not have a notable effect on any of Taal's various measured sensor data.
Taal for context during 2021 erupted 15 days before the earthquake and also 3.5 months
after the quake.
Several aftershocks are likely to follow in the vicinity around the main shock quake in
the next several days to weeks, but these will decrease in abundance over time, somewhat
closely following the expected 1 over x rate of aftershocks.
Since the average largest aftershock of any given earthquake is generally 1.2 orders of
magnitude lower on average than the main quake, the largest aftershock that should occur in
this sequence will average as a magnitude 5.1.
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