On December 2nd 2023 at 10:37 pm local time, a major magnitude 7.6 earthquake struck offshore
of the Philippines.
As the ground shook for nearly 50 seconds and notable damage occurred, it became clear
that what had just occurred was no ordinary earthquake.
Not only was this earthquake the largest to strike the Philippines in 33 years, but it
was also a type of earthquake that when it occurs as a large magnitude should always
be feared; a megathrust earthquake.
This major earthquake was centered offshore of the highly populous island of Mindanao,
where it was specifically 24 kilometers northeast of the city of Hinatuan.
While the megathrust quake was felt by millions of people across an area of more than half
a million square kilometers, you might notice that the epicenter was located in close proximity
to a deep submarine feature known as the Philippine Trench.
This marks the boundary of two tectonic plates where the Philippines Sea Plate to the east
is subducting underneath the Philippine Mobile Belt to the west.
Over lengthy periods of time, generally decades to centuries, the pull of the downward moving
subducting slab can create compressional stresses on the overlying plate, also causing it to
deform downwards.
Eventually, this downwards motion reaches a critical point, creating a major earthquake
which causes the overlying plate to all of a sudden snap back upwards, often by more
than 5 to 10 meters.
This temporarily uplifts large swaths of seawater, creating what can potentially be a wide ranging
and destructive tsunami.
As a result, notificiations were initially sent out in the Philippines telling many people
to evacuate to higher ground, with warnings being sent of a potential tsunami as far away
as Japan and Malaysia.
Luckily, the tsunami which was generated was rather small, not reaching any higher than
8 centimeters or 3.15 inches in height in the Philippines.
However, perhaps due to near shore topography, a much larger 40 centimeter or 15.75 inch
high tsunami was measured several hours later on Japan's Hachijojima island more than 3,000
kilometers distant!
The reason why such a large tsunami was not generated is that the earthquake occurred
at 32.8 kilometers or 20.4 miles depth, which is deeper than many megathrust earthquakes.
Had this earthquake instead occurred at about half that depth, than the eastern coast of
the Philippines would likely be actively facing a 3 to 4 meter high tsunami.
While I have been unable to verify the extent of the damage which has occurred, early U.S.
Geological Survey models suggest about a 70% chance that between 10 million and 1 billion
U.S. dollars worth of damage occurred.
While it is likely that some landslides occurred, there is a potential that the damage may be
much more severe than initially thought.
As, the same aforementioned model suggests that several hundred thousand people were
in an area that could have experienced a liquefaction hazard.
Per a direct quote from the U.S. Geological Survey, "Liquefaction takes place when loosely
packed, water-logged sediments at or near the ground surface lose their strength in
response to strong ground shaking.
Liquefaction occurring beneath buildings and other structures can cause major damage during
earthquakes".
Liquefaction is most likely to occur in parts of the world which have significant yearly
precipitation, large grain sized soils rich in sand, significant groundwater, and of course
earthquakes.
A notable long time fan of my channel happens to live in the area, don't worry he is fine,
but he noted that some large cracks had appeared in the ground, which seemingly expanded every
time a major aftershock struck the region.
These cracks were each several meters long and each trended almost perfectly in a north
to south orientation.
The reason why these cracks appeared in such an orientation is that a large swath of the
overlying Philippine Mobile Belt moved in an eastward and upward direction, causing
cracks to form in a perpendicular orientation as the region unevenly moved.
In the next few days to weeks, more aftershocks, or earthquakes triggered by the main shock
are likely to occur, with a 1 over x frequency compared to the rate of aftershocks on day
1 with x representing the number of days which have passed since the main quake starting
with a value of 1.
Statistically speaking, the average largest aftershock is 1.2 magnitudes lower than the
main quake, which in this case means there is a 50% chance of at least 1 magnitude 6.4
or larger quake occurring as an aftershock.
So far, two aftershocks larger than this have occurred including a magnitude 6.4 and then
a magnitude 6.9 quake.
As a final note, I would like to thank my new YouTube channel member Elaine O' Connell
for supporting this channel!