The date was November 15th of 2017, and it was a regular day in the busy country of South
Korea.
Yet, unknown to everyone a major disaster was about to strike the country which was
actually caused by human activity.
As, at 2:29 pm local time, a major magnitude 5.5 earthquake struck just west of the city
of Pohang.
As heavy shaking commenced, which moved the ground at a maximum acceleration of 0.58 times
that of gravity, items tumbled off of shelves and broke on the floor, while some brick chimneys
collapsed on to cars below, and several bridges were damaged.
By the time this earthquake ended a mere 15 seconds later, more than 2,000 homes had been
damaged, 90 people were injured, and a total of as much as $226 million U.S. dollars in
damage had occurred.
Yet, this was no ordinary natural disaster, as it was not simply caused by the natural
tectonic and regional forces that trigger more than 1,000 earthquakes every day.
As instead, this earthquake was caused by human activity, and this is how.
It has been known for some time that geothermal power plants can trigger earthquakes, with
this being a lesser known potential downside which sometimes but not always occurs during
their operation.
However, these triggered earthquakes are typically quite small, being below magnitude 3, with
the magnitude 5.5 representing the largest such geothermal power plant caused quake to
occur.
Before I explain why this earthquake occurred, we need to first briefly understand how geothermal
power plants work and also why this specific section of South Korea contains a geothermal
plant.
For context, geothermal power plants are a low carbon electricity generation system which
uses the Earth's heat to generate power.
This is most often done in volcanically active areas where shallow to moderate depth magmatic
intrusions are fairly common in the crust, which heats the adjacent crustal rock to temperatures
higher than they otherwise would be.
Thus, if these temperatures reach above a certain threshold, a borehole can be made
and power can eventually be generated.
First, water is injected into the ground where it is rapidly heated.
It then flashes to steam due to the intense heat, causing it to rise.
After it rises, the steam enters a series of turbines which spin and generate electricity,
before cooling and then being pumped back into the ground.
As to why the region around Pohang was chosen for a geothermal plant, believe it or not
this is not a recently volcanic area, unlike say the Hallasan volcano on the island of
Jeju which produced the tuff cone you are looking at a mere 5,000 years ago, with no
volcanic eruptions in or around Pohang in millions of years.
Instead, for a varying number of reasons, the middle depth crust around Pohang is naturally
warmer than average rock, which can sometimes occur far from volcanic areas.
This is for example how Croatia operates a successful geothermal power plant, despite
not having a single active volcano in the country.
As for why this large earthquake occurred, the reasoning it quite similar to how the
oil and gas industry can cause earthquakes; wastewater injection.
In this case, the wastewater is instead useful water injected into the ground so it flashes
to steam before rising again.
Since the water is either pumped into an area which previously did not have as much water,
or is drained beyond what used to exist there, this process can cause stress changes at regional
fault lines.
As a result, pressure builds up over time before finally being released as an earthquake.
Possibility two is that water injected back into the ground seeped into a fault and lubricated
it, allowing it to more easily slip and triggering an earthquake, in a process which is almost
identical to how some volcano tectonic earthquakes occur.
As a result, small magnitude earthquakes, typically below magnitude 3 are incredibly
common at some geothermal power plants, such as the Geysers geothermal field in California,
which is why it might look like the Clear Lake volcano is always producing so many earthquakes,
as these are not volcanic but caused by geothermal power plant operation.
In the case of the Pohang quake, it happened to trigger a previously unknown and buried
fault line which was several kilometers in length.
Only about 3 kilometers of a fault line need to move at once to generate a magnitude 5.0
quake, and such fault lines whether buried or exposed on the surface are surprisingly
common around the planet.
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