Why are earthquakes so hard to predict? - Jean-Baptiste P. Koehl
In 132 CE,
Chinese polymath Zhang Heng
presented the Han court with
his latest invention.
This large vase, he claimed,
could tell them whenever an earthquake
occurred in their kingdom–
including the direction
they should send aid.
The court was somewhat skeptical,
especially when the device triggered
on a seemingly quiet afternoon.
But when messengers came
for help days later,
their doubts turned to gratitude.
Today, we no longer rely on pots to
identify seismic events,
but earthquakes still offer a unique
challenge to those trying to track them.
So why are earthquakes so
hard to anticipate,
and how could we get better
at predicting them?
To answer that,
we need to understand some theories
behind how earthquakes occur.
Earth’s crust is made from several vast,
jagged slabs of rock
called tectonic plates,
each riding on a hot, partially molten
layer of Earth’s mantle.
This causes the plates to
spread very slowly,
at anywhere from 1 to 20
centimeters per year.
But these tiny movements are powerful
enough
to cause deep cracks in the
interacting plates.
And in unstable zones,
the intensifying pressure may
ultimately trigger an earthquake.
It’s hard enough to monitor these
miniscule movements,
but the factors that turn shifts into
seismic events are far more varied.
Different fault lines juxtapose
different rocks–
some of which are stronger–or weaker–
under pressure.
Diverse rocks also react differently to
friction and high temperatures.
Some partially melt, and can release
lubricating fluids
made of superheated minerals
that reduce fault line friction.
But some are left dry,
prone to dangerous build-ups of pressure.
And all these faults are subject to
varying gravitational forces,
as well as the currents of hot rocks
moving throughout Earth’s mantle.
So which of these hidden variables
should we be analyzing,
and how do they fit into our
growing prediction toolkit?
Because some of these forces occur
at largely constant rates,
the behavior of the plates
is somewhat cyclical.
Today, many of our most reliable clues
come from long-term forecasting,
related to when and where earthquakes
have previously occurred.
At the scale of millennia,
this allows us to make predictions
about when highly active faults,
like the San Andreas,
are overdue for a massive earthquake.
But due to the many variables involved,
this method can only predict
very loose timeframes.
To predict more imminent events,
researchers have investigated the
vibrations Earth elicits before a quake.
Geologists have long used seismometers
to track and map these tiny shifts
in the earth’s crust.
And today, most smartphones are
also capable
of recording primary seismic waves.
With a network of phones around the globe,
scientists could potentially
crowdsource a rich,
detailed warning system that alerts
people to incoming quakes.
Unfortunately, phones might not be able
to provide the advance notice needed
to enact safety protocols.
But such detailed readings
would still be useful
for prediction tools like NASA’s
Quakesim software,
which can use a rigorous blend of
geological data
to identify regions at risk.
However, recent studies indicate
the most telling signs of a quake might be
invisible to all these sensors.
In 2011,
just before an earthquake struck
the east coast of Japan,
nearby researchers recorded surprisingly
high concentrations
of the radioactive isotope pair:
radon and thoron.
As stress builds up in the crust right
before an earthquake,
microfractures allow these gases
to escape to the surface.
These scientists think that if we built
a vast network of radon-thoron detectors
in earthquake-prone areas,
it could become a promising
warning system–
potentially predicting quakes
a week in advance.
Of course,
none of these technologies
would be as helpful
as simply looking deep inside
the earth itself.
With a deeper view we might be able
to track and predict large-scale
geological changes in real time,
possibly saving tens of thousands
of lives a year.
But for now,
these technologies can help us prepare
and respond quickly to areas in need–
without waiting for directions
from a vase.
