Earthquakes

Do we really understand
earthquakes? Lets see.

The crust of the earth
together with the uppermost part of mantle is called the lithosphere. Earth’s
lithosphere is divided into sixteen larger tectonic plates, and several smaller
plates. These plates are constantly moving, driven by convection currents in
the planet’s mantle. To understand earthquakes, we’re interested when two
plates either converge or slide past each other. Let us first see what happens
when two plates “converge”.

When that happens, the
heavier plate slides below the lighter plate in a process called “subduction”.
Usually the heavier plate is oceanic, and the lighter plate continental.
(Because continental crusts are lighter than and higher than oceanic
crusts…as if floating on them). Anyway, the descending oceanic plate is rich in
hydrous minerals which release water when subjected to pressure and temperature
prevalent at such depths, causing flux-melting of mantle, forming liquid magma
which buoyantly ascends to form an arc of volcanoes on the overriding plate,
parallel to the subduction zone. Examples of such volcanic arc mountains
include the Andes, the Andaman and Sunda island arcs, Japanese archipelago,
Philippines, Aeolian islands and many others.

This region of collision
and sinking of the heavier plate is called “subduction zone” or “oceanic
trench”. These trenches are the deepest points on Earth. The deepest one being
the Mariana Trench, more than 11 km deep, is located in the subduction
zone between the Pacific plate and the Mariana plate. The inclined plane
representing the interface between the two plates is called the Benioff Zone. This
zone is an area of intense seismic activity and has lots of earthquake foci
(Remember the focus of an earthquake is the point where it originates). A good
analogy is to compare the two plates as two sandpapers rubbing against each
other. The sandpapers have uneven surfaces that grip onto each other at various
points. As they slide, these contact points are continuously locking
and unlocking along the Benioff zone, releasing seismic energy.

The depth of the
earthquake focus is of vital importance in understanding the severity of the
quake. This is determined by the angle at which the Benioff zone is reposed. A
small angle indicates shallow foci, and a steep angle indicates deeper quakes.
This angle depends on the difference in densities of the two plates. Greater
the difference, greater the angle. So if the overriding plate is young, hot and
light and the subducting plate is old, cold and dense, the angle of dip is high
resulting in deeper quakes. But even in such steep angles, there is some
probability of the focus being shallow. So what determines the exact depth of
the earthquake. Well we can only say that once the quake has happened !

Upto depths of 100 km (which is the thickness of the lithosphere), seismic energy is
released due to friction and thrusting between the sliding plates. Beyond that
internal deformation and dehydration of the subducting plate becomes the main
reason for release of seismic energy. For deep-seated earthquakes below 300km
and upto 700 km, mineralogical changes in the subducting plate is the dominant
earthquake mechanism. Beyond 700km the lower mantle begins and we have no
earthquakes.

Here on Earth’s surface,
we aren’t much worried about these deep earthquakes because by the time the
shock waves reach the surface, they’re pretty weak to cause much damage. So
even though the “Magnitude” of these Earthquakes in Richter scale in terms of
energy released might be large, the perceived “Intensity” here on Earth’s
surface in the “Mercalli scale” would be small for these earthquakes.

The shallow earthquakes
are what really bothers us, because its shock waves can rock us into disaster, especially
when it is close to human habitations. The effects of shallow earthquakes can
also be amplified in areas with soft or loose soil, which can liquefy and cause
buildings and other structures to collapse.  So lets look at some examples to understand.
The infamous 2004 Sumatra Earthquake was caused by subduction of the Indian
Ocean plate beneath the Burma plate. The Indian Ocean
plate is estimated to be around 75 million years old and
the Burma plate is estimated to be around 130 million years old. The
difference is not a lot, which is why the earthquake focus was at a shallow
depth of around 30 km from the ocean floor. The earthquake had a magnitude of
9.1–9.3 and it triggered a series of devastating tsunamis along the coasts of
most landmasses bordering the Indian Ocean, killing over 200,000 people in
fourteen countries.

The 2011 Japan
earthquake was caused by subduction of the Pacific plate beneath the North
American plate. The Pacific plate is estimated to be around 135 million
years old and the North American plate is estimated to be around 200 million
years old. Again the difference is not huge, which is why the earthquake focus
was at a shallow depth of around 25 km from the ocean floor. It was the most
powerful earthquake ever recorded in Japan, triggering a tsunami and also causing
the Fukushima Daiichi nuclear disaster.

The recent Turkey-Syria
earthquake of February 2023 is different from what we’ve discussed so far. It
was not a subduction earthquake. It resulted from the Anatolian and Arabian
plates sliding past each other in what is called a “transform fault”.