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”.