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Where Earthquakes Happen, and Why.

Earthquakes are a tragic natural disaster and one that cannot be avoided, such is the force of mother nature. The recent magnitude 7.8 earthquake that devastated Nepal on April 25th, took place, as most large earthquakes do, on a boundary between two of the Earth’s tectonic plates – in this case, the Indo-Australian plate and the Eurasian plate.

The Economist looks at why these earthquakes happen and where they are most likely to hit.

Tectonic plates are basically rigid structures that are made up from the Earth’s crust and the upper layer of its mantle which move around as single entities. There are seven large plates – The African, the Antarctic, the Eurasian, the Indo-Australian, the North American, the Pacific and the South American. The speed in which they move with respect to each other is relatively similar to the growth rate of a fingernail, which is to say a few centimetres every year. However, because the plates are so rigid, pressures and strains at the boundaries between them often build up for centuries before getting released all at once in a tremendous surge of power. Earthquakes are the result of this.

Science bit: Plates can move in three main ways with respect to one another they can draw away from each other; slide past each other; or crunch into each other head on. Each possibility offers up a different type of earthquake.

Plates that move away from each other are often seen in the oceans along which new oceanic crust – thinner and denser that of which the continental bits of plates are composed – is made. The earthquakes that appear at these boundaries are often frequent, small and relatively untroublesome due to their remote location and the quakes typically do not trigger tsunamis.

Locations where these plates slide past each other are a bit more hazardous. An example of this is the San Andreas fault which can be found in California, where the Pacific plate rubs against the American plate, both moving in opposite directions.

The most damaging of earthquakes however are produced when two plates move towards each other. Typically when this happens, one plate loses the battle and starts sliding under the other. The friction which this subduction creates produces large earthquakes, such as the tsunami-generating one that Japan endured in 2011. The subduction zones at the edge of the Pacific plate are the reason for the volcanic “ring of fire” that girdles the ocean

The Nepal earthquake was created by a sudden movement between the Indo-Australian plate moving north and being pressed down by the Eurasian plate. The sudden movement was a system of faults parallel to the front of the Himalayas that is taken as marking the boundary between the plates. These large shocks are expected to continue for millions of years on and off, although not on a common basis.

Subduction is relatively easy when one at least of the plates involved is made of dense oceanic crust, and is thus able to sink into the underlying layers of the mantle. When the Indo-Australian plate in which India is embedded started moving towards the Eurasian plate tens of millions of years ago, that was what was going on: oceanic crust on the same plate as India, but to the north of it, was subducted under the southern edge of the Eurasian plate. During this process a lot of once-seafloor sediments were added to the northern continent’s edge, like mud to the blade of a bulldozer. When the more buoyant bit of the plate carrying India proper reached Asia, though, things changed. No longer able to simply ride over the Indian plate, the Eurasian plate reared up—and the seafloor sediments stuck to its leading edge became the world’s mightiest mountain range. The speed with which the two continents were converging slowed as the resistance increased. The thickness of the crust grew ever greater as they pressed into each other.   At some point however, the limit to the extent to which the Indian continent can be pushed any further north or downwards will be reached. When that happens, the Indo-Australian plate will itself come asunder. The eastern part, which carries Australia, will continue to plough north into Indonesia. The western part which carries India will grind to a halt and a new boundary, and with it a new scope for earthquakes will arise.

Although the UAE does not get earthquakes per-se and does not sit upon a destructive fault line, it does and has been the victim to strong aftershocks that can be felt throughout the region such as the last one in 2014 in the wake of 5.5 magnitude earthquake that hit Iran.

Source: The Economist

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