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The Science of Earthquakes

Photo Credit: Dailymail.co.uk
Earthquakes are caused by seismic waves of energy released as a result of contact between tectonic plates
By Fawad Ahmed Najam

The death toll from last month’s earthquake in Mexico has been reported at 370. Most of these casualties were in Mexico City, where 45 buildings collapsed from the impact of the 7.1 magnitude earthquake whose epicenter was about 160 kilometres away from the city near a town named Puebla.

Twelve years ago,  on October 8, northern areas of Pakistan were jolted by an earthquake above 7.6-magnitude. Its epicenter was about nineteen kilometres north-east of Muzaffarabad. The ‘big one’ turned thousands of neighbourhoods in the northern areas, including an apartment building in Islamabad, into rubble. At least 85,000 people died as a result and thousands were left physically injured and deprived of livelihoods.

Ever since that disaster, the fear of earthquakes has remained inscribed in our collective memories.

Why do earthquakes occur?

Our planet’s inner structure is made of three spherical layers: the core, the mantle, and the crust.

The core is the innermost layer and is roughly the size of earth’s moon with a temperature as high as that on the surface of the sun (6000°C). Geologists further divide the core into two parts: an inner core (solid) and an outer core (liquid), mainly made of molten alloy of iron and nickel.

The second layer, i.e. mantle, surrounds the outer core and is relatively stiffer, mainly comprising partially-molten pressurized rocks.

The outermost layer is the crust.It is a very thin sheet of relatively cold, solid, and rocky materials. On the crust lie continents and oceans. Compared to the overall size of the earth, the crust is only as thick as the peel on an apple.

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The crust is not a single, continuous and solid layer. In fact, it is cracked at many places around the globe. Instead of one big rigid shell, the crust is made of several rocky segments or ‘plates’ floating on the relatively softer mantle. These segments are called tectonic plates. Around 94 percent of the surface area of earth (both land and oceans) is made of just seven major plates, while there are many minor plates as well. These plates are moving slowly with a rate as low as a few centimeters per year. The heat from mantle and outer core constantly provides the necessary force for the plates to move (through a phenomenon known as convention – the same phenomenon which causes the warmer air to rise). At their boundaries, they sometimes slide by – or grind past – one another and occasionally bump into one another. Wherever edges of any two plates get stuck together due to friction (or due to the relative difference in the rates of their movement), the energy that normally enables their movement starts getting building up at that point. And after hundreds and thousands of years, when the force of the moving plates finally overcomes the friction in their jagged edges, all the stored-up energy gets released in the form of seismic waves, like ripples in a pond. These waves can travel and shake the crust over hundreds of kilometres. We feel them as earthquakes.

Quick Fact: Earthquakes also occur on the moon. They are called “moonquakes”.

The intensity of an earthquake is measured in Richter scale. It tells us about the amount of energy released during an earthquake in the form of a constant number called the ‘magnitude’. Though, intensity is not the only factor which affects how an earthquake affects a particular region. Distance from epicenter of an earthquake also matters. The shaking intensity gets minimised the further away one moves from the epicenter, just like the brightness of a fluorescent light bulb.

An important detail about the intensity of an earthquake is that it increases exponentially. For each additional unit of magnitude, the intensity  of an earthquake increases 10 times and the amount of energy released increases 32 times. This means that a 9-magnitude earthquake will shake the earth’s surface 10 times more than an 8-magnitude and 100 times more than a 7-magnitude earthquake (for points located at similar distances from the epicenter).

The most high intensity earthquake ever recorded was a 9.5-magnitude from southern Chile near a place called Valdivia on May 22, 1960.

Refer to the following animations to visualize how tectonic plates slide past, crash into, and pull out from one another. Use the mouse wheel to zoom in and out, and move the mouse pressing the left click to rotate view.  (courtesy: jig.space)

When the two plates SLIDE PAST each other (Transform Plate Boundary)

When the two plates BUMP into each other (Convergent Plate Boundary)

When the two plates PULL AWAY from each other (Divergent Plate Boundary)

How do earthquakes occurs under the ocean?

When tectonic plates under the ocean slide past, crash into, or pull out from one another, the water above the point of origin gets displaced from its normal position. The result is what we call a tsunami. Generally, it is caused by earthquakes generated from convergent plate boundaries, i.e. where a plate tries to crash into or under the other plate.

The following animation demonstrates the process.  It shows an oceanic tectonic plate is slowly crashing under the upper continental tectonic plate, causing it to deform over hundreds of years. Upon the sudden release of stored energy, i.e. an earthquake, the water on the oceanic plate is displaced, resulting in a tsunami.

This is part I of a two part series to mark the 12th anniversary of the earthquake that struck northern Pakistan on October 8, 2005. The second part goes over Pakistan’s geology and existing building codes. It will conclude with recommendations for building codes to ensure earthquake-proof construction. 

Fawad Ahmed Najam has a PhD in Structural Engineering from the Asian Institute of Technology (AIT) in Bangkok, Thailand. He currently teaches Civil and Structural Engineering at the National University of Sciences and Technology (NUST) in Islamabad.

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