World earthquakes

For millions of people in the western part of Sichuan province in China, the morning of 12 May 2008 started out as a day like any other. People left their homes for work as usual, saying goodbye to family members without any thought that they would never see them again. Children packed into their school classrooms, their minds on lessons and games. It seemed like just another busy day.

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Damage from the 2008 Wenchuan, China, earthquake. Photo by Raymond Koo, courtesy of EEFIT, UK.

At 28 minutes past two in the afternoon, catastrophe struck. There was no warning – just a sudden terrific roaring sound, as buildings bucked violently, sending down clouds of plaster dust, and then giving way completely, as the heavy ceilings came crashing down. People outside were the lucky ones. For them, the ground itself rocked violently like the deck of a boat in a storm. All around, clouds of dust arose from collapsing buildings, amid the sounds of roaring, of crashing masonry and of screaming people fleeing into the streets.

These scenes were repeated in the same instant across a huge area, nearly 300km long, at the edge of the Sichuan plain where the mountains of Longmen Shan rise up abruptly from the flat farmland. In the narrow mountain valleys, steep, rocky slopes came tumbling down, as massive rockslides added to the devastation and blocked the roads. In a matter of a couple of minutes, over 70,000 people were killed and another 370,000 injured. Around five million people were left homeless. It was the worst earthquake disaster in China in over 30 years.

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Damage from the 2008 Wenchuan, China, earthquake. Photo by Raymond Koo, courtesy of EEFIT, UK.

An earthquake is a calamity like no other. Most natural disasters can be seen coming. When Hurricane Katrina struck New Orleans in 2005, meteorologists had been tracking it for days. Everyone knew in advance when it would hit, where it would hit and how hard it would hit. No-one could claim to be surprised by it. Volcanoes are harder to predict in advance, but an eruption usually starts off with minor activity that shows something is going on, well before the volcano blows its top. The biggest killer amongst natural disasters is probably drought – and that kills slowly and gradually (through famine) over a period of years.

Only an earthquake has the capability to kill so many people, so quickly, over such a large area and so suddenly. And, therein, lies some of the terrible fascination of earthquakes. They are always there, lying in wait.

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Damage from the 2008 Wenchuan, China, earthquake. Photo by Raymond Koo, courtesy of EEFIT, UK.

So what exactly is an earthquake and why do they occur? Well, it may sound surprising, but unlike most other types of disaster, there is a good side to earthquakes. In fact, a world without earthquakes would be good news for fish, but not for us. This is because the land on which we live is perpetually being eroded by the forces of wind and water. Mountains are worn down into soft silt that is washed away by rivers and dumped on the ocean floor. Furthermore, this process has been going on for millions of years – so why isn’t the Earth one big shallow sea by now?

The answer is that the downward processes of erosion are balanced by upward processes of mountain-building; and earthquakes are a necessary part of the huge upheavals needed to push up great chains of mountains like the Alps or Himalayas.

The ultimate cause of all these movements lies deep within the Earth, in the region known as the mantle. Here, heat from the planet’s molten core creates vast, slow convection currents turning over the mantle material, similar to what you see in a saucepan of soup heating on a stove. The Earth’s outer layer, on which we live, is a thin crust of lighter rocks, not much more than 30km thick in most places that floats on top of the mantle. The currents in the mantle beneath this crust have long since broken it up into chunks (called plates), which now move independently of one another, sometimes moving apart and, at other times, scraping past each other or colliding. And, where they collide, rocks are scrunched up under pressure, broken, folded over and pushed upwards into mountain chains.


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