Buildings collapse during an earthquake. An earthquake happens when the tectonic plates which form the Earth’s outer layer suddenly slip past each other, snap or make some other rapid movement, especially along their edges or at cracks (faults). There are about 100,000 earthquakes each year. Scientific devices called seismometers on continuous “quake watch” detect them all. Nine out of ten of these earthquakes are too small to cause damage, or they occur in very remote regions of the world. Another 30 to 40 cause small-scale problems, while up to about 20 cause major damage and a few make headlines. Every five or ten years, a massive quake results in great loss of life and devastates a wide area.
How an earthquake happens
The outer layer of the Earth is made up of a number of giant slabs, called tectonic plates. These are always on the move. This movement is very slow—no more than about a centimetre a year—but the pressure is enormous. When plate edges grind against one another, they send out shock waves through the ground. We feel these as vibrations, and call them earthquakes. Most are small tremors that do no damage. But when plates “lock” together, pressure starts to build up in the rocks under the ground. Eventually that pressure becomes too much for the rock to withstand. They suddenly snap, causing a major earthquake.
The sudden jolt of a quake usually lasts no more than a few minutes and may be over in just a few seconds. Shock waves, also called seismic waves, spread out from a place called the focus. A shallow focus is down to 70 kilometres (about 40 miles) below the surface, an intermediate focus 70–300 kilometres (40–200 miles), and a deep focus below 300 kilometres (200 miles). The shock waves reach the surface first at the epicentre, the point on the Earth's surface directly above the focus, and are usually strongest here.
P-waves (1) and S-waves (2)
Shock waves radiate from the epicentre like ripples on a pond. The immensely powerful vibrations of a massive earthquake travel around and through the whole planet, making it tremble and shake for up to 20 minutes. Speeding along at about 18,000 km/h (11,000 mph), they can be detected thousands of kilometres from the epicentre.
There are different types of shock waves. P-waves (primary waves, 1) travel quickly through the Earth, although the Earth's inner layers bend the waves. S-waves (secondary waves, 2) are slower and cannot go through the liquid outer core. Close to the earthquake's epicentre, P-waves squeeze and stretch the rocks. S-waves shake them up and down and from side to side.
Effects of a quake
The shock waves hurtling through the crust cause the ground to shake. What happens next depends on how strong the shaking is. If quite mild, items may fall off shelves, cupboard doors swing open and tables and chairs slide around the room. If the shaking is stronger, cracks appear in walls and on the ground, concrete slabs fall off buildings, glass panes shatter, electricity supplies are cut off and pipes burst.
Ground rupture is the splitting open of the Earth's surface along a fault line. A block of land slips down on one side of the fault—up to several metres in the case of major earthquakes. It can cause major damage to roads, buildings, dams, bridges and power stations.
When shock waves from an earthquake pass through solid rock, it is completely unaffected. But shock waves passing through sediments such as moist sand or gravel cause them to become almost like a liquid. This is called liquefaction. Buildings sink into the liquefied sediments and topple over.
Mexico City was built on old lake sediments, so many buildings were badly damaged when an earthquake struck the region in 1985. The districts of San Francisco that were damaged in the 1989 quake had been built on top of debris from the 1906 earthquake that had been dumped in the waterfront area.
After an earthquake
When the epicentre of an earthquake is on the sea bed, a large submarine landslide can result. This produces a series of fast moving waves, travelling at around 800 km/h (500 mph), called tsunamis. In deep water, they are small, but as they approach the shallow coastal waters they slow down and build up in height. Some tsunamis are tens of metres high as they crash on to the shore. Tsunamis can cause great damage to coastal areas.
A map of earthquakes (red dots)A diagram of the San Andreas FaultMost earthquakes happen along the edges of the Earth’s huge plates, especially where these are actively moving. This motion often results in volcanoes, too. Many earthquakes occur along subduction zones where one plate with oceanic crust pushes down below the edge of a continent. They also occur on transform faults, such as the San Andreas Fault in California, where two plates grind alongside one another in opposite directions.
High-risk earthquake regions are the “Pacific Rim”, around the shores of the Pacific Ocean, Southeast Asia (Philippines and Indonesia), from northern India west to southern Europe. Japan, on the west coast of the Pacific, experiences many earthquakes because it is lies close to a subduction zone. Most of the quakes are so small they are not even noticed. But every now and then there will be a major earthquake that has devastating consequences: for example, the 2011 Tohoku earthquake and tsunami, which led to the Fukushima nuclear disaster.
Earthquakes may also happen away from the plate edges. Some may result from the building of dams and the creation of reservoirs. These add a significant weight to the rock layers underneath, which may trigger earthquakes.
Valdivia, Chile, after the 1960 earthquake
The Great Chilean Earthquake
The most powerful earthquake ever recorded, measuring 9.5 on the Magnitude Scale, occurred in Chile in 1960. On 22nd May of that year, a giant fault deep beneath the ground in western South America, some 600 kilometres (375 miles) long, suddenly slipped about 20 metres (65 feet). A vast area of land in southern Chile was violently shaken for nearly four minutes. The Chilean town of Valdivia was devastated, with many of its buildings reduced to rubble.
The ocean floor dropped, causing the sea to rush away from the shore, then return in several giant tsunami waves, 10 metres (more than 30 feet) high, which smashed into the shore and flung ships far inland. A smaller tremor about ten minutes earlier had sent most people rushing into the streets. This saved many lives when the main quake came, although still around 5000 people were killed.
Consultant: Ian Fairchild