What is an earthquake?
The ground beneath our feet seems rock-solid, but our planet’s surface is in fact a dynamic grid of slowly moving sections, known as tectonic plates. Normally, this motion is imperceptible to humans, showing itself only on geological time scales (North America and Europe, for example, are drifting apart at the rate of just five centimetres per year). The movement causes stress to build in the crust. If the stress rises beyond a critical threshold, a portion of the crust will give way, shifting suddenly and violently. This sudden motion usually occurs along a fault or fracture – a zone of weakness in the earth’s crust. The result is what we call an earthquake.
“You can only push a rock so far,” explains Joseph Wallach, a geologist at the University of Ottawa. “At a certain point in time, the rock will fail, and you’ll have an earthquake.” 90 per cent of earthquakes occur along the boundaries of tectonic plates, but they can also happen in more stable regions, thousands of kilometres from the plate edges.
The destructive power of an earthquake depends on the depth of its point of focus. For the most common quakes, known as “shallow-focus” events, the focus is just a few dozen kilometres below the surface. Because the crust is in motion so close to the surface, these are the most powerful and deadliest quakes. In contrast, “deep focus” earthquakes originate hundreds of kilometres below the surface, and cause less damage. The spot on the earth’s surface directly above the focus is called the “epicentre.” Earthquakes can also occur beneath the ocean; such oceanic quakes can trigger enormous tidal waves, or tsunamis. These waves can travel great distances at speeds of up to 1,000 kilometres per hour, attaining heights of up to 15 metres as they reach the shore.
Date Location Magnitude Deaths
April 18, 1906 San Francisco 8.3 503
Aug. 16, 1906 Chile 8.6 20,000
Dec. 16, 1920 China 8.6 100,000
Sept. 1, 1920 Japan 8.3 100,000
May 22, 1927 China 8.3 200,000
March 2, 1933 Japan 8.9 2,990
Jan. 15, 1934 India 8.4 10,700
Jan. 24, 1939 Chile 8.3 28,000
May 21-30, 1960 Chile 9.5 5,000
April 10, 1972 Iran 7.1 5,350
July 28, 1976 China 7.8 to 8.2 242,000
Sept. 16, 1978 Iran 7.5 to 7.9 15,000
Sept. 19, 1985 Mexico 8.1 9,500
June 21, 1990 Iran 7.7 35,000
Dec. 26, 2003 Iran 6.5 26, 271
Dec. 26, 2004 off the coast of Indonesia 9.0 200,000 (includes tsunami victims)
March 29, 2005 Indonesia 8.7 1,000
Oct. 8, 2005 Pakistan-administered Kashmir 7.6 20,000 (Official toll as of Oct. 11, 2005)
May 27, 2006 Java, Indonesia 6.3 5,135
May 12, 2008 Sichuan province, southwest China 7.8 More than 8,500
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1 comment
Naveen says:
March 2, 2010 at 2:51 am (UTC -6)
I wud like to know everything about the major earthquakes location..I mean the weather , astronomical significances etc…
Maybe there is something common in all these disasters ..
A set patern kind of thing…