September 21, 1999. Copyright 1999. Graphic News. All rights reserved.

BAD YEAR FOR QUAKE CASUALTIES

LONDON, September 21, Graphic News: THE EARTH is not being shaken by more earthquakes than it has in the past, it is just suffering an unlucky run of deadly ÒtremblersÓ near vulnerable populated areas, according to scientists at the British Geological Survey.
TuesdayÕs massive earthquake that rocked Taiwan and has claimed more than 1,500 lives came only a month after devastating tremors that killed some 15,000 people in Turkey and at least another 122 in Greece.
ÒEarthquakes happen all the time, and this year we are not seeing more than usual. WeÕve just had a bad year in terms of the casualties,Ó said Alice Walker, a seismologist at the geological centre in Edinburgh.
The quake which struck at 1:47am measured 7.6 on the open-ended Richter scale Ð larger than the earthquake that caused widespread damage in Turkey last month and one of the strongest quakes to shake the island of Taiwan, which is prone to earthquakes.
The quake was centered 90 miles south-southwest of Taipei. There have been more than 1,000 aftershocks, keeping the people of Taipei on edge.
While three major quakes in just over one month may seem like a lot, the world is hit by an average of 18 earthquakes measuring 7.0 or greater every year, and four or five above the very dangerous 7.6 level, according to the geological centre.
Earthquakes happen as slabs of the earthÕs crust Ð known as tectonic plates Ð collide as they float in different directions on an underlying sea of magma. Most strike harmlessly at sea or in remote unpopulated areas. But as the world becomes more populated, casualties are bound to increase.
ÒThe main factors in terms of damage are proximity to a city, how deeply the tremor is buried, and the quality of the construction in the nearby cities,Ó Chris Browitt, another seismologist at the centre, said.
TaiwanÕs strict building codes and newer buildings may have prevented the quake from killing as many people as in Turkey.
The three recent large quakes were all probably different in nature, geologists said, but all struck near heavily populated areas. TaiwanÕs was a ÒsubductionÓ quake, where one plate slides under another. TurkeyÕs was a case of two plates grinding along each other and in the case of Greece, it was probably two plates pulling apart.
Subduction zones occur because when new sea floor is created from the earthÕs molten mantle the old sea floor must have somewhere to go.
As one plate slides under the other trenches are formed along the boundary. One plate bends downwards at about a 40¡ angle and plunges under the other plateÕs leading edge, eventually to melt back into the liquid mantle below.
Where the subducting plate bends downward, there is left a gap in the ocean floor running along the plate boundary Ð thus the common term Òtrench.Ó The Marianas Trench off Guam in the western Pacific plunges to a depth of over 36,000 feet, (11,000 metres) compared to an average ocean depth of 13,000 feet (3,950 metres), and is the lowest point in the ocean floor.
/ENDS
Sources: Reuters, UPI, U.S. Geological Survey, British Geological Survey