Alex Salkever, Special to The Christian Science Monitor
Three major temblors in the space of a little more than a month - in Turkey, Greece, and now Taiwan - are enough to set millennium doomsayers crowing. And it's prompting some people to wonder: Are we seeing a kind of global seismic chain reaction?
But the pace of big quake activity is
normal, scientists say. Sudden slippages of the earth's fractured and evermoving
crust trigger an average of 18 quakes of magnitude 7 or more each year.
Most strike harmlessly at sea or in unpopulated areas. But when megatremors
hit urban areas, the damage is greater.
The 7.6 quake that shook Taiwan early on the morning of Sept. 21, killing at least 1,500 people, is the biggest there in 64 years. But it is not connected to the quakes in Turkey and Greece, according to current research. "People have looked for links between big quakes and never found them," says Gerard Fryer, a geophysicist at the University of Hawaii.
Nonetheless, the similarities and distinctions between the massive tremors provide scientists and civil authorities with lessons for the future.
The Taiwan temblor was more powerful
than the Turkish quake, but the outcome was far less severe for the island
nation - partly because the epicenter was farther away from the big cities.
The majority of buildings held fast and the death toll will likely be several times smaller than that from the Turkey quake, where hundreds of buildings collapsed. In the end, a confluence of geography, economic development, and government preparedness in Taiwan have prevented a far greater loss of life and property.
What both nations share is a history of big quakes in this century. In 1939 and 1976, temblors in Turkey measuring 7.9 killed 30,000 and 4,000 respectively. A magnitude 7.4 quake hit Taiwan in 1935 and killed more than 3,000 people, while another, more powerful but less deadly quake measuring 7.8 hit Taiwan in 1984 but claimed only 15 lives. Both the Sept. 21 Taiwan quake and the disaster a month ago in Turkey occurred in the predawn blackness, a time when sleeping victims are easily trapped inside buildings and darkness adds to the confusion of people trying to escape and rescuers trying make sense of the situation.
In Turkey and Taiwan, the quakes appear to have been close the earth's surface - meaning the impact on surface structures is greater. "Most very destructive earthquakes are shallow with epicenters of 10 miles to 20 miles or less," explains Robert Urhammer, a research scientist at the University of California at Berkeley Seismology Laboratory.
But crucial differences exist. The most important being the location of the epicenter. The quake in Taiwan was centered in a sparsely peopled mountainous region 90 miles south of the capital, Taipei, far from the homes of most of this island nation's 22 million residents. In Turkey, the quake originated near a heavily populated industrial zone on the northwest coast of the country.
Geography was another difference. The
rock-solid mountain terrain of the Taiwan epicenter may have lessened the
local impact of the quake. In contrast, the hardest hit part of Turkey,
around the city of Izmit, consists largely of soft soil that shifted dramatically.
"If it's very soft fill, it shakes like a bowl of Jello. If it is
hard rock, it shakes relatively little," says Mr. Urhammer.
Government preparedness and building codes also likely had something to do with the big difference between the two quakes' aftermaths. Natural disasters such as typhoons and tsunami are standard fare for Taiwanese, who are better versed in civil emergencies than the Turks, who had difficulty organizing relief efforts and calling out the army to assist.
"The Taiwanese tend to be very well prepared for national disasters. They build very strong facilities and infrastructure. And preparing for a possible conflict with China has probably prepared them in many ways for an Earthquake," says Allan Clark, a geologist at the East-West Institute in Honolulu, Hawaii.
Added to stronger building codes are newer buildings resulting from Taiwan's vibrant economy over the past decade. "The more recent construction in the cities over the last 20 years has adequately accommodated large earthquakes. Some of the older construction is less stable and less likely to survive if you had an earthquake of that magnitude," says Mr. Clark.
From news accounts, the buildings that went down were often smaller residential units or older buildings.
Even though Taiwan is emerging from this disaster far faster than Turkey, the true death toll may take weeks to tally. "This is a big enough earthquake that there could well be villages where there has been damage that we don't know about yet and the death toll is unquestionably going to rise," says University of Hawaii geophysicist Gerard Fryer.
Nor is this likely to be the last quake in region. The island is rocked by dozens of quakes each year. Earthquakes of this magnitude are not uncommon, although they are usually centered deeper in the ground and out at sea. The majority of the world's quakes occur around the Pacific Rim in a region called the Ring of Fire, where massive continental plates collide and submerge under one another in what geologists call subduction zones. Taiwan sits on the edge of the Ring of Fire. "It's more active than all of California in terms of numbers of big earthquakes. It's part of the island arc system," says Urhammer.
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