Dams and Earthquakes

What Makes A Tremor So Destructive?

(May 15, 2008) As China reels following Monday’s earthquake, scientists are just beginning to figure out the complex mechanics that triggered a temblor of such destructive force and widespread reach.

The magnitude 7.9 quake in China’s southwestern province of Sichuan has left nearly 15,000 people dead and ravaged entire villages near the epicenter.

Scientists say a number of factors appear to have come together to make this quake particularly damaging.

Sichuan lies near the boundary of two tectonic plates, the Indian and Eurasian landmasses, making it a particularly quake-prone area. India, once an island before colliding with the Eurasian plate some 50 million years ago, grinds northward by about two inches a year — a quick pace in geological terms.

This fast movement means more energy is pent up along the plates’ rims, triggering a violent release of the force as earthquakes. Tremors of this magnitude happen every 50 to 100 years, according to the U.S. Geological Survey.

What is more, a seismic team at Japan’s Tsukuba University led by researcher Yuji Yagi says Monday’s quake could have erupted in two distinct phases along the fault line — a rare occurrence. This may have strengthened the quake’s intensity and prolonged the shaking.

The team says the first motions along a fault known as the Longmenshan fault line caused a rift of as much as 23 feet and lasted some 50 seconds. This was likely quickly followed by a smaller slip along a separate section of the fault that lasted 60 seconds.

That means the region experienced intense shaking for two minutes, Dr. Yagi says. In comparison, a temblor that ravaged the Japanese city of Kobe in 1995 and killed more than 6,000 people lasted just 20 seconds and released less than 1/30th of the energy unleashed in Monday’s quake.

Other scientists point to the relative shallowness of the quake’s epicenter, which was just six miles underground. Shallow quakes cause more damage because the energy is released closer to the earth’s surface and triggers stronger shaking.

Moreover, much of the crust that lies beneath China is brittle and uniform because of the relative old age of the landmass. That allowed the destructive waves to travel for hundreds of miles without losing their strength, says Teruyuki Kato at Tokyo University’s Earthquake Research Institute — much like light transmits through an optical fiber. Shaking was felt as far away as Shanghai, some 1,000 miles from the epicenter.

Meanwhile, shaking closer to the epicenter was made worse by a thick layer of sediment deposited in the Sichuan Basin by the Yangtze River, which makes the ground there unstable. This unsettled layer magnified the power of the quake, researchers say; the fertile basin is also heavily populated, increasing the human toll.

To be sure, there are still some unanswered questions. Seismic research in the area is hindered by frequent landslides, which cover up much of the visible tectonic evidence, such as rifts in the ground or exposed ridges, scientists say.

“We need more studies to figure out the exact mechanics,” Dr. Kato says. “It’s clear that we’re looking at a very seismically active region.”

Hiroko Tabuch, The Wall Street Journal [PDF], May 15, 2008

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