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China Quake Delivered Seismic One-Two Punch

Shino Yuasa in Tokyo
Associated Press
May 15, 2008
 
The fault line that caused this week's devastating earthquake in China probably buckled in two stages, and the hardness of the terrain contributed to the wide reach of the damage, Japanese scientists said Thursday.

The magnitude 7.9 quake on Monday struck in Sichuan Province but rattled buildings as far away as Beijing, Shanghai, and Thailand. (See photos of the earthquake's destruction.)

The quake has affected ten million people and may have killed as many as 50,000, according to a revised death toll estimate released by the Chinese government today.

Yuji Yagi, a seismologist at Tsukuba University, said data show the 155-mile (250-kilometer) Longmenshan Fault tore in two sections.

The first section ripped about 7 yards (6.4 meters). The second sheared 4 yards (4.6 meters).

(Related: "Pandas, Tourists Safe After China Quake; Local Towns Leveled" [May 14, 2008].)

But it wasn't the two stages that made the quake so damaging. The shallowness of the epicenter—only ten miles (ten kilometers) underground—contributed most to the temblor's destructive power, Yagi said.

"The damage was very severe because the quake's epicenter was shallow and the quake occurred in densely populated areas," Yagi said.

The other distinguishing factor to the quake was the firmness of the terrain in central China, which allowed seismic waves to travel large distances without losing their power, Tokyo University seismologist Teruyuki Kato said.

Kato contrasted the hardness of the land in China with that in Japan, where patches of soft terrain can often blunt the reach of earthquakes.

"The strata are very hard in the quake-hit region, and that allowed the seismic waves to travel far, while their strength remained intact," he said.

Japan is one of the world's most quake-prone nations, and media there have focused on the possible role of shoddy construction methods in the large death toll in the China quake.

Japan's most destructive quake in recent times was the magnitude 7.3 temblor in the western city of Kobe, which killed more than 6,400 people in January 1995.

Yagi calculated that the energy level of the China quake was 30 times that of the one that struck Kobe.


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