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 7.9-magnitude quake on Monday struck in Sichuan province but rattled buildings as far away as Beijing, Shanghai and Thailand, affecting 10 million people and killing at least 15,000.

Yuji Yagi, a seismologist at Tsukuba University, said data show the 155-mile Longmenshan Fault tore in two sections, the first one ripping about seven yards, followed by a second one that sheared four yards.

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Despite the two-stage quake, which he estimated lasted for about two minutes, it was the shallowness of the epicenter — only 6 miles — that contributed most to the temblor's destructive power, he said.

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

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 said the hardness of the land in China contrasted with 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 7.3-magnitude 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.