Aftershocks of 1976 Quake Strike China

bow in front of the memorial wall bearing names of victims of the 1976 earthquake in Tangshan, north China's Hebei Province. Tangshan lost 240,000 lives to a devastating 7.8-magnitude earthquake on July 28, 1976, which was believed to be one of the deadliest natural disasters of the 20th century. Click to enlarge this image. Yan Jun, Corbis
Yan Jun, Corbis

Recent quakes in a city less than 100 miles from Beijing are likely the aftershocks of a catastrophic earthquake from 36 years ago.

Recent quakes in a city less than 100 miles from Beijing are likely the aftershocks of the catastrophic earthquake that destroyed the city 36 years ago, new research finds.

The nature of these quakes in Tangshan suggests they could roam across the North China Plain and potentially devastate China's capital.

As evidence that the recent quakes are aftershocks, the researchers noted that seismic activity in the Tangshan region since its 1976 quake has remained higher than is normal for the plain. This seismic activity also has been subsiding in a manner typically seen with aftershocks, albeit in a more drawn-out way, said researcher Mian Liu, a geophysicist at the University of Missouri.

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North China has a history of calamitous earthquakes. The magnitude-8.0 Hongdong earthquake in 1303 killed more than 470,000 people. In 1556,  the magnitude-8.3 Huaxian quake killed about 830,000, making it the deadliest quake in human history, Liu said.

In 1976, a devastating magnitude-7.8 earthquake struck Tangshan, an industrial city just 90 miles (150 kilometers) from Beijing. Tangshan was leveled entirely and more than 242,000 people died. Poor construction and a soft foundation that amplified ground vibrations helped ensure the city's destruction, Liu said.

A new Tangshan city was built on the remains of the old one, with higher construction standards for earthquake resistance. However, a recent series of small earthquakes in the region, including a magnitude-4.8 event in May and a magnitude-4.0 event in June, naturally was a source of alarm.

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Different kind of quake

Some researchers are skeptical of the idea that these new quakes are aftershocks of one more than three decades ago. Aftershocks usually die out within a few years of the main quake. [Video: How Earthquakes Lead To Aftershocks]

However, other scientists contend that quakes such as the one that ravaged Tangshan are different from most earthquakes geologists are used to.

Most earthquakes happen at the edges of the tectonic plates that make up Earth's surface, resulting from the energy released when such plates slam against or pull apart from each other. These quakes usually repeat along a few specific faults.

However, when it comes to earthquakes in the middle of continents, energy often builds up comparably slowly in complex systems of interacting faults over large regions. This slow buildup, combined with the fact that rock underneath the middle of continents is often sluggishly viscous, could spread aftershocks over long spans of time.

"Mid-continent earthquakes work in a way fundamentally different from those at plate boundaries, such as the San Andreas Fault in California," Liu told OurAmazingPlanet. In these regions, mid-continent "faults tend to be dormant for a long time, and spur up to produce a cluster of earthquakes before they go back to sleep again, and the next large earthquake would happen on some other faults."

Roaming earthquakes

Two other large earthquakes have struck the North China Plain in the past century: the magnitude-7.2 Xingtain quake in 1966, and the magnitude-7.3 Haicheng quake in 1975. The average seismic activity in these areas, as in Tangshen, is also higher than in the rest of the North China Plain. Also, Liu said, the aftershock pattern in the 1975 Haicheng temblor resembles that seen in the 1976 Tangshen quake (The 1966 Xingtain earthquake struck before seismic networks were established in the region.)

The fact these large temblors result from many interacting faults suggests earthquakes "roam" in northern China, popping up in one spot and then another, largely in places that previously saw relatively little tectonic activity and had no record of large earthquakes. Over the past 2,000 years, more than 22 temblors of magnitude 7.0 or greater have hit North China, but none repeated on the same pieces of fault during this period, Liu said.

It might be possible that a large earthquake will hit Beijing.

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Broader monitoring needed

"In 1679, a magnitude-8.0 earthquake occurred less than 50 kilometers (30 miles) from Beijing," Liu said. (7 Most Dangerous Places on Earth)

The fact these large earthquakes can roam long distances makes it hard to predict when they might happen or what risks areas face. New computer models of earthquakes are needed for mid-continents, Liu said.

"We cannot focus our attention only on places where recently large earthquakes occurred," Liu said. "We need to monitor stress and strain in broader regions than previously thought as necessary."

Such research "will force us to rethink about how earthquakes work in other mid-continents, including the central and eastern U.S.," Liu added. Already geologists are discovering evidence of ancient earthquakes in these regions where no recent seismic activity has been recorded, he explained.

Liu and his colleague Hui Wang detailed this research in the Nov. 6 issue of Eos, the weekly newsletter of the American Geophysical Union.

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