Newfound L.A. Fault Threatens Major Quake

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Scientists believe that up to half of the energy stored in this process could be released on the Puente Hills fault.

"The good news is that major earthquakes along this fault are very infrequent, it may not happen again for thousands of years," said Dolan. "The bad news is that it could be very strong when it does happen."

An earthquake with a magnitude of about 7.5 on the Richter scale probably occurred on the Puente Hills fault 7,000 to 8,000 years ago. That means it was 15 times stronger than the 1994 earthquake that hit Northridge, north of Los Angeles, and killed 51 people.

The Northridge earthquake, which measured 6.7 on the Richter scale, caused U.S $44 billion in economic damage and is the largest natural disaster in U.S. history.

The temblor proved that an earthquake smaller in magnitude can cause greater damage than a more powerful earthquake. The shaking in Northridge was some of the worst ever felt.

Bowl of Jello

In bigger earthquakes, shaking lasts longer and is felt over a larger area. There is a difference in frequency content between small and large earthquakes. Small earthquakes have higher frequency energy and can be particularly harmful to homes. Big earthquakes have low frequency energy and may cause more damage to large structures like skyscrapers.

What makes Los Angeles particularly vulnerable to any earthquake is that part of the basin the city is built on is filled with weak sediments. "The fault will pump energy directly into the basin and cause it to shake like a bowl of Jello," said Dolan.

Mexico City has a similar problem. That city was severely damaged in a 1985 earthquake, even though the epicenter lay in far-away Acapulco.

Establishing what kind of earthquakes could happen is critical for seismic hazard zoning, emergency response, and risk mitigation strategies.

"You want to find out as much as you can about the threat," said Dolan. "That gives you a batter chance to prepare for it."

The main challenge is to build earthquake-proof structures. As seismologists are fond of saying, "Earthquakes don't kill people, buildings do."

A summary of the research appears in the current issue of Science, the journal of the American Association for the Advancement of Science.

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