California Quake Zones May Be More Lethal Than We Thought, Studies Say

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An earthquake of magnitude 7.5 to 7.8 is considered "major" (between "strong" and "great") and could cause severe damage to the Los Angeles metropolitan area.

"The shaking and damage from such an event could possibly exceed that of the 'Big One' on the San Andreas Fault," said Anderson. "But it's important to keep in mind that this event is very rare." Scientists refer to the major earthquakes along these known faults as the "Big One." The San Jacinto fault has large earthquakes every 100 to 300 years. The Cucamonga fault has large earthquakes on average every 500 to 1,000 years. The central Sierra Madre fault has large earthquakes perhaps once every several thousand years.

"So we know that at most a small fraction of the large earthquakes on the San Jacinto fault might grow into the much larger event," said Anderson.

The San Andreas Fault

The Duke University scientists studied the structure of the notorious San Andreas Fault near Parkfield, California, using seismic waves measured from a 7,100-foot-deep drill hole.

The hole, known as the San Andreas Fault Observatory at Depth Pilot Hole, is part of the $20 million National Science Foundation Earthscope initiative to monitor the processes controlling earthquake generation in a seismically active fault.

By studying readings from a chain of 32 seismometers installed in the hole, Chavarria and colleagues determined that the San Andreas Fault, which produced the devastating 1906 San Francisco earthquake, is perhaps more complex than previously thought.

"Using very unique recordings of microearthquakes in the pilot hole, we have been able to obtain images of the San Andreas Fault at depth that show a system of four secondary faults adjacent to the main San Andreas," said Chavarria.

The scientists created those images by tracing the complex paths that earthquake waves took after scattering off the possible underground structures.

"Going deeper in the earth we are closer to the source of earthquakes and therefore we can understand these phenomena better," said Chavarria.

Next summer, scientists will extend the drill hole into the fault zone to construct a major earthquake "observatory."

"This knowledge will allow seismologists to better determine which processes happen right before an earthquake starts," said Chavarria. "This will help us to better understand the risks associated [with] earthquakes as well to better prepare for such events."

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