State overdue for Big Ones, but Japan didn't increase risk

SEISMOLOGY: Experts say temblor is a wake-up call, but not an alarm of new danger.

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California is overdue not just for a cataclysmic earthquake but a blockbuster double feature - two of them - that could overshadow the destruction of the Northridge or Sylmar quakes, experts said Monday.

The devastating quake in Japan, experts say, is a wake-up call to Californians that they are living on the so-called Ring of Fire, where large numbers of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean.

"We are due for not one, but two large, potentially destructive earthquakes in California - one in the San Francisco Bay Area and one in Southern California," said seismologist Barbara Romanowicz of the University of California, Berkeley.

Experts also said that last week's earthquake in Japan throws out all preconceived notions about their strength - among them the belief that the seismic faults under Southern California are too short and shallow to generate an 8-magnitude temblor.

"There have been some assumptions in the past that earthquakes couldn't be bigger than a certain amount that's related to a fault on which they occur," says geophysicist Dave Jackson of UCLA. "But this recent earthquake ... and a number of others have shown that this idea breaks down.

"And earthquakes can be bigger, and we don't know how big they can be."

Experts estimate that Southern California experiences a major earthquake - one measuring close to 8.0 - every 45 to 145 years. With the last significant quake

occurring 154 years ago - the 7.8-magnitude Fort Tejon quake in 1857 - the area is overdue, they warn.

By comparison, the Sylmar quake in 1971 measured 6.6 and the 1994 Northridge quake was 6.7. They were far more destructive to human life and property than some other California quakes because of their location near urban areas.

Heightening the concerns is that California is in the northeast quadrant of the Ring of Fire, which is now the only corner that has not been hit by a catastrophic temblor in the last two years.

The Japan earthquake occurred in the northwest quadrant of the Ring of Fire, while the southwest corner was rocked Feb. 22 by a 6.3-magnitude temblor in New Zealand. Chile, in the southeast quadrant, was devastated on Feb. 27, 2010 by an 8.8-magnitude quake.

Would that mean that, in domino-like effect, a similar catastrophe could occur on the 810-mile long San Andreas Fault, which stretches from San Francisco to Palm Springs?

The idea caught fire Monday when Simon Winchester, an author and earthquake expert, told NBC's "Today" program that a quake in that fourth corner is "inevitable."

"It's becoming abundantly clear that these major earthquakes ... tend to occur in clusters," Winchester said. "A cluster seems to have happened in the last year in the Pacific plate."

But seismologists and geophysicists said such a prediction is just restating the obvious.

"These events are simply a reminder to whoever is living on a plate boundary, like we are on the San Andreas, that plate tectonics goes on unstopped," said Romanowicz, who is also director of the Berkeley Seismological Laboratory.

Seismologist Lisa Grant Ludwig of the University of California, Irvine, says what has happened along the Ring of Fire comes as no surprise.

"What is happening now, is that recent events are highlighting the significance of seismic issues, in graphic and tragic detail," said Ludwig, who led a breakthrough study a year ago documenting that earthquakes strike along California's San Andreas Fault more often than scientists previously thought.

"The calculated probability of earthquakes in California is quite high, and this has been known for years."

But other experts caution against residents going into panic mode.

"The earthquake in Japan has no direct physical connection with earthquakes in California," Jackson said.

"They're both caused by motion of the Pacific Plate relative to other plates. But a slip of the section, even though it's in the enormous section of the fault compared to Japan, it's a small section of the fault compared to the size of the whole Pacific Plate. And the physical effect here in California is pretty small."

Roland Burgmann, a geophysicist at University of California, Berkeley, agreed.

Although the very largest earthquakes can cause seismic activity long distances away, Burgmann said, it is not due to events being on the same plate.

"There is no reason to assume that the San Andreas system is more likely to have an earthquake now, because of these earlier events along the Ring of Fire, around the Pacific Plate," he said. "We should be much more worried about adjoining sections of the plate boundaries near the Japan and Chile earthquakes, that may be more likely to slip now because of these large events."

Selita Ebanks Amber Brkich Cindy Taylor Monica Bellucci Talisa Soto