segunda-feira, 23 de agosto de 2010

O Big One pode ocorrer antes

A ciência não é capaz de prever quando ocorrerá um terremoto, mas um estudo recente traz evidencias que o Big One, o grande terremoto, previsto para ocorrer na área do pacifico nos Estados Unidos está próximo. Será

Strong earthquakes along the San Andreas fault are a lot more frequent than previously thought.
Over the last 700 years, powerful quakes have struck the region every 45-144 years.
The last big 7.9 magnitude earthquake hit Los Angeles 153 years ago; the next is overdue.
If the "Big One" strikes, 2,000-50,000 people could lose their lives.
Although individuals, regulators and the emergency services can prepare, the unpredictable nature of LA's highway grid could hinder relief efforts if the worst should happen. Click to enlarge this image.

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Strong earthquakes along the San Andreas fault in southern California are more frequent than previously thought, so the dreaded "Big One" could be just around the corner, US researchers said Friday in a study.

University of California at Irvine and Arizona State University scientists examined the geological record stretching back 700 years along the fault line 160 kilometers (100 miles) northwest of Los Angeles.

They found that strong earthquakes -- between 6.5 and 7.9 magnitude -- shook the area every 45-144 years, instead of the previously established 250-400 years.

Since the last big 7.9 magnitude earthquake struck southern California in 1857, or 153 years ago, scientists believe the next "Big One" could happen at any time.

The scientists on Friday provided an abstract of their study, which will be published in full in the September 1 issue of the magazine Geology.

"What we know is for the last 700 years, earthquakes on the southern San Andreas fault have been much more frequent than everyone thought," said the study's lead author Sinan Akciz.

"Data presented here contradict previously published reports," he added.

With 37 million people living in southern California, chiefly in the cities of Los Angeles, San Diego and Anaheim, a major earthquake could kill between 2,000 and 50,000 people and cause billions of dollars in damage, scientists said.

UCI seismologist Lisa Grant Ludwig, the study's chief investigator, said people in the area should already be taking precautions.

"There are storm clouds gathered on the horizon. Does that mean it's definitely going to rain? No, but when you have that many clouds, you think, I'm going to take my umbrella with me today. That's what this research does: It gives us a chance to prepare," she said.

For individuals, that means having ample water and other supplies on hand, safeguarding possessions in advance, and establishing family emergency plans.

For regulators, Ludwig advocates new policies requiring earthquake risk signs on unsafe buildings and forcing inspectors in home-sale transactions to disclose degrees of risk.

Some things, she added however, remain unpredictable, especially Los Angeles' troublesome highway grid, which in the best of times gets hopelessly choked in traffic.

Ludwig said the new data "puts the exclamation point" on the need for state residents and policymakers to be prepared.

Article abstract

Century-long average time intervals between earthquake ruptures of the San Andreas fault in the Carrizo Plain, California

  1. Sinan O. Akçiz1,
  2. Lisa Grant Ludwig1,
  3. J Ramon Arrowsmith2 and
  4. Olaf Zielke2
+ Author Affiliations
  1. 1Program in Public Health and California Institute for Hazards Research, University of California–Irvine, Irvine, California 92697, USA
  2. 2School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA


Paleoseismological data constrain the age, location, and associated magnitude of past surface-rupturing earthquakes; these are critical parameters for developing and testing fault behavior models and characterizing seismic hazard. We present new earthquake evidence and radiocarbon analyses that refine the chronology of the six most recent earthquakes that ruptured the south-central San Andreas fault in the Carrizo Plain (California, United States) at the Bidart Fan site. Modeled 95 percentile ranges of the earthquakes prior to the A.D. 1857 earthquake are A.D. 1631–1823, 1580–1640, 1510–1612, 1450–1475, and 1360–1452. The average time interval between the last six earthquakes that ruptured the San Andreas fault in the Carrizo Plain is 88 ± 41 yr. This is less than the time since the most recent A.D. 1857 earthquake, less than all reported average intervals of prehistoric earthquakes along the entire San Andreas fault, and significantly shorter than the 235 yr average used in recent seismic hazard evaluations. The new chronological data combined with recent slip studies imply that the magnitudes of the earthquakes that ruptured the southern San Andreas fault in the Carrizo Plain since ca. A.D. 1360 were variable, and suggest that the widely held view of rare but great surface rupturing earthquakes along this portion of the southern San Andreas fault should be reevaluated.

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