Wednesday, December 31, 2003

Now Here's a Cheery Thought ...

Scientists speculate that the 'pregnant' San Andreas Fault could be ready to deliver.
Scary beauty surrounds Cameron Barrows. He works in lush groves of fan palms that erupt like mirages from moonscape terrain. Hot springs bubble beneath them. Sand dunes drift nearby.

"It's an amazing place," said Barrows, director of the Coachella Valley Preserve east of Palm Springs. The 20,000-acre sanctuary owes its splendors to the San Andreas fault, the frightening part of the bargain.

Many scientists say the Coachella Valley is where the 750-mile San Andreas seems most prone for an epic earthquake, a monster that would be enormously more powerful than the recent temblors in San Simeon, Calif., and Bam, Iran.

As the possible generator of the feared Big One, the San Andreas once dominated the quake worries of Californians. But that was before "subsidiary faults" in locales such as Loma Prieta and, especially, Northridge reordered popular anxieties. They flattened buildings and buckled interstates while the San Andreas remained relatively quiet, as it has since the great San Francisco quake of 1906.

Now, with the 10th anniversary of the Northridge temblor approaching, and after much study of those second-tier faults, scientists again are highlighting the San Andreas as the rupture without rival — a slumbering beast napping on borrowed time.

"The primary fault in California — the big dog — is the San Andreas, and it's important for people to remember that," said Doug Yule, a geologist at Cal State Northridge, which was badly damaged in the Jan. 17, 1994, disaster. "The San Andreas will produce the largest earthquakes."

Yule and his colleagues have dug trenches along the southern section of the fault to carbon-date its buried fissures in hopes of determining just how "pregnant" it is. Their best guess: The San Andreas, from the Salton Sea to San Bernardino, is at term.

The San Andreas last slipped in the region 191 years ago. That is 40 years beyond the average interval for the southern segment, based on estimates that stretch back 12 centuries.

The observatory — it's a widely cast installation of sensors, not a building — will collect data from points along the San Andreas and through the Cascadia subduction zone to Canada.

The subduction zone is where the North American and Juan de Fuca plates meet. Subduction quakes are vastly more violent — magnitude 9s are possible — but far less frequent than those on the San Andreas.

Such events occur in subduction zones once or twice every 1,000 years; the most recent on the Cascadia was in 1700.

Uhrhammer said the observatory would help scientists determine where the San Andreas might be lurching toward a 1906-strength quake.

He doesn't expect a repeat in the Bay Area anytime soon.

On the northern San Andreas, "it could well be another century or so before you get another 1906 event," Uhrhammer said.

The outlook is not as reassuring down south.

Yule, the Cal State Northridge geologist, has found evidence in his paleoseismic trench that a massive quake could be in store for the Coachella Valley — and north into San Bernardino and beyond.

But like all fault excavators, he notes, "We just don't know."

Peachy. So, does this equate to a yellow or an orange alert? On that note, have a happy and safe New Year. 'til next year ...


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