Science of Loma Prieta: Earthquake expert describes temblor

Early autumn was in the air the other morning atop Loma Prieta, the dark, brushy hill that is the tallest point in the Santa Cruz Mountains.

On a clear day, it feels like you can see forever from up here, 3,687 feet above the sea. On one side is Silicon Valley, on the other, Santa Cruz. Off in the fog and haze to the northwest is San Francisco. Oakland is due north.

"This would be a hell of a place to watch an earthquake," said Allan Lindh, a seismologist with the U.S. Geological Survey.

"You couldn't stand up, of course," he said. "There would be all this energy. Off to the right -- wham! -- back to the other side -- wham! It would knock you down.

"You'd have to be sitting down."

Lindh sat down on a cement pad that once was the footing for a radio tower. He grabbed hold of two pieces of steel sticking up out of the cement that once anchored to a tower, one in each hand, as if he could feel that big earthquake. He leaned to the right, then quickly to the left. "Wham! As much energy as a big nuclear weapon. Bam! More than the Hiroshima bomb."

''This would be a great place to see it." He meant to see the land move, to feel the power.

This is earthquake country; an ordinary visitor to the top of Loma Prieta sees the dramatic landscape of the coastal ranges, but a scientist can read the land -- the San Andreas Fault in a long, long rift valley not a mile away. A scientist sees the little hills, the minor landslides, the small slumps in the land forms, the bands of green trees that mark smaller faults like the Sargent Fault. A scientist sees -- almost feels -- the energy stored up in the rocks.

"Like a coiled spring," said Lindh.

Ten years ago tomorrow, on an absolutely still, hazy autumn afternoon, the spring let go and Loma Prieta was ground zero for the biggest Northern California earthquake since 1906.

30 years after Loma Prieta

This story as been republished online as part of The Chronicle's Loma Prieta 30th anniversary package. See more from the project here.

The earthquake began when the San Andreas Fault slipped at latitude 37.036 degrees north of the equator, longitude 121.883 degrees west of Greenwich, at a depth of 19 kilometers under a redwood-covered ridge called the Forest of Nisene Marks.

That was what seismologists call the hypocenter. The energy radiating out from the center, Lindh said, is what did the damage. It moved out from Loma Prieta southeast and northwest, at 2 1/2 kilometers a second; on the Summit Ridge, above Santa Cruz and below Loma Prieta, it was like standing in front of a train. Houses collapsed. The ground split open.

To the southeast, in a few more seconds, it hit Santa Cruz and Watsonville. In 40 seconds, more or less, moving through the ground, it hit the Bay Area, smashing San Francisco's Marina district, breaking the Bay Bridge, ruining the Cypress portion of the Nimitz Freeway in Oakland.

In 15 seconds it did enough damage to kill 67 people.

Lindh knew it was coming. He had written papers in 1983 and again in 1988 saying that the region with "the highest likelihood" of a large earthquake was what he called the Loma Prieta segment of the San Andreas Fault. Lindh and his colleagues knew the quake was coming and knew where it would come. But when? That's the trick.

Near the top of Loma Prieta, Geologist Allan Lindh used gestures to describe the movement that caused the quake.

How the earthquake happens, Lindh said, is fairly simple. "Nothing complicated about it, really," he said. The San Andreas marks the dividing line between two plates deep in the earth. One side is moving northward.

"The plates are moving and storing energy," Lindh said. The rocks bent as one side moved away from the other, bent under the pressure, bent and bent, and on October 17, 1989, they let go.

"Right now," he said, "the energy is building up, like energy of tons of TNT."

Lindh has studied the Santa Cruz Mountains and Loma Prieta for 20 years, looking and measuring and studying. He knows the history of the mountains, the folds of the ridges, the rocks underneath.

Away from the summit ridge and rising up toward the peak, the mountain is "god-awful rough country," Lindh calls it, good for fierce fires and rattlesnakes. "Without roads," he said, "You couldn't go a mile a day in the brush."

Loma Prieta was named by the Spanish and Mexicans who looked up at it from the Pajaro Valley and the Mission of Santa Cruz nearly 200 years ago. They saw a high brushy hill, covered with dark, bluish-green chaparral, a loma prieta -- dark hill. The Americans tried to call it Mount Bache, after a geologist, but the name did not stick.

It is a kind of geologic frontier, a place where the land is always slipping. Atop the peak, now spiky with radio towers and telephone relay equipment, are formations of old rock, chert and graywacke of the Franciscan formation.

The rocks were formed under ancient oceans, twisted and uplifted, split and shifted north.

"Here," Lindh said, picking up an ordinary looking dirt-brown rock. "This is graywacke, laid down a hundred million years ago at the south end of the San Joaquin Valley," he said.

These rocks are fractured by the movement of the earth, ground up and moved again. "The other half of this stuff can be found near the Los Trancos woods parking lot, over up there, near Palo Alto, about 30 kilometers away, he said. "It took a million and a half years to go that 30 kilometers."

The land, the mountains modern people think of as eternal, have moved, slid away. Point Reyes, at the western edge of Marin County, was off Monterey 10 million years ago. In a day's drive, Lindh said, one can see evidence of land moving apart: the old granitic soil of Bodega Head, the headlands of Point Reyes, the Farallon Islands.

All of it has moved north, along a fault that runs up under Loma Prieta, under the Crystal Springs lakes on Interstate 280 on the Peninsula, out to sea near Mussel Rock, back again between Stinson Beach and Bolinas, up the long valley near Olema. The beautiful Tomales Bay is on the fault line.