Travel science: Earthquake trail

The blue post (right) is one of many that mark the trace of surface rupture by the 1906 earthquake, as seen from Earthquake Trail, a paved .6 mile walk in the Point Reyes National Seashore that "explores the San Andres Fault Zone."

It was a seismic jolt that reverberated from Los Angeles to Oregon and ruptured nearly 300 miles of earth. San Francisco suffered the most damage, with burst gas mains fueling over 50 fires that incinerated 500 blocks. Even more than a century later, vestiges of the terrible temblor remain an hour's drive northwest of the city - if you know what to look for.

Belle Philibosian is among the savviest observers. On the bucolic meadow path named for the forces that were unleashed - Earthquake Trail - the U.S. Geological Survey scientist points out the telltale features: troughs, ridges and valleys that follow a straight line. Amid the tranquility of Point Reyes National Seashore, all are signs of an active fault.

Once you're schooled in geology, says Philibosian, who studies past quakes to predict future hazards, "you get a whole new appreciation for the landforms and the story that the land is telling you."

Geologists got the education of their lives when two gigantic sheets of Earth's rigid outer layer, the Pacific and North American plates, muscled past each other before dawn on April 18, 1906. In the aftermath, the scientists scoured the length of the fault for clues to explain the stunning wreckage. What they found laid the foundation for modern seismology.

Not for more than another half-century would their profession accept plate tectonics, the unifying theory that explains the planet's ever-changing geophysical forces. But when they saw fissures appear along a straight line for miles on end - though probably not deep enough to swallow a cow, as one rancher back then told reporters eager for dramatic measures of magnitude - a mental lightbulb went off.

"Even without understanding that plate tectonics was driving faults all around the world," Philibosian says, "the 1906 earthquake allowed scientists to start tying earthquakes to individual fault lines in the ground."

That's because they saw "blindingly obvious" features on the post-quake landscape that hadn't been there before.

Any structures that straddled the San Andreas fault - which had been identified and named little more than a decade earlier - were suddenly cocked several feet sideways. The geologists inventoried roads, bridges, tunnels and pipes. Corrals that once held horses and cattle had gaps big enough to drive a semi through, as if a livestock-loving giant roaming the countryside had pulled fences apart.

Most traces of the 1906 quake now lie buried under roads, high-rises and other trappings of civilization. The Earthquake Trail is a rare public place where remnants of the rupture are still visible.

The terrain here continues to yield rich scientific insights, too. It has helped geologists determine that the San Andreas is the fastest-slipping fault along the plate boundary - and hardly the only one.

"Here in the Bay Area, you're pretty much always close to a fault," Philibosian says, framed by a ridge that hitched a ride, inch by inch over millennia, from about 100 miles south.

With no survivors of the last Big One still alive to share their memories, complacency can easily take over. "The Earthquake Trail keeps that memory alive," she believes.

You just have to know what to look for.

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