This Walking Tour Is Always on Shaky Ground

Half a dozen turkey vultures are doing lazy loops overhead, eyeing the hilly Northern California countryside for a quick meal, as we follow silently behind our guide.

It’s a bright, sunny afternoon with only the slightest whisper of a breeze. The calmness, however, contrasts sharply with the jumbled and disordered terrain, which was once the scene of epic violence--not of the human sort, but by the hand of nature--and will surely be again.

Watching the vultures, a visitor wonders whether these birds of prey sense the danger underneath them. But these reveries are disrupted by our guide.

“And here,” the guide announces cheerfully (too cheerfully, the visitor thinks), “you see where the earth cracked open.”

He points to a row of blue wooden posts on a grassy rise dead ahead.

We were walking over a particularly memorable part of the San Andreas fault, the great geological scar that slashes like a dagger through much of California.

In the early morning hours of April 18, 1906, the ground first ripped apart near here, unleashing the horrible events that virtually destroyed San Francisco some 50 miles away (as the crow, not the vulture, flies).

About three dozen of us, of all ages, have joined our guide to walk the Earthquake Trail, a living textbook in seismology created by the National Park Service to provide some lessons about quakes, as well as to debunk the myths about them.

The trail, only seven-tenths of a mile long in the very heart of the 1906 fault zone, is one of the less well-known delights of the Point Reyes National Seashore, a great natural wonderland of wildlife and scenic vistas overlooking the Pacific that attracts some 2.2 million visitors a year.

Most come for such pleasures as watching the powerful surf crash onto the miles of beaches, or viewing the annual migration of gray whales from a famed century-old lighthouse atop brooding headlands, or hiking along the park’s thickly wooded ridges and ravines.

Yet the Point Reyes peninsula, hooking out into the Pacific like a long-noised gargoyle on a medieval cathedral, has another distinction. It sits on the eastern edge of a portion of the earth’s crust known as the Pacific plate, whereas most of California, along with the rest of the United States, occupy the North American plate.

The San Andreas fault, running along the eastern edge of the peninsula, marks the boundary between the two plates.

“Here you can have one foot in North America and the other is Asia,” says our guide with slight exaggeration as he playfully straddles the path with his legs far apart.

Actually, the fault zone isn’t a single, narrow crack, but a fractured landscape of ridges, small hills and rolling meadows hundreds of yards wide. But in one way, he’s right. Geologically, Point Reyes isn’t really part of North America.

As we followed the Earthquake Trail’s meanderings through the fault zone, we passed by old, moss-covered trees, some of them still tilted from the 1906 shake. We paused at a meadow where the park service raises big, friendly Morgan horses. Then we strolled along a weathered scrap that still shows where the two plates moved past each other.

There was no need for hammer and pick on this field trip; we were getting our geology effortlessly. The gentle path had been paved, and signs along the way explained the earthquake phenomenon in words, diagrams and old photos, e.g., an old steam locomotive tossed on its side by the 1906 jolt.

They even provide safety tips on what to do when the earth starts shaking while you’re at home (stand in a doorway, turn off electrical and gas lines), though, disturbingly, none for while you’re walking along a major fault.

The trail is open to walkers free of charge year-round, as is the rest of the seashore. It begins just a few steps from the seashore’s Bear Valley Visitors Center, a splendid little natural history museum about the rich flora and fauna of Point Reyes, off Route 1, near the town of Olema.

Although walkers can take the trail on their own at any time, on this day, as on every Sunday at 2 p.m., the National Park Service provides one of its ranger-naturalists as an instructional guide.

After last October’s Loma Prieta quake that shook the Bay Area, traffic on the trail picked up in a sudden burst of public seismic awareness, even though Point Reyes escaped the earth’s angry wrath.

Still, the rugged landscape along this section of the San Andreas fault is as good as any for learning how a quake can reshape terra (not so) firma.

Our leader was an affable young Georgian, Ed Amerson, who admitted he was not a seismologist but had plainly done his homework.

He told us that geologists, in jest, refer to Point Reyes as “an island in time” because it will eventually break free from California and became an island. “But don’t worry,” he added. “That won’t happen for another five million years.”

This isn’t millinerian science fiction, Amerson assured us. It’s a fact of plate tectonics, geology’s new orthodoxy, which holds that the earth’s surface is composed of a half-dozen continent-sized plates, as well as a number of smaller ones.

They move constantly, bumping and grinding against each other and slowly changing the planet’s face, as well as creating spectacular geological curiosities like Point Reyes.

For a long time, geologists wondered why the bedrock of Point Reyes was essentially hard granite, while just across the fault was softer, pulverized stuff called Franciscan sediment. Now, thanks to plate tectonics, says Amerson, they know why.

Point Reyes once was part of the Sierra Nevada, east of Los Angeles, but as the Pacific plate slid past and under the North American plate, it ripped Point Reyes away from its mountainous granitic birthplace and, in the past 150 million years, carried it 350 miles north, like a parcel on a conveyer belt.

This slow-motion journey, as the tossed landscape shows, wasn’t smooth, however. It progressed in fits and starts, accompanied by earthquakes. In the big one of 1906, Point Reyes lurched northwestward about 20 feet.

Ever the tutor, Amerson whipped out two little kitchen sponges to demonstrate the plate action.

“Let’s imagine one of them as the Pacific plate and the other as the North American,” he said as he brought the edges of the sponges firmly together.

“The plates have bumped, just like these sponges. Normally, they’re supposed to slide smoothly past each other, the North American plate moving in a westerly direction and the Pacific toward the northwest.

“But here, along this part of the fault, they’ve snagged, warping the rock as the strain builds from the pressure of the plate movements.”

As he demonstrated with the sponges, you could see them bend and swell, storing up the energy from the force he exerted on them.

Then--snap! He suddenly let the sponges break free. The effect was so artfully timed that some of the visitors gasped.

“When the rock has stored its maximum energy,” he says, “the earth cracks, right along the plate boundary, and the ground on each side of the fault lurches in opposite directions to relieve the pressure. The sudden break sends out seismic waves in all directions. It’s these waves that we feel as an earthquake.”

Signs along the trail tell us that we are standing on the epicenter of the 1906 quake--the place directly above the original break in the ground. Not so, according to seismologist Bruce A. Bolt of the University of California at Berkeley, who says that the park service is a little shaky in its facts.

Recent investigations have located the epicenter a few miles south of Point Reyes, under the waters of the Pacific just outside the Golden Gate.

But the Australian-born Bolt doesn’t want to quibble with the authorities of his adopted country. “The earthquake walk is certainly at the site of the greatest measurable ground movement,” he says. “Besides, it’s a jolly good place to see what quakes can do.”

You can still see signs of that movement in an old wooden fence that had been built directly across the fault. One section of the fence is now offset some 16 feet from the other, vividly demonstrating how much the plates lurched past each other.