A Crack in the Edge of the World (23 page)

However, it was measurement—or one measurement in particular—for which Gilbert is perhaps best remembered. And he measured anything he could lay his eyes on, particularly anything that seemed to
have been displaced across the trace of the fault. He measured fences, stands of eucalyptus trees, roads and farm lanes and tracks—and, from the displacements he found, he came up with a litany of earth movements that has been exceeded only by a very few other earthquakes in history. One farmer's fence snapped, and its posts were shifted thirteen feet apart, declared Mr. Gilbert. A line of eucalyptus trees near Bolinas Lagoon was broken and moved, also by thirteen feet. A road southwest of Point Reyes Station was displaced by twenty feet. The rough gravel pavement of the Sir Francis Drake Highway was broken clean across, and one part of the road was moved—Gilbert photographed it, memorably—just over twenty feet relative to the other. Not up or down but from side to side: If you stand and look at where the centerline of the road might be, it suddenly ends, and then reappears twenty feet off to the right, with the highway then continuing and vanishing over the horizon as though nothing had happened.

Although the small Northern California village of Olema saw the greatest displacement of the earth—this roadway was shifted some twenty-one feet by the right-lateral motion of the San Andreas Fault—it has lost its much-prized position as the epicenter of the 1906 event. That distinction now goes to a point under the sea off Daly City, forty miles south.

To the right. The effect of movement on a
right-lateral fault:
This is how the fault is officially known, as a
right-lateral strike-slip
fault. Stand anywhere and look across the fault—and the land on its far side will have been moved to the right. Hills, streams, roads, lines of trees—all of them, if they are on the distant side of the fault, will be to the right of those that lie on the side near the observer.

Today, at the Point Reyes Visitors Center, the fault is plotted through the meadows with a line of blue posts stuck into the ground. In one place the posts show the fault spearing underneath the barn that once belonged to a local farmer named W. D. Skinner. He had told Gilbert many things about the event (including the story about the cow) and showed him how his barn had been torn apart, his neat rows of raspberry bushes offset by fourteen feet, his fences ruined and scattered. And indeed there still is a fence on the property dating from long before 1906. At one point the fence posts seem suddenly to vanish—until you look and spot them once again, half hidden in a patch of woodland. The fence had been built with one continuous line of hastily carved redwood posts, with pine or redwood crosspieces between. But at the moment of the earthquake this fence line was snapped and sundered by the force of the event, with the western end of the fence moving north. And it moved a lot—to such an extent that the two ruptured ends that are visible today have been left no less than twenty-one feet apart.

This figure is most often cited, and impressively so, as illustrating the maximum amount by which the San Andreas Fault, and by association all of California as well, had moved on that extraordinary April morning.

But still, Olema was not the epicenter.

EARTHQUAKES EMIT WAVES
that ring through the solid earth just as sound rings through a bell made of brass. Moreover, they emit a number of different types of waves that, most fortunately and most crucially, move through the earth at very different speeds. It is the difference in speed between the two fastest-spreading kinds of waves (which are known as body waves because they travel through the entire body of the planet) that allows us to determine generally where each earthquake has its point of origin.

All that is required is a minimum of three recording stations, equipped with seismometers, that can observe the earthquake, measure the arrival of those two waves, and record the time between them. The first of the two body waves to arrive—and so the faster of the two—is a pressure wave, a P-wave, one that presses the rock and releases it, presses and releases, as if a Slinky toy were being stretched and then given a hard shove along its main axis. The second, slower body wave is the shear wave, the S-wave, which ripples horizontally through the rock strata—just like the sideways ripple that can be made to course through a Slinky.

(When terrified observers speak of the ground rippling toward them, or rising and falling in great fast-moving wavelike motions, they are almost certainly seeing the next family of waves—surface waves—that propagate more slowly because they only involve the outer surface of the earth. These most destructive waves—which are very easy to see on a seismograph because they are very large and have relatively large amplitudes—are further subdivided into what are known as Rayleigh Waves and Love Waves; the behavior of these, though they can be devastating to buildings and lethal to people, generally has less relevance to determining the location of earthquakes.)

Providing that both the P- and the S-waves travel through the same kind of rocks on their way to the observation station—which of course they would; but it is worth pointing out that waves run at very different speeds depending on whether they pass through granite, say, or shale—then there is always the same differential in their velocities. The farther the origination point from where the earthquake is felt and measured, the greater that differential. An earthquake originating
in Olema might show, on the seismographs in the USGS machines in Palo Alto, a differential between the two waves' arrival times of just ten seconds; that same earthquake, noted by the machines in Columbia University's famous Lamont-Doherty Earth Observatory in New York City, might show a differential of eight minutes. Each single second of separation in the waves' arrival times equates to about five miles of distance. This is similar to the rule-of-thumb approach that is used to compute how far away a storm might be: One sees the lightning flash and then counts the number of seconds until the thunder is heard—that number is very roughly how many miles separate you from the storm. So from Palo Alto the quake would be 50 miles distant, and from New York, 2,400 miles.
*

So, taken singly, any receiving station can tell, simply by noting the difference in the arrival times of the two waves from an earthquake, just how far away the earthquake's center is. But, armed with at least three receiving stations, one can then triangulate these data and find out not just how far away but also exactly where the earthquake is. The exercise is simplicity itself: All one does is draw a series of circles representing the distance (calculated from the time differential) that the earthquake appears to be from each one of the three stations. If the time differential indicated the earthquake was 50 miles from Palo Alto, then draw on the chart a circle with a 50-mile radius around Palo Alto. If the time differential suggested the same quake had happened 2,400 miles away from the Lamont station in New York, then draw a second 2,400-mile-radius circle around New York; and finally if a Calgary observatory measured the quake as occuring 1,200 miles from its seismometer, draw this third circle with a 1,200-mile radius. The three circles intersect at just one point—and this point where they meet is the epicenter—the place on the earth's surface that is directly above the originating rupture. And the exact point of that rupture, and the depth of it below the surface, is something that can itself be determined
by performing the same P-wave, S-wave, and time exercise all over again, but this time in three dimensions. That will give the hypocenter, the true originating point of the event. All, in other words, extraordinarily easy, for anyone with three seismographs and three accurate stopwatches.

Which is what a Berkeley seismologist named Bruce Bolt had—although actually he had many more seismographic records than these—when, in 1968, he recomputed the data responsible for the widely believed notion that the 1906 earthquake had its epicenter in the Marin County countryside near Olema. The 1906 event was recorded not merely by three observatories but by ninety-six seismographs around the world, and just about every single one of them had an accurate recording clock, each tuned to what was then Greenwich Mean Time. Moreover, a prodigious number of the local clocks—ordinary timepieces on living-room mantels, long-case clocks in parlors, clocks on church towers, clocks displayed on public buildings—were stopped dead by the force of the event, further evidence for the time of arrival of at least the strongest jolt of the quake. (It does take some technical expertise, however, to work out whether a clock from a historic earthquake has been stopped by a P-wave or by the subsequent S-wave, or perhaps even later, when a brick might have fallen on it. Generally, though, the shear waves are stronger, and so calculations on stopped clocks often assume that the stoppage was due to the arrival of the S-wave.)

However questionable some of the data might have been, Bolt had a very great deal of them, enough to calculate what is now generally accepted to be the true epicenter of the 1906 event. And so, according to the paper that he published in 1968, the recalculated epicenter turned out to be in the sea, a little more than a mile offshore, southwest of the Golden Gate Bridge and northwest of a particularly crumbled section of the coast near the sprawling suburban community of Daly City.

It has long been recognized that the track of the San Andreas Fault passes into the ocean a few miles south of Olema, close to that expanse of sand and shingle where so many San Franciscans go to swim known as Stinson Beach. The coastline is indented eastward from there on
south, forming a funnel toward the Golden Gate and the main shipping entrance to San Francisco Bay. The fault, however, remains unpersuaded by this deflection, and continues to roll on southeastward in its usual die-straight line. It scythes on through the sea to the immediate west of where the Golden Gate Bridge now stands, and reconnects with the coastline five miles farther on, in a particularly wretched part of the suburban mass of Daly City called Mussel Rock, where—under the fault's malign influence—houses seem always to be sliding into the ocean, landslips are an all-too-frequent occurrence, and the coast is a veritable construction site of seawalls, patched roads, and fractured gas lines. The benighted residents of Mussel Rock have paid a stinging price for the luxury of being able to watch the sun set over the Pacific each evening.

These days the community of Mussel Rock is the site of a ceaseless and wearying three-way battle going on between those residents who like to live there and gaze dreamily out at the foggy blue sea; those who say that the fault below them is moving so fast and so unpredictably that nature will not allow for indolent dreaming, or at least not for long, and that no one should be allowed to live there; and city officials who claim that the fault's energies can all be sapped or countered by man's cunning energies, and that Mussel Beach can be made safe and permanent. It can be made as enduring, some say, as that similarly situated rock in the Mediterranean, Gibraltar—and, in any case, Daly City people are perfectly at liberty to live just where they want to. They may not get good insurance, but they can live there if they feel they must. The tussle continues.

And then there was the row that broke out over Daly City's reputation. Just as soon as Mr. Bolt proved that the epicenter lay offshore a mile from Mussel Beach, a local historian, inspired by the news, designed a heavy brass plaque that memorialized what now appeared to be Daly City's historic importance in the event. His plan was to erect this monument—which he had bought and paid for—on a nearby beach.

But the mayor of Daly City, an elected official who at first blush appears to be a man with rather little sympathy for history, said no. He
didn't, he said, want people to associate his city with an earthquake. “We don't need to put a blemish on Daly City's shine,” he declared. “There's no reason for it.” So the plaque could not be erected anywhere in Daly City.

The historian tried three further times, even appearing before the city council armed with the support of a host of the region's geologists and seismologists. But the councillors remained adamantly intransigent, too, with the result that Daly City will not now be publicly associated in this way with the events of 1906.

Except that it will be associated with the event—thanks to a much more acceptable story that is already well known locally. Refugees from the earthquake's firestorms fled south across the city line and camped on plots beside what were then the meadows of one John Daly's dairy farm. Rather than return and risk another tragedy, many of them then set up home there. Daly City is, in other words, a place that owes its very existence to the 1906 earthquake, is in its very essence a memorial to the tragedy of 1906, and probably does not want to be reminded any further, thank you very much. Besides, the crumbling of the cliffs at Mussel Rock provides a daily reminder of the presence of the fault, which streaks under the entire length of the community on its way south.