A Crack in the Edge of the World (22 page)

The activity was first noted as being concentrated in a valley south of San Francisco, which lay right on top of the line. This valley had been named the Valle de San Andreas by its Spanish discoverer, because he found it on November 30, 1774, the feast day of the apostle most English-speakers (like most Russians) know as Saint Andrew. Once it was realized that the valley lay on (and indeed was caused by) the very fault line where all this activity was concentrated, the fault itself was given the name of the valley: It became the San Andreas Fault.
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The San Andreas Fault is a feature so well known to geologists around the world that it is generally referred to by its initials, SAF. More important for this account, movement along the San Andreas Fault was the fundamental event that all but destroyed San Francisco in 1906. It is also the phenomenon that, on some unpredictable day in the future near or distant, will surely destroy any city built by those improvident enough to site it nearby.

THE STORY OF THE BIRTH
of the San Andreas Fault is far from simple, and interpretations change as new information is uncovered. It can be summarized, however, and in essence it appears that the mechanisms that gave rise to the fault can conveniently be said to have begun about 150 million years ago, out on the edges of a Pacific Ocean that was very much larger than it is today.

A submarine chart of the ocean today shows a line of relatively shallow water that extends southward for several thousand miles from a point close to the west coast of Mexico, via the Galápagos Islands and Easter Island and the Juan Fernandez Islands,
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all the way down to the Antarctic. The island groups that lie along it are volcanoes, some active and some not; these, and the submarine ridge of which they are the visible peaks, are the manifestations (just as the Mid-Atlantic Ridge is a manifestation on the farther side of the world) of a part of the earth that is splitting open, where plates are moving away from each other, and where new material is being oozed out onto the planetary surface (in this case most of it invisibly, beneath the sea). The ridge is called the East Pacific Rise—not least because it is the dominant feature of the eastern half of the Pacific Ocean.

But 150 million years ago it was not in the east: It was much more in the middle of what in those days was the very much wider Pacific. And just like the Mid-Atlantic Ridge, this then Mid-Pacific Ridge was also a spreading zone, a place where two tectonic plates were moving
steadily away from each other. The plate on the western side of the ridge was the Pacific Plate that we know today, and it was moving northwestward. The plate on the eastern side of the ridge, which geologists have named the Farallon Plate, was moving southeastward. To complicate matters further, the North American Plate, which was being forced by pressure from the upwelling of magma at its eastern edge, at the Mid-Atlantic Ridge, was shifting steadily westward, in the direction of these two spreading plates, the Pacific and the Farallon.

The consequence of movements like this one—the collision of the North American and Farallon Plates—is the familiar one of subduction, the phenomenon that is known in every corner of the world where there are active and very violent volcanoes, from Java and Sumatra to Japan, from Kamchatka and New Zealand to Alaska—places where a light continental plate hits a heavy oceanic plate square on, and the heavier plate is subducted beneath its continental collision partner. Wherever such a collision takes place, volcanoes and earthquakes are created and break out in dangerous abundance. In this case the heavy and oceanic Farallon Plate subducted below the light and continental North American Plate—causing many of the geological features, Eldridge Moores's ophiolite sequence in particular, that make up the American West.

The North American Plate's westward movement continued until, sometime around 30 million years ago, it hit a snag: It ran into the selfsame East Pacific Rise. The rise then began subducting below the North American Plate as well—and this created the kind of tectonic confusion that is much better seen on a pool table. (Physicists with a penchant for divining the way that a number of colliding forces all moving in different directions can impinge on and interact with one another work out this kind of thing mathematically; in
The Hustler
, which some say it parallels, it was more a matter of intuition, and gin.)

Once this was done, or under way, a portion of the North American Plate started to collide directly with the Pacific Plate—most of the Farallon had by then vanished and was wallowing deep below what would in time be Southern California. As it did so, the relative motion of the plates changed—just as the directions of cannonading billiard
balls do. The two plates did not hit each other square on, one of them moving west to east, the other east to west: The North American Plate ran out of steam and essentially stopped in its tracks, while the Pacific Plate began to move northward. It began to slide up and along the outer edge of the North American Plate. It began to
slip
along its
strike
, as the geologists who first discovered the phenomenon declared; it began to slip along the line that marked the edge of its outcrop. And it began to do this a little less than 30 million years ago. Where the plates scraped against each other, the land, up on the surface, became crazed and fractured with a pattern of faults.

At first these faults, marking the strike-slip movement going on below, were some distance away from their present-day track, and ran in different directions from it as well. Near the Californian seaside town of Santa Cruz there is a fault called the Gregori-Hosgri that represents one of the early sliding-plate tracks; and another called the San Gabriel Fault a little way north of Los Angeles also shows, in what can be thought of as a fossilized way, where the plates used to slide alongside each other. And both of these go off in a very different directions from the more recent fault, which displays the more recent relative plate movements.

About 10 million years after the process had first begun—about 20 million years ago, in other words—the relative motion of the two plates settled down, running essentially along the line of one principal fault. And though even today in most places the plate-against-plate contact line cannot quite be pinned down to a matter of inches—usually it is more a zone than a line, and is some hundreds of feet or, in places, even miles wide—today's center point, the zone where the maximum annual slip between the two plates is noticed, is the track of that most infamous darling of seismicity, the San Andreas Fault.

It should be noted that this plate-on-plate strike-slip zone extends between two “triple junctions”—places where the two principal plates meet up with two small relict pieces of the old Farallon Plate that did not get themselves subducted. (These parts were not subducted because, in essence, they were too far north or too far south of the main westward thrust of the North American Plate to be affected.) So the fault zone technically runs from what is called the Mendocino Triple Junction, off California's Cape Mendocino—where the Farallon Plate's little relict piece is called the Juan de Fuca Plate
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—down to the Rivera Triple Junction, off Mazatlán and Puerto Vallarta on the Mexican west coast—where the tiny relict piece of the Farallon is called the Rivera Plate.

These two little plates—the Juan de Fuca up north and the Rivera down south—are still subducting, as their predecessor once did. And, sure enough, where they do subduct, there are, as always in such situations, volcanoes. In the north, as a consequence of the beautifully named Cascadia Subduction Zone, there is a fleet of active volcanoes, with Mount St. Helens being the most recently notorious; and in the south there are volcanoes such as Mexico's Colima and Paricutín, the former old and still active, the latter young and, in spite of spectacular eruptions in the 1940s and 1950s, now apparently quite defunct.

Between these two triple junctions, then, runs the San Andreas Fault—this 750-mile-long zone marked by its near ceaseless activity and occasionally by very lethal seismic outbursts. The land on each side of the fault is moving, all the time—though not everywhere along the fault's length at quite the same rate. Overall it is moving at a speed that in real terms seems very slow: Up at a place called Telegraph Creek, close to Cape Mendocino and the northern triple junction, the Pacific Plate is moving northward at about an inch and a half every year. In the terms that geologists understand, however, an inch and a half a year is something approaching raceway speed. According to the USGS, a velocity like this makes the San Andreas a fault like very few others, anywhere in the world. So, by most geologists' lights, it is very fast, very interesting, and very, very dangerous.

Its effects have been noted for a very long while and noted, more-
over, in places far removed from where the fault makes itself topographically obvious, such as in the San Francisco Valley where it was first named. Its real extent was noticed in 1906, for instance, just after the San Francisco Earthquake, when a man named F. E. Matthes was sent up north to Humboldt County to help the California State Earthquake Commission see if what had happened in the Bay Area had spread into the north of the state. He found that it had—and moreover he then mapped what he imagined to be the trace of this selfsame fault, finding that all along its never-before-noticed path there was a pattern of instantly recognizable breaks, shears, landslips, and a host of other peculiar and damaging phenomena.

All of this evidence convinced Matthes that this was not an ordinary fault line, but one that had been occasioned by two sets of rocks
sliding past each other
. To him it was a revelation. Back in 1906 most geologists assumed that faults operated only vertically, with huge forces throwing rocks either sharply upward or downward. But here, 200 miles north of San Francisco, there was evidence of the fault that had caused all the trouble—and it was a new kind of fault, a strike-slip fault as it came to be known, and one that was all too obviously an extension of the San Andreas. It was making itself dramatically felt up among the mountains, the clouds, and the mists of far northern California, farther away from the most gravely affected areas than most scientists of the time could ever have imagined.

This was the first time that geologists fully realized the extent of the fault's spread. It was the first time that the whole of California was seen to be playing host to so threatening and so massive a danger.

CLOSE TO SAN FRANCISCO
the fault, and the drama it causes when it moves, is very much more obvious than it is in the tortured coastline and hills of the state's far north. A town called Olema, forty miles north of the city (
ole
is “coyote” in the language of the coast Miwok Indians), is generally reckoned to be the place on the fault that moved most dramatically during the 1906 earthquake—and such is the local certainty of this that, quite wrongly, local residents like to
think of Olema as having been the event's epicenter. There is a small clothes shop that calls itself the Epicenter. In it and other local stores you can buy caps adorned with the words
OLEMA
and
EPICENTER
. I have one. I wear it. But what it implies is not true.

For a general truth should be pointed out here. An earthquake's
epicenter
is not necessarily the same as the place of maximum ground displacement. The epicenter is the point on the earth's surface directly above that point where the seismic energy of the quake begins to radiate outward, the earthquake's originating point. But the parts of the earth where displacement and damage occur depend on a number of details—most important of all the nature of the underpinning rock, the kind of soil, and the slope of the land. And in Olema—where the fault zone is half a mile wide, and filled with soft soils, crumbling limestones, rotting granites, and generally pulverized rocks, much of it lying on well-watered slopes—the sudden shock of a sideways-moving fault rupture did indeed shift the ground in a dramatic and dangerous way. But that does not mean the earthquake was centered there. It means merely that it had an impressive effect there—with few casualties and less publicity, however, since it was a place where few people lived.

Grove Karl Gilbert, the distinguished geologist who, it will be recalled, was roused by the quake as he lay in his bed in Berkeley, was among the first of the state commission members to go to investigate the situation in the Olema Valley. The devastation he found was most impressive. Buildings made of redwood and oak were crumbled as though fashioned from balsa chopsticks, huge water towers had been tossed around like cruets, long fissures spread across fields. The local dairy farmers told him fanciful stories, including one that was widely reported in the papers: A cow supposedly fell into one fissure, which swallowed her up as it closed shut behind her; she then suffered the postmortem indignity of having her tail eaten off by packs of marauding and hungry dogs.