"When you get a lot of earthquakes, you get a lot of earthquakes." — Charles Richter

It is often hard to see the obvious, unless you are someone who brings a new perspective to a problem. Stanford professor Amos Nur, an expert on earthquakes and in the more general field of how rocks fracture when subjected to high pressure, saw something at the ancient Greek city of Mycenae that countless others had simply overlooked.

It was 1993 and Nur was attending a conference on archaeo-seismology, a relatively new term to describe how archaeologists and seismologists were trying to join forces and benefit from each other’s work. But as Nur later recounted, the conference was a disappointment. The archaeologists and the seismologists seldom mixed, except during short breaks when both groups would indulge in drinking strong Greek coffee, and during an occasional day trip when they were taken to see one of the nearby ancient sites. It was during one such trip that Nur and the others visited Mycenae.

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Here a quick diversion explains an unexpected circumstance that links Mycenae to California and its geology. In 1851, Heinrich Schliemann, then a German businessman, made a trip to California to claim his dead brother’s estate. His brother had been one of the first to arrive in the gold fields, though instead of searching for gold, the brother made a quick fortune buying and selling claims. When Schliemann arrived, he too sensed an opportunity and opened a bank in Sacramento, where he traded in gold dust. The venture was short-lived; local agents were soon complaining that they were receiving short-weight consignments, and Schliemann, feigning illness, quickly left. He eventually made his way to the eastern Mediterranean, where he used his brother’s fortune—and whatever additional money he had accumulated while in California—to finance archaeological work at Mycenae and Troy and other soon-to-be-famous sites. More than a century later, Nur, standing at Mycenae, recognized a feature that Schliemann had unearthed that would change the way seismologists determine earthquake risk in California.

What Nur saw is along the entranceway to the ancient city and within sight of the famous Lion Gate, where a stone relief depicts two lionesses in upright heraldic positions. It was through this gate, so tradition says, that Agamemnon, a Mycenaean king and one of the main characters in Homer’s Iliad, marched his army and led them on a ten-year siege of Troy. Just outside the gate is an immense stone wall that sits atop a head-high steep incline of highly polished rock. To most people, the incline conveys a sense of rock-solid security. To Nur, it was evidence of a past calamity.

Nur recognized the rock incline as a fault scarp—a line along which a past earthquake had fractured and thrust the ground upward. That meant the Mycenaeans had built their city over an active fault. Fortunately for them, this particular fault has not moved in thousands of years, but earthquakes are common in the region. Nur realized that the ancient city of Mycenae, which was at its greatest influence during the Bronze Age, must have been subjected to repeated seismic shakings. But what effect might such subterranean activity have had on the city’s history? Might the sudden abandonment of Mycenae around 1200 B.C. have been caused by an earthquake?

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Archaeologists said no. Though many major cities in the eastern Mediterranean—including Thebes in Greece, Knossos on Crete, and Troy in western Turkey—were also destroyed around 1200 B.C., and though every major site—from Pylos on the Peloponnese Peninsula, to Aleppo in Syria, and Ashkelon in southern Israel—shows some damage consistent with seismic shaking happening around 1200 B.C., the widespread destruction that brought about the end of the Bronze Age—“the worst disaster in ancient history, even more calamitous than the collapse of the Western Roman Empire,” according to one noted classicist—was not instantaneous but had occurred over several decades.

For that reason, archaeologists argued that the end of the Bronze Age was probably caused by several factors, including invasions by foreign peoples and by internal political strife. But Nur proposed another idea.

Since a single catastrophic earthquake could not have been the cause, Nur suggested that several major earthquakes had struck a broad region of the eastern Mediterranean over a period of several decades. But was there any evidence that such a sequence of major earthquakes anywhere in the world had occurred in quick succession? Nur pored through catalogues of ancient earthquakes and discovered that, indeed, there was.

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One such period of increased seismic activity had started in A.D. 343, when an earthquake struck northeast Turkey. Then in A.D. 358, a second earthquake happened to the west. Others followed, also in northern Turkey, in 362 and 368. Then between 394 and 412 A.D., six earthquakes occurred near Constantinople, modern-day Istanbul. Also during the late fourth and early fifth centuries, major earthquakes shook the southern Italian peninsula, the island of Sicily, and Libya in northern Africa, as well as the Holy land and Cyprus. One of the largest events occurred in 365, when the southern shoreline of Crete was pushed up as much as 27 feet, comparable to the maximum amount of uplift recorded along the coast of Alaska in 1964—meaning the 365 earthquake had been a colossal event. In all, during the second half of the fourth century and the first few decades of the fifth century A.D., at least a dozen damaging earthquakes hit the central and eastern Mediterranean region. Nur’s examination of earthquake catalogues also showed that the centuries immediately before and after were periods of relative seismic quiet.

But was there a more recent—a more obvious—example of a series of major earthquakes that had detailed information about the location and size of individual events? Yes, there was.

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In 1939, after two centuries of quiescence, the North Anatolian Fault, which runs across northern Turkey roughly parallel to the coastline of the Black Sea and which is a boundary between the African and Eurasian plates, came to life. By 1999, 13 major earthquakes had occurred. What is even more remarkable, 7 of the 13 ruptured the North Anatolian Fault in a systematic way: Each successive earthquake ruptured a segment of the fault that was immediately west of the previous earthquake.

The sequence began in northeast Turkey—as it had in A.D. 343—near the city of Erzincan, where, on December 26, 1939, the shaking was so severe and the damage so great that the old part of Erzincan was abandoned and a new city center was soon built to the north. Then three years later, in 1942, the next earthquake happened, immediately west of Erzincan, and a year later yet another earthquake west of the 1942 event. In all, the sequence of seven west-migrating earthquakes ruptured a 600-mile-long continuous segment of the North Anatolian Fault.

Each of these prolonged releases of seismic energy—in the fourth and fifth centuries A.D. and again in the 20th century— lasted several decades and occurred after a millennium of relative seismic quiescence. This supported Nur’s suggestion that a similar series of major earthquakes could have occurred over several decades around 1200 B.C., nearly a millennium before the recorded quakes of the fourth and fifth centuries. It was a new phenomenon, one that Nur called “an earthquake storm.”

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But why would seismic energy be released as a series of large earthquakes lasting for decades?

Nur had an answer: stress transfer.

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I once spent an intriguing afternoon watching an artisan prepare colored glass panes for a stained-glass window. The secret, he revealed, was not to cut completely through a thick pane, which could shatter the glass into shards, but to etch each one with a cutting tool that left the geometric curve he wanted the edge of a pane to have. Then, by the appropriate application of heat and cold, by twisting the pane ever so slightly, and by relying on the weakness of an etched curve, he could induce a glass pane to break as a series of arcuate cracks and produce a pane of any desired shape.

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The purpose of the application of heat and cold and of the twisting was to induce a specific pattern of concentrated stress that enabled the artisan to control where the pane was mostly likely to break.

It is an art that few people have ever mastered. If one substituted the induced thermal and twisting stresses for the buildup of stress in the earth’s crust by the movement of tectonic plates, and substituted the sequence of cracks produced in the glass panes for earthquakes, then one can understand how an earthquake storm could be produced.

Think of it in another way. Imagine that a giant zipper is holding together two tectonic plates. As the two plates tug against each other, a segment of the zipper sudden slides open, but, as a zipper is apt to do, it snags occasionally. As the tugging continues, the zipper again slides, then snags again. Each time, the sliding zipper represents an earthquake and the tugging of the plates becomes concentrated at another place along the zipper.

Or consider another example—one that Nur prefers. Take a wide rubber band and cut a few short slits in it. As the band is stretched, each slit in turn opens up and the ends of the slits lengthen. The sequence that the slits open and by how much depends on how the stress pattern gets transferred and concentrated at new locations across the rubber band.

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If this seems complicated, rest assured it can all be explained mathematically by applying what is known as the Coulomb-Navier failure criterion, a well-established physical law widely used by engineers to design buildings, bridges, and other monumental structures. The Coulomb-Navier failure criterion tells how much an object—or the Earth’s crust—can be pushed or pulled, twisted or sheared, before it breaks. And the criterion has probably never been more thankfully applied—at least in a geologic application—than during the recent earthquake storm along the North Anatolian Fault in northern Turkey.

In 1997, using the Coulomb-Navier failure criterion, a forecast was made, based on the sequence of recent earthquake ruptures, that there was a 12% chance that a magnitude-7 or larger earthquake would strike near the city of Izmit, 40 miles east of Istanbul, during the next 30 years. Two years later, a magnitude-7.6 earthquake did devastate Izmit, killing more than 25,000 people and causing $65 billion in damages. Within months after that earthquake, another forecast was made, this time for the area around Düzce, 60 miles east of Izmit. Some school buildings, thought to be in danger of collapse by seismic shaking, were closed. Then on November 17, 1999, another earthquake hit, flattening school buildings.

Such success gives credibility to earthquake forecasting, or rather to the idea that the probability of a future earthquake—identifying the magnitude and a time period—can be given based on stress transfer. It also prompted a search for other examples of earthquake storms throughout the Earth’s history.

An earthquake storm probably ran up and down the Italian peninsula during the late 17th and throughout most of the 18th centuries. It began with two damaging earthquakes that originated beneath the Apennine Mountains east of Naples in 1694 and 1702. The activity then migrated north to a region east of Rome with three major earthquakes in early 1703. By the second half of the 18th century, activity had returned to southern Italy, where five major shakings occurred along the toe of the Italian boot, in Reggio Calabria.

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A more recent storm occurred in eastern Mongolia between 1905 and 1957, when four magnitude-8 events struck. And an earthquake storm is happening now along the Xianshuihe and adjacent faults along the northern edge of the Tibetan plateau in southwest China, where 11 major earthquakes have happened in the last 120 years.

After decades of almost no seismic activity, the Xianshuihe Fault became active in 1893 when an earthquake rocked the Tibetan district of Kada, destroying the Dalai Lama’s Grand Monastery of Hueiyuan and seven smaller monasteries. In all, 74 Buddhist priests and 137 Chinese and Tibetan soldiers were killed. Since then, ten more strong shakings have occurred, including a magnitude-8.0 shock on the nearby Longmenshan Fault in 2008. The most recent event occurred on April 14, 2010, in Qinghai Province when many Buddhists were killed when a 12th-century monastery collapsed.

This, of course, raises the question: Has an earthquake storm ever occurred in California? Here we are hampered by a historical record that spans barely 200 years. But using the techniques of paleoseismology, evidence has been revealed that two storms may have occurred in a place that few people associate with devastating earthquakes—Hollywood.

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Though it is one of the most densely populated regions of California, Hollywood offers an unusual opportunity to recognize and walk along an active fault. The area was urbanized in the 1920s, before the widespread use of mechanized earth-moving equipment, so much of the original topography is still intact, even subtle features such as alignments of low hills and shallow troughs that record the trace of recent earthquakes. In essence, the network of winding streets and the placement at odd angles of apartment buildings and commercial enterprises, as well as the occasional abrupt slope across one of the sprawling lawns in Hollywood and nearby Beverly Hills, are subtle evidence of an original jumbled ground surface. And by finding the appropriate steep incline, one can follow the Hollywood Fault.

Begin at the corner of Hollywood and Vine and look north along Vine Street beyond the 13-storied cylindrical tower that houses Capitol Records. Just beyond Capitol Records, just before Vine Street reaches the Hollywood Freeway, the roadway ramps up a steep hill. The hill is there because the ground was pushed up by repeated earthquakes. Along the base of the hill is the Hollywood Fault.

From that point, the fault can be followed west along the base of the same hill, running parallel to and maintaining a distance of a few blocks north of Hollywood Boulevard. It runs along the base of the low hill where the Magic Castle, a private club of magicians and the home of the Academy of Magical Arts, is located. Farther west, the fault runs directly beneath the house where the Nelson family lived and where the opening scene of their famous 1960s sitcom—the series was called "The Adventures of Ozzie and Harriet"—was filmed.

Continuing west, close to where Hollywood Boulevard ends, the fault angles to the southwest and crosses just south of the busy intersection of Sunset and La Cienega Boulevards. This is a neighborhood of fine restaurants and fashionable boutiques. On the north side of Sunset Boulevard one can find, after considerable searching through the urban construction, an occasional outcrop of hard granite. This is the rock that comprises the Santa Monica Mountains to the north; high up the mountainside is the famous hollywood sign. South of Sunset Boulevard, there are no rocky outcrops; instead, one stands on a deep layer, several hundred feet thick, of loose sediments that washed out of the canyons of the Santa Monica Mountains and that fill the Hollywood Basin. It is this discontinuity—granite outcrops north of Sunset Boulevard and deep sedimentary fill to the south—that, here, defines the Hollywood Fault.

The western end of the fault lies somewhere near the grounds of the Beverly Hills Hotel, just north of the intersection of Sunset Boulevard and Rodeo Drive. From there, if one walks a mile or so south along Rodeo Drive to Santa Monica Boulevard, then turns right and continues to Wilshire Boulevard, one will now be standing at the eastern end of another fault—the Santa Monica Fault—which continues to the ocean’s edge and beyond.

Now return to where the Hollywood Fault crosses under Vine Street and head east. From here, the fault runs close to Franklin Avenue, then along Los Feliz Boulevard. At the east end of the Santa Monica Mountains—that is, at the southeast corner of Griffith Park, home of the Los Angeles Zoo and Griffith Observatory—the fault disappears under the floodplain of the Los Angeles River. What lies on the other side?

There is another fault—the Raymond Fault—which is, perhaps, a continuation of the Hollywood Fault and which runs eastward through southern Glendale and across the San Gabriel Valley, through South Pasadena to the foot of the San Gabriel Mountains. Kinks in the Raymond fault are responsible for the low hill where the luxurious Langham Hotel—formerly Ritz-Carlton—is perched and for the shallow depression that is Lacy Park. The Raymond Fault is also responsible for the low hills on the north side of the Santa Anita Racetrack, visible from the grandstand.

What do the Santa Monica, Hollywood, and Raymond Faults have in common? Besides lying along what seems to be a continuous line, all three ruptured about 10,000 years ago and again about 1,000 years ago.

Unfortunately, paleoseismologists have not yet determined whether the earthquakes along the Santa Monica, Hollywood, and Raymond Faults occurred as a single colossal event or as a series of relatively quick earthquakes, happening over years to centuries, the latter being an earthquake storm. (Unfortunately, the techniques used in paleoseismology are not yet sufficiently refined to distinguish, in this case, between years and centuries.) But there is a curious coincidence: All three faults did rupture at about the same time; then, after a period of several thousand years, all three ruptured again, lending further credence to Richter’s statement: “When you get a lot of earthquakes, you get a lot of earthquakes.”

Moreover, other nearby faults have a similar history.

James Dolan at the University of Southern California has dug trenches and sunk holes large enough for him to climb down to examine the Hollywood Fault. He has also dug trenches and sunk holes into the Puente Hills Fault that runs southeast from Griffith Park, the Whittier Fault that runs east of downtown Los Angeles, and the Newport-Inglewood Fault that runs south from close to the Beverly Hills Hotel to the city of Long Beach and may merge with the Rose Canyon Fault that continues to San Diego. At all of these faults, Dolan has determined a similar rupture history: major earthquakes along each one about 10,000 years ago and again about 1,000 years ago. And in each case, the earthquakes that ruptured these faults were much larger than the most recent damaging earthquake to strike the Los Angeles area, the 1994 Northridge earthquake, which killed 60 people, injured more than 7,000, and caused $44 billion in damages. In short, according to Dolan, there have been two “bursts” of seismic activity in Los Angeles and the immediate surroundings in the last 10,000 years.

Fortunately, the time interval between such “bursts,” or earthquake storms, in this particular region of California is thousands of years, so it is highly unlikely that one will occur in the near future; thus this region is in a “seismic lull.” But that is not true elsewhere in California.

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In 2008, a report was issued by the Working Group on California Earthquake Probabilities—a group that then consisted of about 50 geologists, geodesists, and seismologists—that said it was “virtually assured” that California would be struck by a magnitude-6.7 or larger earthquake during the next 30 years. Such a claim is not profound when one considers that a dozen such events occurred somewhere in California during the previous 100 years. What was profound was that the group was able to identify which faults were the most likely to rupture. The 2008 report has now been updated to indicate how severe the ground shaking might be and the chance of multiple earthquakes.

In northern California, the most probable destructive seismic event is along the Hayward Fault, which runs along the east side of San Francisco Bay. The previous event, in 1868, occurred when only 24,000 people lived near the fault. Today more than 1,000,000 people live within five miles of the fault trace. Hundreds of homes and other structures are built close to the fault, and freeways, water lines, and power lines cross it at several points.

The 1868 event was a moderate earthquake, based on the extent of the damage, probably a magnitude-6.8 earthquake. According to the Working Group, which considered, among other things, how fast stress is accumulating along the Hayward Fault, there is a 31% chance of a repeat of the 1868 earthquake or a larger event in the next 30 years. A larger event would probably involve rupture along the Rodgers Creek and possibly the Maacama Faults to the north, or rupture along the Green Valley-Concord and the Greenville Faults to the east, or the Calaveras Fault to the south. The group also noted that the city of San Francisco is almost equal distance from the Hayward and San Andreas Faults, so a significant earthquake along the Hayward Fault could produce shaking in San Francisco as severe as in 1906.

Elsewhere in northern California, a major earthquake along the subduction zone between Cape Mendocino and Vancouver Island—a region known to geologists and seismologists as Cascadia and which the Working Group gave a 10% chance of rupturing in the next 30 years—will almost certainly be followed within decades, perhaps even within hours, by a major earthquake along the northern segment of the San Andreas. Such an earthquake-pair sequence—rupture of the Cascadia subduction zone followed by rupture of the northern San Andreas Fault—has happened 14 times in the last 3,000 years.

(Contrary to popular opinion, when a major earthquake happens, the chance of another major event happening soon after does not decrease, but increases dramatically. For example, for any three-day period, the chance of a major earthquake occurring somewhere in California is 1 in 100,000; however, if a major earthquake has just happened, then the chance of another earthquake of equal or greater magnitude striking in the next three days is 1 in 10, a sobering statistic and one that needs to be known by anyone involved in rescue operations after a major seismic event.)

Before considering where the seismic risk is highest in the densely populated regions of California, it is important to note that there is a significant risk of one or more major earthquakes along the Walker Lane Seismic Zone in the eastern part of the state. In particular, the Working Group identified the Carson Range, Mammoth Lakes, Owens Valley, and Death Valley as places where one or more major earthquakes might occur—giving a probability of 4% in the next 30 years—potentially causing damage in the greater Reno or greater Las Vegas areas.

In southern California, the San Jacinto Fault, which runs from Cajon Pass through Riverside and continues to the southeast, was identified by the group as a “tectonic time bomb.” In fact, according to the 2008 report, the probability that this fault will rupture in the next 30 years is the same as for the Hayward Fault—31%. In either case, the result will cause extensive damage and disrupt millions of lives.

But neither the Hayward nor the San Jacinto Fault represents the greatest seismic risk in the state. That distinction, so say the experts in the Working Group—who have evaluated the geologic and paleoseismic work, examined the current levels of seismicity, have conducted extensive geodetic surveys checking to see how fast the North American and Pacific plates are moving today, and have done calculations using Coulomb-Navier stress equations—belongs to another feature, one that, according to the same experts, is “the most dangerous fault” in California.

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The Palm Springs Aerial Tramway, the steepest cable ride in the United States, carries passengers up 6,000 feet from the sweltering heat of Palm Springs to the refreshingly cool and at times snow-covered hillsides close to the summit of Mount San Jacinto. The 12-minute ride gives one ample opportunity to study the mountain face and take note of the rapid change in flora from a desert floor to an alpine peak. One is also given the opportunity to look in the other direction—the entire floor of the tramcar rotates, making two revolutions during the ascent—so one is treated to an increasingly expansive view of the rugged Sonoran Desert. In the distance, from the top of the tramway one can see Mount Charleston 200 miles away near Las Vegas. To the south is the Salton Sea. Immediately in front, seemingly at one’s feet, is the broad Coachella Valley, the northern extension of the Salton Trough. And running close to the axis of Coachella Valley is the San Andreas Fault.

The fault is easy to see. To the northeast, on the valley floor, is a dark green patch, the community of Desert Hot Springs. This and many other oases in Coachella Valley exist because impermeable fault gouge along the San Andreas Fault has forced groundwater to rise close to the surface.

South of Desert Hot Springs is a 20-mile-long ridge, Indio Hills. Here the San Andreas Fault consists of two strands, running on either side of the ridge. Indio Hills exists because earthquakes along both strands have pushed up the ridge.

Near the southern end of Indio Hills, the two strands merge at another oasis, Biskra Palms. From there, the San Andreas Fault is a single strand, its trace easily identified by following the eastern edge of the dark patch of irrigated fields that surround the farming communities of Indio, Coachella, Thermal, and Mecca. South of Mecca, the fault continues in a straight line to its southern end at Bombay Beach on the east side of the Salton Sea.

In all, nearly 100 miles of the San Andreas Fault can be seen from the top of the Palm Springs Aerial Tramway. It is this, the southernmost segment of the fault, that worries seismologists because this is the only segment that has not ruptured in historical time. And there are additional reasons to be concerned.

Four deep trenches have been dug across this segment, and from a detailed examination of the walls, paleoseismologists have identified five, possibly seven, major ruptures that have occurred during the last 1,100 years, the most recent in 1690.

Moreover, this is one of the most seismically active segments of the San Andreas Fault. More than a dozen moderate earthquakes have occurred since 1935. Such persistent activity could presage a major event—in the same way that a wooden board broken over a knee begins to crack before it breaks.

(More than a dozen moderate earthquakes occurred in the San Francisco area during the 70 years before the 1906 earthquake, but only one during the 70 years after the event.)

To add to the concern, geodetic measurements across this segment of the fault show that points on opposites sides of the fault are sliding slowly and continuously—those on the west side of the fault moving to the north and those on the east side to the south—at an average rate of 1.5 inches a year. This means that, since the last major earthquake in 1690, 27 feet of crustal movement has accumulated on opposites sides of the fault—without any movement yet along this part of the fault. That is a buildup of an enormous amount of seismic energy that has yet to be released.

All in all, the paleoseismic evidence of five or seven major earthquakes in the last 1,100 years—showing that large events are not unusual—and the current high level of seismicity and the steady buildup of seismic energy along the fault point to one thing: A major earthquake will occur soon along the southernmost segment of the San Andreas Fault.

Thomas Jordan, director of the Southern California Earthquake Center at the University of Southern California located in Los Angeles, and a member of the group that produced the 2008 report and an update, has put it bluntly: This segment of “the San Andreas Fault is locked and loaded and ready to rumble.”

But when?

Jordan and others say there is a 59% chance that a magnitude-6.7 or larger earthquake will occur along the Desert Hot Springs–Salton Sea segment of the San Andreas Fault in the next 30 years. That is almost twice the probability for a comparable earthquake happening on the nearby San Jacinto Fault or on the Hayward Fault.

Jordan and others have also considered what they have termed a “doomsday” scenario in which the entire southern half of the San Andreas Fault—from the Salton Sea to Parkfield—ruptures as a single earthquake. Parkfield is considered the northern limit because the rupture of a single event will probably not be able to propagate farther north where the stress on the fault is being relieved constantly by fault creep—evident by the slow pulling apart of the walls of the DeRose Winery—and by the frequent occurrence of magnitude-6 earthquakes, four in the last 100 years with the most recent in 2004.

Such a “wall-to-wall” rupture would involve 350 miles of the fault, considerably more than the 270 miles that ruptured in 1906, and the earthquake would be proportionally much bigger. Jordan and others estimate that such a cataclysmic event would correspond to a magnitude-8.2 earthquake and release about ten times more energy than the one in 1906.

Fortunately, the probability of such an event is low: less than 1% during the next 30 years. But considering that shaking would last more than a minute and be severe both close to the fault and in communities built over sedimentary basins—which would include but not be limited to San Bernardino, Los Angeles, and many towns in Ventura County, where, according to Jordan, the ground will shake “like a bowl of jelly”—there is still reason for concern.

Because small earthquakes are more common than large ones, a more likely scenario is that only the southernmost segment of the San Andreas Fault will rupture, at least at first, relieving some of the built-up seismic energy but not all. To understand what may follow, it is important to return to the North Anatolian and the Xiashuihe Faults and compare them to the San Andreas Fault.

All three are transform faults along plate boundaries. In all three cases, the relative plate motions on either side of the faults are the same, about 1.5 inches a year, so stress is increasing along the faults in all three places at the same rate. All three have significant strands that split off a main strand—in California, the Hayward and the San Jacinto Faults; in China, the Longmenshan; and in Turkey, north and south strands that run west of the city of Düzce and that are responsible for the creation of the sea lane known as the Dardanelles. There is also a segment of the North Anatolian Fault that creeps, just as the San Andreas Fault does north of Parkfield. Whether the Xiashuihe Fault also has a creeping segment is not known; that fault has not been studied as intensely as the other two. And two of these faults have had earthquake storms. By analogy, it seems a third earthquake storm along the San Andreas is possible.

How would such a storm evolve?

Again, from studies of the North Anatolian and the Xiashuihe Faults, it would probably begin at one end, perhaps by a rupture of the southernmost segment of the San Andreas, and proceed along the main strand and some adjacent faults.

The initial rupture would change the stress pattern—just as the 1992 Landers earthquake did and which led to the 1999 Hector Mine earthquake—so that there would be new places where stress was now concentrated. Eventually, because so much stress would be released in southern California, stress concentrations would form in northern California, jumping the 100-mile-long creeping section north of Parkfield. So the northern segment of the San Andreas Fault would also be involved.

Here a new concern arises. As the stress pattern along the San Andreas Fault changes with each successive earthquake, so does the stress pattern change along adjacent faults, causing some of them to rupture out of their previous “pattern.” For example, in 1939, the earthquake that leveled the city of Erzincan broke along the main strand of the North Anatolian Fault, as well as the nearby Sungurlu-Ezinepazari fault. In China in 2008, after decades of earthquakes along the Xiashuihe Fault, a rupture occurred along a nearby parallel fault, the Longmenshan Fault. The same would happen in California.

In particular, in southern California, the Cucamonga Fault, which runs west from Cajon Pass and along the southern base of the San Gabriel Mountains, could rupture simultaneously with or soon after a major earthquake along the San Andreas Fault. And that would lead to stress changes along the Raymond Fault—which is at the western end of the Cucamonga Fault—and from that to other faults in the Los Angeles region.

In northern California, the Calaveras Fault splits off the main strand of the San Andreas just south of San Juan Bautista. So a rupture of the northern San Andreas Fault could lead to a rupture of the Calaveras—or the Hayward or the Greenville or the San Gregorio Fault.

All this is to emphasize an important point: The exact sequence of future ruptures, and hence major earthquakes, along the San Andreas and its many adjacent faults cannot be predicted—which is why Jordan and others issued probabilities in their reports. The series of quakes would not disseminate out in a necessarily coherent direction.

But one thing is certain: The last 100 years in California—which happen to correspond to a period of rapid urban growth—have been a period of seismic calm. That cannot continue.

The damaging earthquakes that have occurred—1925 Santa Barbara, 1933 Long Beach, 1952 Long Beach, 1952 south of Bakersfield, 1971 San Fernando, 1989 Loma Prieta, 1992 Landers, and 1994 Northridge—were moderate events, seismically speaking; they released only a minuscule amount of the stress that has built up between the North American and Pacific plates. This enormous amount of stress and, thus, seismic energy can only be relieved one way: as a series of large earthquakes.

And that could occur as an earthquake storm.

Excerpted from “Earthquake Storms: An Unauthorized Biography of the San Andreas Fault” by John Dvorak. Copyright © 2014 by John Dvorak. Reprinted by arrangement with Pegasus Books. All rights reserved.