by John Dvorak
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.”
But why would seismic energy be released as a series of large earthquakes lasting for decades?
Nur had an answer: stress transfer.
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.
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.
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.
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.
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. Farthe
r 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.*
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.*
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 l
evels 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.
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.
