by John Dvorak
A snapshot of the speed and the direction that various points in North America are moving can be determined quite precisely by use of the Global Positioning System, the familiar satellite-based system used to navigate cars or, with a handheld electronic device, to find the nearest Italian restaurant or specialty boutique. To determine precise movements of the Earth’s surface, highly sophisticated equipment is used and complex data-handling procedures are applied. The results are impressive because after a year or so of measurements, speeds as low as a few tenths of an inch per year can be determined for points separated by thousands of miles.
Such measurements show that the Pacific plate is moving to the northwest at a steady rate of 1.9 inches per year relative to the interior of North America. Two-thirds of the movement—1.4 inches per year—is occurring across the San Andreas Fault and its subsidiary faults, such as the San Gregorio-Hosgri, Hayward, and San Jacinto Faults. A quarter of the relative plate motion—0.4 inches per year—is occurring across the Walker Lane seismic zone, which is consistent with the lower seismic activity of the Walker Lane compared to the San Andreas system. But that still leaves a small amount of plate motion—0.1 inches per year—to be explained.
And GPS measurements have revealed this motion: It is occurring across yet another seismic zone—the Intermountain Seismic Belt—that bisects Utah from south to north, includes the Wasatch Fault east of the Great Salt Lake, which broke in at least five distinct major earthquakes between about 400 and 1600 a.d., and that may run as far north as Idaho and Montana. Major historical earthquakes have occurred along this seismic belt, most recently as a magnitude-7.3 earthquake near Hebgen Lake, Montana, in 1959 and a magnitude-6.9 earthquake near Borah Peak, Idaho, in 1983.
Furthermore, the broad regions between these various seismic areas—the triangular block comprising of the Great Valley of California and the mountainous Sierra Nevada that is bounded on the west by the San Andreas Fault system and on the east by the Walker Lane, and the elongated block comprising most of the state of Nevada and that is situated between the Walker Lane and the Intermountain Seismic Belt—are, essentially, aseismic and are moving as rigid blocks. And that, to sum it all up, is what is happening now to the western part of the United States today.
As described earlier in the chapter, for more than 100,000,000 years huge crustal blocks, such as the Sonomia and Smartville Blocks, to name two, collided against North America to form much of the western United States. That changed 25 million years ago when the earliest San Andreas–type fault formed. Since then, increasingly large amounts of the western United States are being sliced apart—the Los Angeles borderlands and basin, the Great Valley-Sierra Nevada, and central Nevada—and these are being slid to the northwest by the same internal force that drives the Pacific plate.
So a very long and very grand moment is being played out in the western United States. The San Andreas Fault is the primary element today in the boundary between the Pacific and North American plates, taking up almost all of the relative plate motion and accommodating it, mostly, by the occasional large earthquake. But it is not the only element. Others include the Walker Lane, where a new San Andreas–type fault might be in the making, and, farther east, the Intermountain Seismic Belt, which in the distant future will become the primary element in the plate boundary.
Whether this will come to pass is, of course, unknown. But if current trends continue for another 10 or 20 million years—and hundreds of thousands of major earthquakes occur—the Salinian block west and south of San Francisco, the Los Angeles borderlands and basin, and the peninsula of Baja California will be several hundred miles northwest of their current positions and separate from the North American continent. By then, the ever-growing Gulf of California might reach into Owens Valley, and Las Vegas could be near the Pacific shore. And maybe a continuous right-lateral, strike-slip San Andreas–type fault might run close to Lake Tahoe and through northern California and part of southern Oregon.
But there is an important element missing in this story: the opening of the Gulf of California that caused an eastward shift of seismic activity and moved the southern segment of the San Andreas Fault to its current location. The 1872 Owens Valley earthquake supports the idea that the opening of the Gulf of California is also responsible for the creation of the Walker Lane seismic zone. But what is happening in between? What is happening across the Mojave Desert?
For years, there was speculation as to what must be happening across the Mojave Desert to support the above story, but nothing was clear until June 1992, when the largest earthquake to strike southern California in 40 years hit. That earthquake started near the San Andreas Fault, then ruptured northward across 60 miles of the desolate Mojave Desert, opening a new chapter in understanding how the San Andreas is evolving.
Undoubtedly, the most important earthquake so far in California history was the 1906 San Francisco earthquake because of the devastation it caused to the city and because of the advancements that followed to the new science of seismology. Arguably, the second most important earthquake is one that did almost no damage and caused only one fatality—that of an unfortunate three-year-old boy who was asleep when a cinder block from a chimney crashed through his house. It is this second important earthquake that seismologists have focused on because it showed what seismologists had long expected—that earthquakes, even those separated by many years, interact.
On June 28, 1992, early morning gamblers in Las Vegas paused for the few seconds it took seismic waves to roll under them. In Hollywood, a man who was awakened by the early morning shaking said it felt, for a full 30 seconds, as if his house were a ship on a rough sea. Thirty miles north of Palm Springs in the town of Landers, where the earthquake originated, a Mrs. Iona Bong, 61 years old, said she started to fly off her bed when, fortunately, her husband grabbed her arm and brought her back to earth. The retired couple spent the next day sweeping up broken glass and smashed belongings and carting them to a huge garbage can. For the seismologists who would study the 1992 Landers earthquake, it was, as one seismologist pegged it, “a revolutionary event.”
First, the earthquake did not occur on one, but on six different faults. The rupture began on a short segment of a previously unknown fault located about 20 miles north of the San Andreas Fault and the northern end of the Salton Trough. From there, it propagated northward along the Johnson Valley Fault, then onto the Landers Fault. After a pause of seconds—which can be read on the seismographs recorded during the earthquake—the rupture resumed and continued to break northward along three other nearby faults in succession: the entire length of the Homestead Fault, the northern half of the Emerson Fault, and the southern third of the Camp Rock Fault. In all, the surface rupture extended for 60 miles across the Mojave Desert along a segment of a line that could be drawn from Lake Tahoe, down the axis of Owens Valley, and to the Salton Trough. Here was confirmation that the system of valleys, such as Owens Valley, which were contained within the Walker Lane seismic zone should be considered a northern extension of the same tectonic forces that were opening the Gulf of California.
The intermediate seismic zone that crosses the Mojave Desert has since been known as the Eastern California Shear Zone. Earthquakes in that zone cause the land to slip to the right, just like those of the Walker Lane and the San Andreas. In short, all three areas are involved in accommodating the motion between the Pacific and the North American plates, and so all three are part of the plate boundary.
Then there is the question as to why so many different faults were involved in a single event. Why did so many individual earthquakes happen on so many nearby faults?
Long before the 1992 Landers earthquake, seismologists had speculated that an earthquake on one fault might trigger earthquakes on other nearby faults. Here was ample proof: The six faults had broken in quick succession, the whole process—again, determined by a careful examination of seismic records—taking about 25 seconds. And that was
followed by the usual occurrence of aftershocks, a swarm of earthquakes smaller than the main shock that originate in the same area.
Then came a surprise.
Three hours and eight minutes after the Landers earthquake, another earthquake rocked southern California. It originated under the San Bernardino Mountains, 30 miles west of the town of Landers near Big Bear ski resort. It was outside the expected area of aftershocks, so seismologists were at first unsure how to classify it. Was it a distant aftershock? Was it an independent earthquake, and the two events had occurred within hours of each other by happenstance? Or had the Landers earthquake triggered, in some yet to be explained way, a significant earthquake beneath Big Bear?
Seven years later, another major earthquake struck southern California, and again it originated beneath the Mojave Desert. This one was in an even more remote location than the Landers earthquake; it was centered beneath an abandoned rock quarry, and it is now known as the Hector Mine earthquake, a magnitude-7.1 event that occurred on October 16, 1999.
Candy McCain of Phoenix, who was feeding coins into a slot machine in Las Vegas, felt the shaking. “I wasn’t sure exactly what was going on,” she told a news reporter, “but then I saw the signs swaying and the leaves on the fake palms rustling, so I thought it must be an earthquake. When it stopped, I started playing again.”
Michelle Fabian, who was in bed asleep on the 18th floor of the Mirage Hotel, was awakened by the shaking. “The whole place was shaking like crazy,” she said.
A westbound Amtrak train, the Southwest Chief, was traveling near the earthquake’s center when the shaking started. Fortunately, it was behind a freight train and was going slower than usual. But the combination of the shaking and the moving train caused several rails to come loose and the Southwest Chief derailed, injuring several people.
Though the damage was slight and the injuries minor, the Hector Mine earthquake is important because it is only the second time in California history—that is, during the last century and a half—that a major earthquake has occurred beneath the Mojave Desert. It and the Landers earthquake—the other major earthquake to occur beneath the desert—happened within just seven years of each other, an unlikely coincidence when one considers that paleoseismic work showed that neither the Landers nor the Hector Mine Fault had slipped in more than 1,000 years.
So something extraordinary happened in the Mojave Desert in the 1990s. Somehow, the 1992 Landers earthquake triggered the Hector Mine earthquake seven years later. It seemed the theory of elastic rebound that Reid proposed after the 1906 earthquake had to be modified and applied to multiple earthquakes. In short, it seemed that large earthquakes should not be considered single isolated events, but part of a sequence or cluster, a conclusion that Sieh and others had already been led to in their paleoseismological work.
As scientific coincidences go, a year after the Landers earthquake and a few years before the Hector Mine earthquake, an expert on the mechanics of earthquakes was in Greece for a scientific meeting. During that meeting, he happened to see something in the archaeological record that convinced him that, yes, indeed, major earthquakes could occur as quick sequences, one triggering the other. And he gave a name to the new phenomenon: He called it an “earthquake storm.”
*It is along the suture between the Smartville and the Sonomia Blocks that the Mother Lode, the stretch of major gold fields of California, is found.
*As mentioned in Chapter 8, the age of 25 million years comes from the oceanographic work by Atwater.
*To further add to the recognition of the San Jacinto Fault, there is a four-tiered freeway interchange at Colton—where I-215 and I-10 meet—that is built directly over the fault.
Chapter 12
Earthquake Storms
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 archaeoseismology, 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.
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?
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 evidenc
e 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.
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.
