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Simulation of the speed of ground movement during a magnitude 7.8 earthquake on the southern San Andreas Fault, represented here by a dotted line. Developed by the Southern California Earthquake Center -
A composite image of the San Andreas Fault — seen here as a line to the right of the Temblor Range near Bakersfield, California — generated from data from the Shuttle Radar Topography Mission and Landsat satellite.
On April 18, 1906, the world awoke abruptly to the existence of the San Andreas Fault. A 7.8 earthquake ripped open the fault’s northern segment just south of San Francisco, shaking many of the city’s buildings to rubble and sparking fires that incinerated the rest. At least 1,000 people lost their lives; the city sustained nearly a half-billion dollars in property damage. It was one of the worst catastrophes in U.S. history; it still ranks among the top 10 deadliest natural disasters to strike on U.S. soil.
Some modern seismologists admit to morbidly wishing for earthquakes, if only to have a chance to observe the forces they have spent their lives studying. A hundred years ago, geologists were no different. The 1906 temblor split a crack in the earth nearly 300 miles long; the earth on either side had slipped north and south, opening up 24-foot gaps in fences at places where they crossed the fault. “It afforded,” as geologist Andrew Lawson wrote at the time, “an exceptional opportunity for adding to our knowledge of earthquakes.”
Without fancy instruments, radiocarbon dating, or even knowledge of plate tectonics, Lawson, who chaired the Department of Geology at the University of California at Berkeley, dispatched teams of geologists and students to study the force that laid waste to San Francisco. They followed scarps, valleys and tumbled-down rocks from Humboldt County, across the Bay to Mussel Rock south of the city and through Southern California’s mountains, all the way to the Coachella Valley, southeast of Los Angeles. By the autumn of 1908, Lawson’s team had constructed a near-perfect rendering of the San Andreas Fault, but for one segment: While Lawson’s chief geologist on the scene, Harold Fairbanks, speculated that the fault continued down to the Salton Sea, he could not follow an accurate line to confirm it. The far southeastern segment remained a mystery.
A century after Lawson completed his report, geologist Ken Hudnut sits in his office at the U.S. Geological Survey and uses Google Earth to examine some of the same features that Lawson’s crew explored on foot and horseback. A tall, sandy-haired 47-year-old with a teacher’s straightforward demeanor, Hudnut heads up what’s known as the Southern San Andreas Fault Evaluation, or SoSAFE. The project aims to log the past 2,000 years of earthquakes along the fault’s southern stretch to get a better sense of what the fault segment might do in the future, and when it might be active. The last major earthquake on the southern segment happened in 1857; geologists’ best guess is that it was centered at a place called Fort Tejon, just north of Los Angeles. And if geology follows any pattern at all, it appears that this segment of the fault is due to rip again soon — right about now, in fact.
“We didn’t appreciate that until recently,” Hudnut says. “For the longest time, we thought that 1857 section of the fault had a recurrence interval of anywhere from 150 to more like 300 years.” Now scientists think that window could be as narrow as 100 years. They’re also beginning to suspect that, over time, the fault has been more prone to massive rips than moderate ones. “Historically, we’ve seen that it can produce a 7.8,” Hudnut says. “But we also have to wonder whether it can break in even bigger earthquakes.” On the logarithmic moment magnitude scale — a refinement of the mathematical formula Charles Richter devised to compare earthquakes in 1935 — a magnitude 8 earthquake releases 32 times the energy of a 7, and is 1,000 times more energetic than a 6. Hudnut thinks the southern San Andreas may have enough energy stored up to produce an 8. “We have no reason to doubt it, but we can’t prove it — yet.”
California has some of the strictest building codes in the world. Since the Long Beach earthquake in 1933, the state has passed rigorous standards for construction, particularly for schools. When a 7.3 quake hit Landers, Calif., in 1992, an elementary school one-half mile from the epicenter held up so well that it served throughout the day as the neighborhood emergency shelter. But even if most buildings stay standing, a 7.8 on the southern San Andreas could still devastate the cities of Southern California, possibly rendering them uninhabitable for weeks, even months, as Hurricane Katrina did New Orleans. The reason: Many of the supply lines and transportation arteries that feed into Los Angeles come through the San Bernardino Mountains, right along the fault. An earthquake of that magnitude, at that location, could shred them all.
SoSAFE is part of the USGS-run Multi-Hazards Demonstration Project, an effort to bring geologists, economists, public health officials and politicians together to figure out how to prevent natural disasters like earthquakes from evolving into catastrophes — and determine whether it’s worth spending billions preparing for something that may never happen. On Nov. 13, another offshoot of the Multi-Hazards project, a consortium of schools, scientists and government agencies called “the Earthquake Country Alliance,” will stage its first public relations event: the Great Southern California ShakeOut. This region-wide drill will simulate a 7.8 earthquake on the southern San Andreas — using a possible location, and potential magnitude, of the long-awaited Big One.
At 10 a.m. that Thursday morning, teachers will order schoolchildren to duck, cover and hold on under their desks, pretending that books and other objects are flying across the room. Business owners will put out their cigarettes and practice turning off the gas. People at home will fill bathtubs with the water that, in an actual earthquake, might provide the region’s only drinkable supplies. Utility companies will hoist temporary towers to replace imaginary downed transmission lines; emergency workers will practice responses to calls for help from trapped people who don’t exist.
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It remains to be seen whether any of it will matter when the first jolt of a real seismic wave knocks Southern Californians off their feet; nothing gets the public’s attention like an earthquake. In that way, nature handed the ShakeOut planners a gift: At 11:42 a.m. on July 29, 2008, a previously undetected fault sprang loose perilously close to downtown Los Angeles, producing a 5.4 magnitude earthquake that knocked cans off grocery-store shelves and set wood-frame bungalows swaying like boats. It was the largest quake in the Los Angeles Basin since 1994, when a 6.7 on the Northridge fault claimed 60 lives, collapsed two freeways and caused $40 billion in damage. Suddenly, Lucy Jones, the USGS chief scientist who became famous when she helped calm Southern Californians during a spate of earthquakes in the ’90s, was back on the radio. “Some people have forgotten they live in earthquake country,” observed Jones, the architect of the Multi-Hazards Demonstration Project. A drill may be the only way to remind them.
For the purposes of the ShakeOut, Hudnut and his colleagues mapped the hypothetical earthquake on the segment of the San Andreas that stretches from the Salton Sea to Lake Hughes north of Los Angeles. No part of this segment has erupted since 1857; some stretch of it slipped in 1812, but nobody knows how much or exactly where. The San Andreas’ far southeastern extreme, the segment Fairbanks couldn’t follow, has been quiet since about 1680.
Geologists still don’t know what to make of that stillness. Faults come in different flavors and run on different schedules; some “thrust” faults only push out of the ground every 1,000 years; some strike-slip faults, where the ground moves laterally on either side of the fault, shift a little bit every 20 years. By digging deep into the ground and looking for signs of past quakes, geologists have recently uncovered a possible pattern in the southeastern strand of the San Andreas. From 800 to 1680, it broke on an average of every 150 years. So what does it mean that it hasn’t moved for the last three centuries?
One of SoSAFE’s goals is to find out. “A geologist once described it as being 10 months pregnant,” Jones says. “But I think it’s more like being 20 months pregnant. At which point you know you’re not dealing with a normal pregnancy.”
Sixty years after Lawson completed his 448-page report on the San Andreas Fault, international networks of seismometers, installed to register underground nuclear tests, unexpectedly revealed the contours of the long seams of tectonic plates that form the earth’s crust. The plates form continents and thrust underneath seas; their natural instability roils up in volcanic ridges and triggers tsunamis. Where they slide up next to each other, the ground springs with thousands of cracks.
The San Andreas Fault marks the slippery boundary between the Pacific and North American plates. Where the fault is relatively straight, it simply creeps: the plates inch in opposite directions a little at a time, and no one can feel what’s happening. Where the edges of the plates form more jagged boundaries, the fault gets stuck, bends and builds up strain. That rock eventually snaps back into place, sending waves of seismic energy reverberating through the earth.
Some segments of the San Andreas build up strain for a century or two before exploding in magnitude sevens. Other segments, such as the geologically famous stretch that runs through the town of Parkfield in Central California, bend and spring back in fives or sixes every 20 odd years. Some have so far eluded geologist’s understanding.
“California only has written records dating back to 1769,” says Glenn Biasi, a research seismologist at the University of Nevada who works on a team of scientists assembling a chronology of quakes on the southern San Andreas. “So we have to go through layers (of earth) to get estimates on the dates of older earthquakes. Then we try to figure out how regular they are.” In other words, to compile a historical record of earthquakes, geologists have to dig.
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Kerry Sieh was a 26-year-old geologist in 1976 when he struck out with his wife, his brother, a handful of graduate students and a couple of shovels for Pallett Creek, a marshy patch in the desert on the southern San Andreas in northern Los Angeles County. They started digging a trench by hand. By noting where the ground had shifted and dating the decomposed vegetation around those shifts, Sieh began to construct a “rupture chronology” of the fault. In time, he pulled together enough funds to rent a backhoe and dig down through two millennia of geological time. The “several meters of strata freshly exposed in the cut before me had seen fully 2,000 winters and summers and autumns and springs,” he wrote in 1980, “the coming and going of the Little Ice Age, generations of forest and chaparral, animals and Indians, and even several great earthquakes.”
Sieh is now known as the father of paleoseismology — the fine art of determining earthquake patterns by logging data revealed in trenches. When he started out, he was able to nail down earthquake dates only to within 30 years. Today, Sieh and other geologists can be more precise. Kate Scharer, an assistant professor at Appalachian State University who works with Biasi, goes into the trenches with a microscope and tweezers to extract just one species of grass from the peat. She then dates it and compares it to another substance in the same layer — maybe a piece of charcoal that burned 50 years before it ended up in the soil. “We’ve been able to get the error ranges on those sites down to plus or minus five years,” Hudnut says. “Now we’re chipping away at it, trying to get it down even tighter than that.”
In 2005, Scharer and Biasi, along with USGS geologist Tom Fumal and Ray Weldon from the University of Oregon (“our visionary,” says Biasi), took the published data on 56 prehistoric earthquakes, derived from 12 sites on the southern 400 miles of the San Andreas Fault, and plotted them in three different scenarios. Their graph, with its brightly colored lines stretching across segments of the fault over time, reveals that earthquakes may have been happening on the southern San Andreas as frequently as once every 100 years. At any rate, no lull in the past 1,600 years has gone on longer than 200 years. It defies logic to think that the current calm will last much longer.
What will happen to Southern California when a 7.8 earthquake breaks open on the San Andreas Fault, exactly where Hudnut calculates that it might? Unstable desert sand will turn into something like barely set gelatin and many of the houses built on it will collapse, as will any remaining unreinforced masonry buildings (known as “URMs” — or, as one seismic engineer calls them, “FPRs”: Future Piles of Rubble). Hillsides throughout the San Gabriel Mountains and on Cajon Pass will collapse in cascades of rock and debris; dams will splinter. Seventy seconds into the rupture, seismic waves will reach deep into downtown Los Angeles, where 55 seconds of sustained shaking, measured from the moment the waves hit, could bring down older “steel-moment” high rise buildings. (By contrast, a 6.7 earthquake that hit Kobe, Japan, in 1995 — one year to the day after Northridge — shook for seven to 10 seconds. One third of the city’s 600-some steel-moment frame buildings came down.)
Fire will likely follow the destruction: The Los Angeles Department of Water and Power anticipates 1,600 ignitions in the urban wildlands interface, hundreds of which will merge into larger blazes before they can be contained. Throw a dry, hot Santa Ana wind into that scenario and Los Angeles, like San Francisco in 1906, may well burn to cinders.
Closer to the fault, it will be mayhem. In 1857, Fort Tejon, the estimated epicenter of that year’s quake, was an outpost inhabited by a handful of military personnel, Native Americans and 28 camels. The 7.8 temblor temporarily changed the courses of several rivers, tossed fish out of nearby lakes and left a surface rupture more than 300 miles long. But only two people were killed, one of them crushed when her adobe house collapsed. Nowadays, however, the Los Angeles exurban area stretches north beyond that fault segment, and several million people live along its edge.
Back on Google Earth, Hudnut zooms in on a construction site teetering at the edge of the fault. He worries. “They want to get them right along the fault because that’s where you have the nice view. It makes me think perhaps our state law is not strict enough.”
Hudnut worries, too, about the “lifeline corridors” entangled with the fault. Interstate freeways run alongside the San Andreas; aqueducts, natural gas lines and electrical transmission corridors crisscross it. Hudnut scrolls through the landscape to find them. “Here’s where the state water project comes down,” he says, tracing his finger along a thin dark line on his computer screen. “You have Interstate 15, two major freight rail lines and a lot of fiber-optic cable running along Old Route 66. You have gasoline pipelines and high pressure gas lines and also high-tension power lines all nested in close together.” All of them thread through the Cajon Pass, a major gateway into the northern Los Angeles Basin. Hudnut fears that a rupture today would leave metropolitan Los Angeles without water or power for weeks, maybe months.
“These lifelines that cross the fault, they were not designed not to break. We don’t know for sure, but we think they’re going to all break. People haven’t thought that through. People haven’t decided yet that they really need to fix these so that they won’t break. I’m not sure they’ll never decide that, but we’re trying to encourage people to think about what the real consequences would be.”
Before he died in 1952, Andrew Lawson built himself a house in the Berkeley Hills that was designed to withstand any earthquake or fire. It has yet to be tested by geology, but the fire that swept through the area in 1923 didn’t even singe a library book inside. A little engineering goes a long way: As seismologist Nick Ambraseys famously said in 1968, “Earthquakes don’t kill people. Buildings do.”
Hudnut might like to amplify that: Earthquakes may not obliterate civilizations, but weak infrastructure can. As many have noted, it wasn’t the shaking ground that destroyed San Francisco. It was the fact that the shaking cut off the city from its water supply, so no one could fight the fires that followed.
In Southern California’s infrastructure, Hudnut says, “We think we’ve identified something like the levees in Katrina. The scientists saw the vulnerability of the levees; they predicted what could happen. But there wasn’t the political will to raise the money to fix them.” He hopes Los Angeles can avoid a similar fate. But time may be running out. And geological forces don’t wait for budgets — or repair crews.
This article appeared in the print edition of the magazine with the headline The coming quake.
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