No wonder, then, that magnitude is a term almost everyone knows but few understand. “It’s even hard for the specialists to keep it all straight,” admits John Vidale, the network’s director. In some ways Charles Richter set an impossible goal in attempting to develop a single scale to measure and compare earthquakes. How can one number sum up something so complex? It’s like trying to measure a hurricane. Should you focus on the storm’s dimensions or on its vortex dynamics? What about wind speed, which varies widely from place to place, just like earthquake shaking?
Richter picked the simplest yardstick he could think of for earthquakes: how much they move the needle on the recording drum of a seismograph. Big quakes will trace out taller wiggles than small quakes, he reasoned. By comparing the height of the tallest wiggles from different quakes, it should be possible to rank them by size. Richter picked a specific type of seismometer for his measurements and developed equations to correct for the epicenter’s distance from the recorder.
What he mainly wanted was a simple answer for the reporters and citizens who bombarded Caltech’s seismology lab with questions after every quake. “We felt a certain responsibility to keep the public informed,” Richter said in a 1982 interview, “particularly as misinformation was often seized upon and twisted in a way that was contrary to the public interest.”
In the early 1930s, when Richter was testing his scale, the Southern California seismic network consisted of seven instruments. A true understanding of tectonic forces was decades away. But it was immediately clear that earthquakes came in such a staggering range of sizes that there was no way to squeeze them all onto the same scale without using numbers so big they would cause more confusion than clarity. “If there was anything you could call an actual discovery that came out of that scale, it was that the biggest earthquakes were enormously bigger than the little ones,” Richter recalled.
At the suggestion of his boss, Beno Gutenberg, Richter resorted to what he called “a device of the devil.” To keep the numbers on the scale manageable, he made it logarithmic instead of linear. The result was the deceptively straightforward numbering system seismologists still use today. And just as in Richter’s time, a system that was meant to inform the public still leaves many people scratching their heads and wondering how earthquakes that are so close to one another in number can be so different on the ground.
An earthquake that measures 4 on the Richter scale isn’t just slightly bigger than a 3. It’s ten times bigger—if by bigger you mean the height of the tallest wiggle on a seismograph. (The instruments are all digital these days. The UW keeps a drum recorder in the basement so television crews have something to film after a quake.) A magnitude 5 is one hundred times bigger than a 3. Jump up to 6 and you’ve got a quake that’s one thousand times bigger than a 3, and so forth, stepping up the scale by factors of ten.
But if you’re more concerned with the amount of energy unleashed by earthquakes—a truer mark of destructive force—then you have to pull out your calculator and multiply by 31.6 for each step up the scale. That makes a magnitude 6 more than thirty thousand times stronger, or more energetic, than a magnitude 3.
Over the years a few lonely voices have pleaded for a more user-friendly scale. In his book The Sizesaurus, Canadian science writer Stephen Strauss pitched a fit. “Why is something that looks as simple as 1-2-3-4-5-6-7-8-9 so bloody confusing?” he asked. An op-ed in the San Francisco Chronicle titled “It’s Time to Dump the Richter Scale” pointed out that computer owners don’t blanch at big numbers, like gigabytes. Why can’t a public that’s well acquainted with trillion-dollar deficits digest an earthquake scale that ranges from 1 all the way up to 20 trillion, asked the author. “We know that a 20 mph wind is twice the velocity of a 10 mph wind. Two inches of rain is twice as wet as one inch of rain,” he wrote. “Our new scale should reflect the actual energy or shaking a person would feel.” Then, as if to underscore his point, the writer was forced to append a lengthy correction because he screwed up describing the current scale. A few scientists have even called for a linear scale. But they might as well insist the United States go metric or dump “The Star-Spangled Banner” because no one can hit the high notes. Maybe it ought to happen, but it’s not going to.
Seismologists don’t have any problem with logarithms, but they were quick to find other faults in Richter’s scale. Designed for the shallow quakes typical in California, it didn’t do a good job of sizing up deep quakes that shake the ground in a different way. Nor was it good at measuring quakes from far away. In fact, as seismologists studied earthquakes in more detail, they realized that “shaking” encapsulates a wide range of motions, from back-and-forth and sideways to up-and-down and everything in between. Some waves hit fast and hard. Others are slow and undulating. Just as the human ear can’t detect all the sounds a dog’s ear can, no single type of seismometer can capture all the different ground motions. And the very biggest quakes simply blow most instruments off the scale.
Richter’s system was tweaked many times, and entirely new scales and instruments were developed to better capture the diversity of shaking. By the 1990s, more than twenty different scales were in use, and scientists were arguing that Richter’s original was obsolete. “The many different magnitude scales are generally all included together in the maddeningly vague term ‘Richter scale,’ which is popular with the press but meaningless to a seismologist,” wrote Tom Heaton, who had joined the faculty at Caltech by then.
The favored replacement, the moment magnitude scale, is a more complete measure of a quake’s fury. Unlike Richter’s it’s based on actual measurements, calculated from seismograms, of the size of the fault rupture that generates a quake. It was only after developing the moment magnitude scale that seismologists were able to fully appreciate just how powerful some past quakes were, including Chile’s world record 9.5 in 1960. But moment magnitude is useless for very small quakes. The signals they create are too weak.
THE MOMENT MAGNITUDE SCALE
Like Richter’s original, the moment magnitude scale seismologists now prefer is logarithmic. Each full step up the scale represents a ten-fold increase in ground shaking. That relationship breaks down for the biggest quakes, which shake longer but not that much harder. A better yardstick is energy released, which increases thirty-two-fold with each step up the scale. (image credits 7.1)
So seismologists engage in a juggling act with every quake. The smallest are sized up on something called a “coda magnitude scale,” which measures how long it takes for ground shaking to die out. Middling quakes of magnitudes 2 to 4 are measured using what is essentially Richter’s scale, though seismologists call it “local magnitude.” (Richter wouldn’t mind. He was never comfortable with his name being attached to the scale.) For quakes bigger than that, the moment magnitude scale prevails.
When the Nisqually quake struck the Puget Sound area in 2001, the regional seismic network was tuned to the coda scale. As a result the first size estimate the system spit out was a paltry magnitude 4.8. Seismologists knew that was way too low, but the network wasn’t set up yet to automatically calculate the magnitude of bigger quakes. They had to do the math by hand. The answer they got was magnitude 6.7, but they still weren’t sure it was right. It was possible the quake was even bigger and had simply saturated the local instruments. Like students comparing answers on a math quiz, local seismologists got on the phone with the USGS National Earthquake Information Center in Colorado, which ultimately put the quake’s official size at magnitude 6.8.
It’s not unusual for different groups of scientists to come up with different magnitudes for the same quake. Numbers for the 2004 Indian Ocean quake range from 9.0 to 9.3. But like Richter, seismologists still want that single number for public consumption. “We just try to give the press a number we’re not going to have to change,” Vidale said. “We don’t want to adjust it later and have to explain why, because it’s almost impossible to explain these magnitude scales.”
CHAPTER 8:<
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IT CAME FROM THE DEEP
NO ONE IN THE PAST THREE HUNDRED YEARS has witnessed a Cascadia megaquake. Not a single soul in the past millennium has weathered a rupture on the Seattle Fault. But hundreds of thousands of people across the Northwest have stories to tell about the third type of earthquake that stalks the region: deep quakes, like the one that struck between Olympia and Seattle in 2001. Some old-timers have lived through three of them, including the biggest quake in Washington’s recorded history.
Brian Schimpf was in the control tower at Sea-Tac Airport when the Nisqually quake of 2001 started rattling the glass just before 11:00 AM on February 28. It was a rare sunny winter’s day. With two planes ready to land and another half-dozen taxiing for takeoff, Schimpf quickly sized up the situation.
“All right, we’ve got an earthquake,” he announced over the radio. As the shaking intensified, ceiling tiles and insulation rained down on the controllers. The Seattle Times reported that Schimpf sounded “almost calm” as he continued broadcasting. “Attention all aircraft in Seattle. We have a huge earthquake going on. The tower is collapsing. I say again: The tower is falling apart.” With a Boeing 767 on final approach, the three-quarter-inch-thick windows began to shatter. “For the first time in my life,” Schimpf told the newspaper, “I thought I was probably going to die.”
On the West Seattle Bridge, streetlights popped and smoked and cars skidded to a stop. Some drivers feared the bridge was falling. Nearly two thousand people were at work in Starbucks’ headquarters, a brick behemoth south of downtown built in 1912. One employee said it felt like the building was slammed with a battering ram. “People were screaming. Things were falling off the walls, flying off desks, and filing cabinets were falling over,” he told a reporter. Chunks of facade and parapet tumbled nine stories to the ground, and water pipes burst. The building’s owner had recently installed massive X-beams to make it more earthquake safe. “If those braces weren’t there, we’d be looking at a pile of rubble,” said one worker. Repairs and upgrades still cost $50 million.
Paula Vandorssen’s home was a total loss. She was lucky to escape with her life when a river of mud sloughed off the slope in her backyard south of Seattle. “It was two seconds I had, and by the third my house was gone,” Vandorssen said. The slide cracked her white frame house in two and filled it with four feet of muck. Her Volkswagen was buried, her cat killed. “If my kids had been home, we would have been dead,” she told a reporter.
The churning mass of mud and logs poured across Vandorssen’s lawn into the Cedar River, damming its flow and threatening to flood neighboring homes. A quick-thinking streets supervisor dashed to a construction site and commandeered backhoes to scoop up the mud and return the river to its banks.
In Olympia state workers dove under tables as two-hundred-pound light fixtures came crashing down. The dome on the capitol building shifted and twisted on its base while sandstone columns pulled away from the edifice. One of the hardest-hit areas was Seattle’s historic Pioneer Square. Merchants were still cleaning up from an ugly brawl during Mardi Gras celebrations the previous night. Like a scene from a slapstick movie, a worker who had just finished replacing broken windows at The Elliott Bay Book Company watched his handiwork shatter. Around the corner bricks rained from an old hotel, crushing cars parked on the street.
“I’m just stunned that there hasn’t been a greater loss of life,” FEMA director Joe Allbaugh said when he toured the state. The only person to die during the quake suffered a heart attack. More than four hundred people were injured, though, and about forty thousand applied for federal assistance, the most from any natural disaster in state history. Official estimates put the total damages at $2 billion. The quake clocked in at magnitude 6.8.
As she cleaned up broken knickknacks after the shaking stopped in 2001, Seattle-area resident Carol Davis recalled riding out similar quakes in 1949 and 1965, and how much she hated feeling the ground shift under her feet. She started to throw out the fragments of ceramic angels and crystal doves, then changed her mind. “I put them back where they were to remind me to be alert and think about how I can help others when things like this happen,” she told a reporter.
It’s entirely possible seismic veterans like Davis will be around for the next Nisqually-style shakeup. The USGS estimates that damaging deep quakes strike the Puget Sound region every thirty years, on average. But that’s just a guess. Some scientists suspect as many as seven or eight have hit the region over the past century, which pencils out to a quake every dozen years or so. Two of them struck a scant three years apart. The odds are the region’s next destructive quake will be of this type. Yet even though deep quakes are the Northwest’s most common big ones—with a small b—they are also the least understood.
Deep quakes emanate from dozens of miles underground, where the laws of geophysics say rocks shouldn’t fracture at all. They leave so few clues on the surface that they’re almost impossible to study. Except for a short historic record, scientists are largely flying blind as they try to estimate how often deep quakes strike and how destructive they can be. One of the only things known for sure is that deep quakes in the Northwest originate inside the oceanic plate being shoved under the continent. But exactly what transpires down there to make the ground shake over hundreds of thousands of square miles remains an enigma.
The depth of the quakes is a blessing, because it moderates their destructiveness. The shallow Northridge quake in 1994 was smaller than the Nisqually quake but did ten times more damage, killing seventy-two people and snapping freeways apart in Southern California. The curse of the Northwest’s deep quakes is that they seem to cluster under the population centers of Puget Sound, though scientists warn that Oregon and British Columbia aren’t immune. Nor was the Nisqually quake the worst case. Despite its multibillion-dollar price tag, the 2001 quake was a feeble example of what the deep Earth can serve up. The 1949 quake Carol Davis experienced as a girl measured magnitude 7.1, the most powerful in the state since record keeping started. Nisqually was about a third as strong.
“Some of the worst damage in the Western Hemisphere has come from these kinds of earthquakes,” said University of Washington geophysicist Ken Creager. A deep quake under central Chile in 1939 killed almost thirty thousand people, far more than died in that country’s world-record magnitude 9.5 megathrust in 1960. One of the most catastrophic quakes in modern history struck deep under the Peruvian coast in 1970, killing seventy thousand people. Most of the victims were buried under a debris avalanche that sloughed off the country’s tallest peak—very much like past lahars from Mount Rainier that roared down valleys where tens of thousands of people live today.
Conventional wisdom in the Northwest holds that deep quakes here can’t get much bigger than magnitude 7.2. But the Peruvian disaster measured 7.9. A similar quake in Japan hit magnitude 8. Could the Pacific Northwest see something that size, more than sixty times as powerful as Nisqually? “It wouldn’t surprise me,” Creager said. “To me, that’s one of the big questions from a hazards perspective: What’s the biggest deep earthquake we should be prepared for?”
Carol Davis was ten years old on April 13, 1949. She remembers playing at a friend’s house and drinking lemonade before lunchtime. School was out for spring break. Davis was just about to head home when the porch started rolling so violently she could barely stand. More than sixty years later, she could clearly summon the feeling of choking on an ice cube as the railings danced in one direction and the steps gyrated in another. “It was horrible.” When her father got home from work, he described racing out of his downtown office building, only to encounter a hail of bricks on the street.
Almost every building in Seattle’s Pioneer Square was battered. The damage was equally extensive at the state capital complex in Olympia. Twenty-five miles south in Chehalis, the shaking left four out of every ten homes and businesses in need of structural repairs. One golfer watched the fairways around him “roll like a shaken rug.”
Seven other people died across the Puget Sound region. Workers rebuilding the Tacoma Narrows Bridge, which had been ripped apart in a windstorm nine years before, clung to safety lines as bolts sheared and the five-hundred-foot towers whipped back and forth. A twenty-three-ton saddle designed to hold the suspension cables in place plunged into the water, crashing through a scow and injuring both men on board.
From his seismology lab at Caltech, Charles Richter classified the quake’s intensity as “near major.” The needle of the University of Washington’s sole seismometer was jolted off the drum by the shaking. Scientists conferred and ciphered for more than two weeks before mistakenly pinning the epicenter on the Olympic Peninsula when it was really near Olympia. Monitoring wasn’t a whole lot better on April 29, 1965, when a magnitude 6.5 quake rumbled up from somewhere between Seattle and Tacoma. Once again the UW instrument, one of only three seismometers in the Pacific Northwest at the time, was knocked offline. As data came in from more distant stations, local scientists realized they were dealing with a deep source, even if they couldn’t quite figure out what it was.
“For decades, really, in the history of seismology these were difficult earthquakes to explain,” said USGS scientist emeritus Steve Kirby. Kirby spent much of his early career in rock labs, subjecting chunks of quartz to bone-crushing pressures and temperatures hot enough to wilt steel beams. That’s how geologists try to simulate what goes on in the depths of Earth, far below the reach of any drill rig. One of the things Kirby was curious about was why deep quakes happen when they aren’t supposed to.
Full-Rip 9.0: The Next Big Earthquake in the Pacific Northwest Page 14