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 ou didn’t have to be a mystic to find signs of apocalyptic portent in the closing months of 1999. Floods, war, a solar eclipse, hurricanes and earthquakes – an appalling abundance of earthquakes. Between August and November, no fewer than five massive temblors shook Turkey (twice), Taiwan, Mexico and Southern California’s Mojave Desert. These quakes measured magnitudes of 7.4 and 7.2 (Turkey), 7.6 (Taiwan), 7.5 (Mexico) and 7.1 (California) on the scale used to compare seismic events.
“Those were big earthquakes,” says USC earth scientist James Dolan, putting things in perspective, “but they were not what we call great earthquakes.” Hardly encouraging words from the expert who traveled to Turkey to study the aftermath of the August 17 earthquake which, together with the November 12 temblor, left as many as 50,000 dead in Anatolia.
Globally, 1999 was actually a slow year for earthquakes. According to a recent Los Angeles Times article, in an average year the earth experiences 15 to 20 events in the 7.0 magnitude range. In 1999, there were only eight such temblors – three of them in remote regions of the South Pacific. It only seemed like a particularly shaky year because so many of the quakes occurred near major urban centers.
While the world trembled, seismic scientists like Dolan shook with excitement. From USC, they fanned out to examine last year’s major quake sites, keen to probe the mysteries of the moving earth. For these researchers, rushing into potentially dangerous situations and scenes of death and devastation goes with the territory.
“Earthquakes are our experiments,” explains Tom Henyey, professor of earthsciences and director of the Southern California Earthquake Center, a National Science Foundation-funded consortium headquartered and administered at USC. “We view them as an opportunity.”
HALIDERE, TURKEY As a half-mile band of shoreline dropped into Izmit Bay, the entire waterfront of this quaint resort town was destroyed. |
Besides Henyey and his USC colleagues, the internationally recognized SCEC is home to an eclectic community of scientists from nine academic institutions. Together, these seismic sleuths hope to determine when and where future damaging quakes will occur in Southern California, calculate expected ground motions and share their findings with the public.
Strangely, it was in pursuit of these distinctly regional goals that they journeyed to far-off Turkey and Taiwan after last fall’s global onslaught of earthquakes.
“We wanted to know what kind of ground breakage and ground motion had occurred, and what kind of damage had resulted,” says Henyey. “Our mission is to help reduce the earthquake hazard in Southern California, and we are learning a great deal from Turkey and Taiwan.”
PERHAPS THE LOST CITY OF TROY - whose ruin lies at the mouth of the Dar-danelles – subconsciously beckons Trojan researchers to Turkey, but scientists also find solid scientific reasons for studying there.
Dolan had been conducting earthquake research in Turkey before the first quake struck, which is how his graduate student Ross Hartleb happened to be on the spot during the big event. In its aftermath, Dolan and Hartleb formed a team, including graduate students Allan Tucker ’97 and Shari Christofferson, to map the rupture.
Other earth scientists from the College of Letters, Arts and Sciences also descended on Turkey. Seismologists David Okaya and Yehuda Ben-Zion looked for subterranean changes in the fault zone, while Ned Field tried to relate the strength of ground motion from the earthquake and aftershocks to resulting damage.
Soon thereafter, civil engineer Costas Synolakis accompanied his graduate student José Borrero ’95, M.S. ’97 and visiting professor Ahmet Yalciner to investigate a possible tsunami (or tidal wave, though the latter is a misnomer) thought to have been triggered by the Turkish quake. Another USC civil engineer, Jean-Pierre Bardet, traveled to Turkey and then on to Taiwan, which was hit by a powerful earthquake September 21. Earth scientist Ta-Liang Teng also hooked up with Bardet in Taiwan, where an estimated 2,300 people had died.
A month later and closer to home, the Hector Mine earthquake shook the Mojave Desert so savagely that its force was felt from Las Vegas to the Southland. Centeredin a Marine Corps training facility at Twenty-nine Palms, the October 16 temblor caused minimal damage and no injuries, though Dolan and other scientists had to be wary of unexploded military ordnance as they pawed the escarpments and fissures the quake had left in its wake.
DESPITE FOLLOWING HARD ON EACH others’ heels, this profusion of temblors appear to be unrelated. With the exception of the two Turkish quakes, the ruptures occurred thousands of miles apart and involved different and mechanically dissimilar faults. Indeed, there’s no physics that geologists currently understand, Dolan says, that can explain a mechanical connection between these seismic events.
SEAWEED tangled around these hand-rails indicates the water-level must have reached al-most 6 feet on this street in Golcuk. Synolakis calls this classic evidence of a tsunami. |
“That doesn’t mean that a connection doesn’t exist,” he notes carefully, “just that we can’t explain one.”
Though unconnected, the quakes do warrant comparison. Turkey’s North Anatolian and California’s San Andreas faults have a lot in common. They are two of the world’s longest strike-slip faults. This type of fault separates two tectonic plates (blocks of the earth’s crust) that are moving horizontally past each other.
Both the North Anatolian and the San Andreas faults are about the same length; the respective plates are moving at about the same speed, and millions of people live close to both of them.
But there’s also a big difference.
“The North Anatolian fault has long been famous for a really spectacular series of earthquakes,” says Dolan. “There have been half a dozen since 1939 – ranging from 6.8 to 7.9 – and each earthquake has occurred west of the previous one, marching toward Istanbul.” The San Andreas is much more complicated and more erratic. It hasn’t produced a great earthquake since 1906.
Yet the North Anatolian fault’s frequent ruptures and clear path is precisely what makes Turkey so very interesting to re-searchers like Dolan.
“This is the most simple and straightforward sequence of earthquakes of any fault in historical times,” he says.
That Turkey also happens to have meticulously preserved historical records is a stroke of fabulous good fortune. Official documents from ancient Byzantium and the Ottoman empire go back at least 2,000 years for Istanbul, and 1,000 years for more remote areas of the country. Thanks to these superb records from antiquity, today’s earth scientists can accurately date ancient seismic events to within a few weeks.
Official records, for example, indicate that between 1650 and 1684 almost the entire North Anatolian fault broke in a series of earthquakes. Earlier documents reveal that the two ends of the fault also had ruptured between 967 and 1050. But what about the fault’s center, located in the Turkish hinterlands?
Two years ago, Dolan and Hartleb began a paleoseismology study in central Turkey to find out. They dug trenches at the fault to collect organic deposits. Using radioactive carbon dating techniques, they identified the age of the earthquake fractures. Detailed Turkish records helped the researchers calibrate their carbon dating to other documented events.
The results confirmed what Dolan had suspected: the central part of the North
Anatolian fault had indeed ruptured in the 11th century. What’s more, fragmentary evidence suggested two previous earthquake sequences – one in the 8th or 9th century and another in the 3rd century.
“We are seeing a pattern here,” says Dolan. “Over a period of a few decades, the entire fault ruptures, and then it sits there for 200, 300 or 400 years. We think the same thing has happened in Anatolia a
IN SMALL TOWNS and big cities alike, researchers saw buildings teetering at precarious angles alongside structures showing little or no damage. |
t least three times, and probably five times.”
What does all this mean to Southern Californians living in their own strike-slip fault zone?
Dolan says it’s a beginning to understanding the San Andreas fault – the southern part of which has been considered “10-months pregnant” for a long time. But the research challenges in California are daunting. The region’s historical records are adequate only from the advent of daily newspapers in the mid 1800s – the blink of an eye in geological time. A few scanty accounts date back to the early Spanish missions. Before that, the state’s seismic history is a blank.
California’s lack of ancient bureaucracy hinders researchers no less than its abundance of modern red tape. It’s a lot harder to get permission to dig trenches here than in Turkey. And it costs a lot more money.
It’s not unusual for geologists studying key faults to be denied access to private property by suspicious land owners. “They’re afraid we’ll see an endangered species and put their land off limits for development,” Dolan says.
By contrast, “the Turks are among the world’s most hospitable people,” Dolan notes. “Families who had lost their homes and were living in a tent on the front lawn would invite us in for tea or to share food. Local municipalities loaned us excavation equipment to dig the trenches at no charge.”
Unfortunately, deconstructing Turkey’s faults is no substitute for learning the twists and turns of the Southland’s own geotectonic terrain. The San Andreas is a far more complicated mechanical system than the North Anatolian fault. In Turkey, about 90 percent of the motion between the two big plates occurs along a single strand of the North Anatolian fault. Along California’s San Andreas fault, the motion between the Pacific and American plates is much more widely distributed among nearby faults.
“We have so many different faults, including big cross-faults that intersect the San Andreas at weird angles, and they all interact in a complex way to cause our earthquakes,” Dolan says.
Still, the Turkish explorations are enormously helpful. “If we can understand the mechanically simple system in Turkey, then perhaps we can bootstrap ourselves up to an understanding of the more complex problem in California. That’s what we are trying to do,” Dolan says.
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