Cover Story : Accurate to a Fault : Seismology: Scientists are slowly unraveling the complicated puzzle of quake activity in the San Gabriel Valley. Their high-tech tools might show which systems to focus on.
For decades, the San Gabriel Valley has been home to much of the nation’s earthquake re search. And although the 6.8 Northridge temblor caused only modest damage locally, its deadly upward punch refocused attention on the theory that serious seismic activity is continuing in the Los Angeles Basin--much of it increasingly centered in or near the San Gabriel Valley.
Within the past seven years, four of six earthquakes in the basin registering 5.0 or larger were centered in the San Gabriel Valley or nearby Upland, researchers say.
“We’ve gone from about zero activity to a lot of activity,” said Richard Morton, a geologist with the U.S. Geological Survey at UC Riverside. “That’s what’s really troublesome.”
Geologists and seismologists sprang into action when the Jan. 17 temblor struck Northridge, killing at least 57 people causing more than $20 billion in damage. Since then, they have busied themselves in the San Fernando Valley, setting up portable seismographs, measuring ground movement and examining cracks in the earth.
Their findings about the Northridge quake could affect how scientists view the faults and earthquake activity 20 miles to the east in the San Gabriel Valley. The research “might change our picture of how these (San Gabriel Valley) faults interact together,” said Lucile M. Jones, a U.S. Geological Survey seismologist. “It might tell us which are more important than others.”
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Scientists have been working for years to unravel the horrendously complicated puzzle of faults in the San Gabriel Valley and much of the Los Angeles Basin. The valley’s fault system is far more complicated than the notorious San Andreas fault zone just over the mountains to the east, seismologists say.
“It’s not clear how the pieces fit together,” said Jones, the seismologist well-known to local television viewers for her savvy translations of what all the shaking means.
Those pieces include active faults like the 60-mile-long Sierra Madre, considered the biggest threat in the valley capable of generating a 7.5-magnitude shock. Shorter, less-threatening faults such as the Raymond, Cucamonga, Whittier, Eagle Rock, San Jose and Clamshell Saw Pit also thread through the valley.
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Complicating the issue are faults such as the 50-million-year-old San Gabriel Fault. The San Andreas of its day, the only energy the San Gabriel generates now is controversy. Scientists debate whether movement 11,000 years ago on this fault means it still should be considered active, Morton said.
At the other end of the activity scale is one of the more recently discovered faults, the Elysian Park Fold and Thrust Belt. A thrust occurs when one side of a slanting fault moves closer to its parallel side.
The Elysian Park belt, skirting the San Gabriel Valley’s southwestern edge, was discovered when it emerged as the prime suspect in the Whittier Narrows quake.
Between the giant lines of these faults are webs of smaller faults, not significant enough to even warrant names.
Buried and unseen faults become known, as did the Northridge fault and the Elysian Park belt, only when an earthquake shakes the earth, Morton said.
Like the Northridge fault, the Elysian Park Fold and Thrust Belt is a blind thrust, meaning the fault line cannot be seen on the surface. Small hills on the surface may give some idea of the existence of blind thrusts underneath, but because their location cannot be pinpointed, scientists may have no knowledge of these faults until an earthquake hits. With the same seismic energy as other faults, these may not be more dangerous but do carry the added punch of surprise.
“These buried systems are very important criteria,” Jones said. “We can have a 6.6 anywhere in Los Angeles. . . . You can understand them, but basically you can’t get the extent of them until an earthquake happens.”
And earthquakes are always happening. Usually their magnitude registers below 3, the level at which seismologists begin studying quakes as major seismic events. From 1900 to 1920, seismologists saw little need to study the Los Angeles Basin, which enjoyed relative seismic quiet.
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But the following two decades--1920 to 1940--saw a cluster of temblors. Big quakes hit the southern end of the county, including a 6.4-magnitude tremor in Long Beach in 1933. Residents as far north as Monterey Park still recall pianos rolling out their back doors.
Then followed 30 more years of quiet, until a 5.2 quake rumbled through the San Gabriel Mountains in 1970. The temblor was noted mainly by scientists because of its remote location on Lytle Creek. But the following year, nearly everyone took notice when the 6.4-magnitude Sylmar quake killed 58 people and caused $511 million in damage.
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It was the only quake this century to rupture the surface of the Los Angeles Basin, a rare event because earthquake fissures typically occur one to 12 miles below the surface.
After that, the cluster continued through the 1970s and 1980s. Quakes in the San Gabriel Valley or nearby included tremors in 1987 in Whittier Narrows, 1988 in Pasadena, 1990 in Upland, 1991 in Sierra Madre and this year in Northridge.
But what it means, no one knows. Are we at the beginning, middle or end of a cycle? “Only history will tell,” Morton said.
Although the Sierra Madre Fault is the biggest danger locally, Jones said she cannot predict when it might strike the San Gabriel Valley, except to say a 5.0 quake is expected in the next 200 years. The problem, she said, is that society wants earthquake hazard information on a human time scale measured in years, instead of a geologic one measured in eons.
One thing the valley has in its favor, and that makes it unusual in the Los Angeles Basin, is the relatively solid geology, Morton said.
The bedrock of the San Gabriel Mountains--except for the lower, newer rock of the hills near Glendora--provides a stable base during earthquakes, Morton said. In the valley floor, deep water tables--below 35 feet--provide safety against liquefaction, the movement of waterlogged soil that plagued Santa Monica in the Northridge quake.
San Gabriel Valley has long been the center of earthquake research in the United States, dating from 1928, when Caltech established a seismology lab in Pasadena. In the 1930s, Caltech Prof. Charles F. Richter developed a scale of measuring earthquakes that made his last name a household word.
In 1943, a group of Caltech scientists, using the name Jet Propulsion Laboratory, received a $5-million Army grant for rocket research. JPL went on to become an internationally known military and space research lab, entering seismic research with satellites in the 1980s.
In the early 1970s, the U.S. Geological Survey affiliated with Caltech, now recognized as one of the world’s premier quake labs.
And the effort to understand San Gabriel Valley’s faults is always under way, using a variety of methods:
Satellite data
A constellation of 24 Defense Department satellites--circling the globe and sending data back to 45 portable receivers worldwide--yields information about the creep of tectonic plates, noting movement as little as a quarter of an inch. The receivers, placed at various sites at yearly intervals, record whether the ground has moved since the last placement.
Based since the 1980s at JPL, the program received a recent boost of $63,000 from a National Science Foundation grant. It enabled Gregory A. Lyzenga, a geophysicist at Harvey Mudd College, to add two receivers to the four in California.
Armed with the laptop computer-size receivers and one-foot-diameter antennas, Lyzenga and students from the Claremont Colleges last fall roamed the mountaintops and valleys in San Gabriel Valley. They busied themselves placing the receivers in various locations so that the satellites could retrieve the data and compare it for movement against data from the same spot recorded a year ago. When the Northridge quake struck, they were sent scrambling again for aftershock data to measure ground movement.
“I’m a little bit of a strange bird,” said Lyzenga, a college undergraduate teacher suddenly plunged into the world of research science to rub arms with scientists armed with doctorates far removed from the classroom. “This is a very modern development. Literally three to five years ago it would have been unthinkable.”
Trenching
Digging into the first five to 20 feet of earth, scientists measure rock displacements and deduce the movement and magnitude of old quakes. They can pinpoint the era of these older quakes using another technology: radiocarbon dating of organic debris captured in rock.
Through radiocarbon dating, Caltech geologist Kerry Sieh and others analyzed the Raymond Fault in the San Gabriel Valley and the San Andreas Fault, both near Wrightwood on the north side of the San Gabriels and 20 miles north into the desert. In one spot, he gathered a history of 12 earthquakes in the last 1,800 years, one of the more complete records available.
Near Wrightwood, evidence of an 1812 tremor and others yielded clues to earthquake patterns. Some major quakes did not seem to occur at both locations, even though they were separated by a mere 20 miles.
Sieh’s trenching also led him to conclude that the southernmost end of the San Andreas Fault, between Indio and the Salton Sea, is the most likely area to next generate an earthquake.
Explosives
Beginning this fall, scientists from the Southern California Earthquake Center at USC and the U.S. Geological Survey will set off 60 small blasts at least 50 feet underground from Seal Beach through the San Gabriel Valley to the Mojave Desert. The blasts equal a magnitude 2.5 earthquake, tremors that occur at the rate of 40 to 50 on a normal day in Southern California, scientists say.
The explosions are usually placed in spots perpendicular to fault lines to give the best readings and to allow differences in the rock to be easily discerned, said Jim Mori of the U.S. Geological Survey in Pasadena.
The northeast route followed by the blasts will roughly parallel the San Gabriel River, with up to 10 explosions set off in the valley, Mori said.
Using ammonium nitrate to create gentle shock waves, scientists will measure wave travel speeds to reveal the underlying geologic structure--the geological equivalent of a human X-ray. The speed at which the waves bounce off underlying rock and travel back to the surface allows scientists to determine what sort of rock and soil lies underneath. Mathematically, Mori said, the information gives a detailed picture of the fault location and the geologic structure.
Seismic networking
Located at Caltech for the past three years, a computerized system stores Southern California quake facts culled from 220 sites. Small sensors the size of a soup can, with accompanying electronic equipment, are piled in garbage can-size barrels. These barrels are scattered 10 to 15 miles apart to allow scientists to carpet the area and get overall readings for the entire region.
A handful of sites are located in the flats of the San Gabriel Valley; another 20 are in the San Gabriel Mountains, where the bedrock gives better readings, Mori said.
“We try to hide them,” he said, adding that solar panels sometimes used to power the equipment are very often hit by bullets from target shooters or are pulled off by thieves.
Sensitive seismographs, capable of recording zero-magnitude quakes--that is, the very smallest quakes--are set up to collect data. After major shocks measuring magnitude 5 or better, additional portable seismographs are hustled out to even more sites.
The system, called a “jukebox” by some scientists, enables researchers to retrieve data via computer and avoid traveling to Pasadena to spend weeks reading through computer tapes.
Jones and her husband, Egill Hauksson of Caltech’s seismology lab, are network experts. Together or with other scientists, the couple have written on most major quakes in the San Gabriel Valley.
Their data has helped reveal the geometric structure of faults in the San Gabriel Valley, the direction of slippage in earthquakes, the geometric and geologic fit of faults and the determination of fault systems. Such information can yield the discovery of potentially hazardous faults or future earthquakes.
Despite all this research activity, much is unknown.
“We don’t know what makes them start or stop,” Jones said. “Or why they happen today instead of tomorrow. We don’t know what is the trigger.”
