Los Angeles Times Special Quake Report: One Year Later : Still Shaken / Lessons : Unsettling New Facts : Scientists Unearth the Hazards

In a few terrifying seconds last January, the Northridge earthquake set in motion a sweeping transformation in how scientists gauge the danger posed by the cobweb of hidden thrust faults underneath the Los Angeles area. After a year and more than 11,000 aftershocks, the scientific reverberations from the most costly temblor to strike a North American city continue to shake up preconceptions about urban earthquakes.

Never had scientists been so well prepared to record the effects of a major urban earthquake. The Los Angeles area was laced with dozens of seismic monitoring stations and scores of ground motion sensors as well as advanced satellite stations that could detect the most subtle shifts as the Earth roiled. In the past year, even more have been installed.

As they sift through their findings, scientists now believe that the hidden geologic mechanisms that shoulder and shift the titanic strains building up in Southern California are more complicated and potentially more destructive than they had ever imagined.

Several researchers suggest that an earthquake of magnitude 7.0 or greater should shake the region every century or so, although no quake of that size has occurred since the founding of Los Angeles in 1781.

“Historically, this was a very important earthquake,” said earthquake geologist James F. Dolan at USC’s Southern California Earthquake Center. “We now understand that blind thrust faults pose a major danger to Los Angeles. We are looking at a hazard we really have not appreciated before.” In a blind thrust fault, one side of the fracture pushes up over the other deep below the surface with no apparent deformation at ground level.

As they come to appreciate the significance of these hidden faults, scientists are reassessing every aspect of seismic safety in the region.

In heated hallway arguments, measured symposium presentations and a series of major peer-reviewed scientific papers, earthquake experts have made the metropolitan area the focus of unprecedented scientific scrutiny. While it may be years before the lessons of the Northridge quake are completely absorbed, the conclusions that geologists, geophysicists and seismologists have settled on so far are sobering.

More Quakes Coming

Scientists today say there is a greater likelihood of major earthquakes in the Los Angeles area than they previously believed. The potential for damage to high-rise buildings is greater than engineers had expected.

Many earthquake experts now believe that the densely inhabited suburbs of the northern rim of the Los Angeles Basin, including the San Fernando Valley, face one of the greatest seismic hazards in Southern California.

“This earthquake occurred on a fault we didn’t even know about, let alone suspect was active,” said Thomas L. Henyey, director of the Southern California Earthquake Center.

“It tells us as scientists that we have to start assessing the fault problems in Southern California in a different way than we have in the past,” he said.

UCLA geophysicist and seismologist David D. Jackson agreed. “With the thrust faults, you can get a nasty earthquake where you can’t see a big fault existing. That was certainly true with Northridge. But even when you know a fault is there, the size of the event sometimes comes as quite a surprise.”

Such was the case with the magnitude 6.7 Northridge temblor. With their more elaborate instrumentation, scientists recorded the most severe ground motions ever detected in an earthquake, with the Earth rippling at speeds of up to five feet per second.

Based on the new data, computer simulations released last week show that even steel-frame high-rise buildings, previously thought to be virtually earthquake-proof, could be seriously damaged or destroyed in a strong quake. Buildings using sophisticated base isolation systems to protect themselves from the ground shocks also could sustain serious damage.

Building codes in the Los Angeles region, adopted long before there was any real data about the effects of a serious earthquake in an urban area, are inadequate, experts have concluded in the aftermath of the Northridge quake. At least 5,900 buildings were seriously damaged in the temblor. The most severe damage was found in buildings built before 1976, the year stricter building codes came into effect.

But much of the damage from the Northridge quake occurred because building codes had not been followed properly and building inspections were inadequate, engineering experts said.

The Tilting Earth

U.S. Geological Survey earthquake expert Kenneth Hudnut in Pasadena said that the Northridge temblor, which measurably raised about 1,900 square miles of the metropolitan area, highlighted the potential danger the tilting Earth poses to power lines, natural-gas pipelines, water supplies and sewer systems in future earthquakes.

“The Jensen Filtration Plant near the L.A. Reservoir was affected by a change of gradient in the Northridge earthquake,” he said. “Had the vertical movement been greater, all the gravity-powered water and sewage systems of the Valley would have been affected. That type of damage to the infrastructure is harder to evaluate and harder to repair than freeway damage.”

Based on measurements gathered during the Northridge temblor, Stanford University experts say the ground shaking caused by a serious earthquake--of magnitude 7.0 or more--would cause fewer casualties but much more structural and economic damage than previously predicted. Economic damage could be as high as $140 billion, they estimated.

Despite their new findings, earthquake experts are hard-pressed to synchronize the Earth’s enigmatic geologic clock--in which millenniums may pass between the most destructive tremors--with the more hyperactive human attention span. Public officials and emergency planners struggle to deal with the long-term risk on the shorter timetable of budget cycles, building code debates and construction deadlines.

Scientists are even less capable of forcing anyone to act on their growing sense of alarm.

“Earthquakes do not kill people; buildings kill people,” said Dolan of USC. “It is all a matter of building codes.”

But public officials and earthquake experts on the state’s Seismic Safety Commission so far do not intend to urge formally that homeowners be required to retrofit their property to mitigate against quake damage.

In its comprehensive report on the Northridge quake--expected within several weeks--the commission also is expected to say that further research is required before precise retrofitting and repair steps can be advocated for buildings whose steel frames were cracked by the temblor.

Nonetheless, the state experts are anxious that the scientific findings from the Northridge quake be translated into more comprehensive safety standards as quickly as possible.

“Although the commission believes California’s seismic building and land-use practices are among the best in the world, there remain weaknesses in our program that result in unacceptable vulnerabilities in the areas of safety and the economy,” says a final report draft by the commission.

“The commission believes that, recognizing these vulnerabilities, California cannot continue with business as usual,” the commission said.

No place in California is completely immune from the possibility of a serious earthquake. Since 1989 alone, there have been four earthquakes in the state that registered more than 7.0 in magnitude, but none of them in the Los Angeles area. All were more powerful than the Northridge temblor. Yet, because of its location in an urban area, the Northridge earthquake caused far more damage.

Experts at the U.S. Geological Survey and the Southern California Earthquake Center say the region should expect at least one magnitude 5.0 or greater earthquake every year for the foreseeable future.

While many earthquake experts agree that more serious quakes are in the offing for the Los Angeles area, they cannot say when or where the next major temblor will strike.

Historical Record

The geologic record for the past several thousand years is confusing--it shows no evidence of any earthquake more powerful than the Northridge temblor in the Los Angeles Basin. Some scientists say that could portend a truly large temblor, while others say it might be evidence that faults in the basin are not capable of generating such a severe quake.

Researchers can only attempt to calculate how much strain is built up in the region and then construct scenarios in which the Earth releases its pent-up energy.

Feeding potentially destructive energy into the system of faults underlying the region is the strain building up at the big bend of the infamous San Andreas fault, which helps compress the Los Angeles area between Palos Verdes and Pasadena at the rate of about half an inch every year.

Caltech earthquake experts Egill Hauksson and Kate Hutton, working in collaboration with USGS seismologist Lucile M. Jones, suggest that more damaging earthquakes will almost certainly occur over the next decade.

They offer four main scientific lessons from the Northridge temblor:

* Potential sources of major earthquakes may be “routinely” missed by researchers investigating the region’s geology because there is no discernible evidence at the Earth’s surface of blind thrust faults like the one responsible for last January’s quake,

* The width of the region’s fault rupture zones may be several miles greater than thought and, therefore, potentially more destructive.

* No matter how devastating the force of the Northridge quake, it was not enough to relieve the tectonic strain built up in the region.

* Because there have been no major quakes--of magnitude 7.0 or more--in the region for at least 200 years, “significant” accumulated strain is waiting to be released by one or more temblors on the faults in the region.

Each big California earthquake teaches scientists lessons about how quakes occur and casts somewhat differently the odds on the seismic future regionally.

Scientists often used to view the rupture process as instantaneous, with the entire affected portion of a fault sliding at once.

In recent years, however, the view developed that the rupture proceeded like the opening of a zipper, with quake pulses moving in a distinct direction along a fault, the rupture beginning at one end and then occurring perhaps seconds later at the other.

Presently, in the wake of the Northridge quake, a simulation of the temblor at the Caltech Seismological Laboratory depicts the progress of the eight-second main shock, from the hypocenter at a depth of 11 miles on the southeast corner of the rupture zone, up and sideways to the west side, at a depth of four to five miles in a somewhat discontinuous series of pulses.

It is clear in the simulation that the rupture is over at the hypocenter by the time it reaches its uppermost finish a few miles along the dipping fault plane.

Differences of Opinion

Scientists remain somewhat at odds over what precise fault system was involved in the Northridge earthquake--whether it was related to the Oakridge fault system to the northwest, the Holser fault to the north or simply on a previously unknown blind thrust fault ramp dipping toward the Elysian Park system cutting across the Los Angeles area from Whittier to the ocean.

One curious feature of the Northridge earthquake was noted by Hauksson, Hutton and Jones, who raise the possibility that two small swarms of quakes that occurred before the main shock in the Santa Monica Bay and on the Holser fault in the Santa Clarita Valley may have been precursory events, instead of isolated tremors, as previously believed.

Another uncertainty, rising to a level of a polite dispute between scientists, concerns whether the 1971 Sylmar earthquake may have added to stresses and triggered the Northridge quake 23 years later. The new distribution of stress created by the Northridge quake, they suggest, could offer a tantalizing hint to the location of the next major temblor.

Considerable attention continues to be paid to the supposed “quake deficit” in the area.

Given measurements of accumulated tectonic strain, most earthquake scientists believe more serious quakes should have occurred in the last 200 years in Southern California than actually have taken place.

The Northridge quake was the strongest in the basin in those two centuries, but the scientists think there should have been many more earthquakes of similar size. Otherwise, the scientists suggest, a much bigger earthquake is required to alleviate the pressure built up as the Earth has compressed over the centuries. Because there’s been so little activity at the Northridge-quake magnitude, they contend, the next significant quake may be that much more powerful.

Jackson at UCLA and Dolan at USC said that there are three possibilities: A truly large earthquake will allow the area to catch up in a single spasm; the tension could be released by a series of smaller quakes, the size of the Northridge temblor, or imperceptibly small, steady ground movements called “aseismic creep” will release the strain more gently.

“I think the occasional bigger earthquake is most likely,” Jackson said.

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LIVING ON THE FAULT LINES

The Northridge earthquake gave scientists a new window into geological forces underlying the region.

* Seismologists have recorded more than 11,000 aftershocks from the Northridge earthquake since the main shock 12 months ago. Like the initial quake, all the aftershocks occur several miles below the surface.

* The actual fault on which the Northridge quake occurred, and its aftershocks, is a plane that dips southward from about 4 miles below the surface to as deep as 11 miles. Earthquake experts are still arguing about just which larger fault complex was involved.

* Since the quake, researchers have mapped how seismic stress increased in some areas and decreased in others, revealing a distinctive butterfly-shaped pattern similar to those detected after previous quakes. Most scientists think the stress changes are too small to outweigh other influences on where future quakes may occur.

* Days before the quake, swarms of small quakes were felt in Santa Monica and near the Holser fault north of the Valley. Their connection to main quake is a topic of speculation.

THE BIG BEND

Just north of Los Angeles, the San Andreas Fault interrupts its northwestward direction and briefly takes a more westerly course along the San Gabriel Mountains before resuming its previous route. This is the known as the Big Bend, whose huge tectonic stresses are responsible for the earthquakes in the Transverse Ranges and the Los Angeles Basin.

THE MAIN SHOCK

Measured in one- to two-second intervals, the actual rupture of the Northridge quake moved from the hypocenter, at the lower right, up the fault plane. By the time quake pulses had reached the upper left corner eight seconds later, the original rupture at the hypocenter had stopped.