WAITING FOR THE QUAKE : IT WILL SURELY BE DESTRUCTIVE, BUT KNOWLEDGE AND PREPARATION OFFER HOPE

As Southern California waits for the Big One that will inevitably hit sometime in the future, the experts are unanimous on one point: The great quake will strike with a vengeance that will leave parts of this region in ruins.

Unlike the 7.1-magnitude Bay Area quake that had a devastating but quite limited impact on Northern California, the monster quake projected for the Southland will wreak havoc over a much wider area. But earthquakes are fickle by nature, so some areas will be destroyed and others spared.

Studies by the federal and state governments show that entire cities will be cut off from emergency help because of damaged highways. Utility lines will be snapped in hundreds of locations, leaving people without power, and gas leaks will pose an enormous threat of fire and explosion. Some hospitals will probably collapse at the time they are needed the most.

Emergency services will be overwhelmed, forcing most people to fend for themselves for the first two or three days until help arrives from distant locations.

Nearly a decade ago, long before some imperiled areas exploded with new homes and shopping centers, a federal study predicted that thousands would die and property damage would run close to $20 billion. Those figures are now as obsolete, as the then barren wasteland along the eastern flank of the San Andreas Fault that is now blanketed with homes.

While no expert seriously doubts that Southern California will be hit with a great quake sometime in the not-too-distant future, there is some debate over which earthquake fault is most capable of generating the Big One.

As for sheer magnitude--the amount of energy released by the quake--there is only one contender: the mighty San Andreas. That is the only fault in this region considered capable of generating an earthquake as great as 8.4 magnitude.

The San Andreas is some distance from the most densely populated regions of Southern California, although it is closer to downtown Los Angeles than San Francisco’s Marina District is to the epicenter of last month’s Bay Area quake. The San Andreas is far enough away, however, that most of the quake’s energy would be dissipated before the shock waves reached heavily populated areas.

That doesn’t help the people of Riverside, San Bernardino and the Antelope Valley--among the fastest growing areas in the nation--because the San Andreas is right next door. But it does mean that when most Southern Californians look for the Big One, perhaps they ought to look a little closer to home.

Ronald Scott, professor of engineering at Caltech and a leading authority on earthquake engineering, said the Bay Area quake shows that the magnitude of an earthquake may not be the most important factor in determining whether it is the one everybody has been waiting for.

“I think it (the Bay Area quake) reinforces my vague feeling that the principal thing to be concerned about is still the 7 locally rather than the big one on the San Andreas,” he said.

A quake of that magnitude, rupturing the region’s most densely populated areas, could cause far more damage and claim more lives than a much more powerful quake on the more remote San Andreas, Scott and others believe.

So the legendary San Andreas is not the only place to watch for the Big One. And Southern California is laced with faults that could be capable of generating a magnitude 7 quake, most notably the Newport-Inglewood Fault that devastated Long Beach in 1933.

No matter which direction the Big One comes from, it will wipe out some areas while sparing many more. Historically, large earthquakes are like that. They reach out and touch some, but ignore others.

“My gut feeling is it’s going to be rather spotty,” said Caltech seismologist Clarence Allen.

But the “spotty” areas, experts agree, will be hit with a fury that is hard to imagine.

The reason for the patchwork quilt effect lies in the fact that Southern California, as one geologist put it, “is a messy piece of real estate,” geologically speaking. The land has been transformed over millions of years as soils washed down from the mountainsides to form the basins below the hills and along the coastline.

The sediments are now several hundred feet deep in some areas, and quite shallow in others, creating a framework for widely differing effects from a catastrophic earthquake. The Bay Area temblor demonstrated this dramatically when it destroyed buildings in the Marina District of San Francisco and spared many more that were much closer to the quake’s epicenter.

While homes and buildings in some areas were destroyed by the Bay Area quake, most modern structures came through it with minimal damage, and most experts believe the same thing would happen here. The skyscrapers that are springing up throughout Southern California would probably survive even the magnitude 8.4 quake that the San Andreas Fault is believed capable of generating, most experts contend, but that conviction has yet to be tested.

In fact, there is no convincing record of just how the ground itself will behave in a catastrophic quake, according to Caltech’s Scott.

Modern buildings are engineered to withstand a certain level of ground shaking, depending on local earthquake probabilities, but the instruments that measure that phenomenon were developed only a few decades ago. Engineers need to know how the ground behaves in a magnitude 8 earthquake in order to design properly for it, but “we have no strong motion records of a greater than magnitude 8 earthquake,” Scott said. “We haven’t had one (great quake) anywhere in the world that was instrumented” at the time of the quake, so the expectation of how intense the ground shakes in a great quake is “just interpretation.”

He said it requires a “jump in engineering imagination” to conclude that a great quake is like a smaller quake, but just bigger and longer in duration.

“Some may feel more comfortable about that than I do,” he said.

Generally, however, officials were reassured by most of what they saw after the Bay Area quake, which suggested that most modern structures should survive even a great quake.

Paul J. Flores, director of the Southern California Earthquake Preparedness Project, said he was relieved when he flew over the Bay Area after the quake because most buildings came through in one piece.

“My first impression at San Francisco was, my God, we’ve come a long way,” Flores said. “I thought, look at this city, it’s intact.”

Clearly, modern construction techniques spared most of San Francisco the devastation experienced by the people of Armenia last year when a quake that was several times weaker than the Bay Area temblor claimed at least 25,000 lives.

Engineers may be able to design buildings that will remain in one piece through even a major quake, but sometimes even that is not good enough. Some of the structures that collapsed during the Bay Area quake failed not because they were poorly designed, but because the ground beneath them failed.

Loosely compacted soils amplify the wave motion from an earthquake, greatly intensifying the shaking. If the water table is close to the surface the shaking can be increased even more, leading to something geologists call “liquefaction” or “ground failure.” During liquefaction, the soil assumes the characteristics of a liquid, causing buildings to topple over or even sink into the ground.

Unfortunately, Southern California is awash with candidates for liquefaction. A series of studies done over the years by the California Division of Mines and Geology has identified many areas that would be particularly vulnerable during a major earthquake. The studies are pegged to specific earthquakes, because they were designed to help officials plan for how they would deal with a major quake, but some generalities can be drawn.

Many areas that are either densely populated today or well on their way to becoming so have all the ingredients for ground failure--loose soils, underlaid by a high water table. This includes the popular boating area of Marina del Rey, the rapidly growing Oxnard region, the coastal belt running from Long Beach to Newport Beach, and many areas across the basin.

Generally, the more hazardous regions were created by the forces of nature over millions of years, and they include ancient river beds, dry lakes, coastal plains and alluvial fans extending out from the foot of various hills and mountains.

“We can identify where it (liquefaction) is going to happen, but there’s not much you can do about it,” said Scott.

But as the Bay Area quake demonstrated so forcefully, not every building in even the most hazardous areas would be expected to fail. Single-family homes, for example, might be damaged, but most are less likely to collapse than old unreinforced brick buildings sitting on bedrock.

Destruction by liquefaction is not the sudden, deadly jolt that most people associate with earthquakes, Scott said.

“By and large, structures sink and they settle, but they don’t fall apart,” he said. “It’s a soft, squishy motion.”

The chances of survival there are quite high, he said, although the building and the utilities might be destroyed, increasing the danger of fire. But chances are, he added, if your home is destroyed by liquefaction you will probably be able to get out in time.

“I would rather be there than in a number of other structures I can think of,” he said.

He suggested, however, that the best thing to do with land subject to liquefaction is to “build a golf course on it.”

One of the more unsettling things about liquefaction is the fact that it does not always happen the way one might expect.

San Jose, for instance, is much closer to the epicenter of the Bay Area quake, and nearly the entire city is built on sediments that accumulated near the southern end of the bay over millions of years, making it a prime candidate for high ground motion. It also has a high water table. But San Jose had far less shaking than the ground beneath the San Francisco airport, which was specifically engineered to limit the amount of shaking.

That was a surprise to geologists and engineers, but they are accustomed to being surprised by earthquakes. The problem is that there are so many components to a destructive earthquake that it is almost impossible to predict the consequences.

Scott said that if the Bay Area quake were to strike again today, at exactly the same magnitude but in a slightly different way, the “shaking intensity at all the same locations might be quite different.” If, for example, the same section of the fault ruptured in a purely horizontal direction without the vertical component of last month’s quake, the energy radiating out from the epicenter would be different, he said.

That is because the fault mechanism--how the fault moves--is the first of three components that determine eventually what will be left standing when the earth finally quits shaking.

When the fault ruptures, Scott said, it sends out a signal in all directions, but the signal is greatly altered by the material-- the second component--it passes through. In some cases it is dampened, and in other cases it is actually amplified, so the strength of the shaking any individual receives “depends on what’s between you and it.”

Third, local geology has the last word. Generally, bedrock is the place to be because the shaking there is less intense.

All of that makes it extremely difficult for engineers to be totally confident that they have designed their buildings to withstand every earthquake. Scott pointed out that the San Andreas is a well understood hazard, so major structures in Los Angeles are designed to withstand a strong quake on that fault.

However, a smaller quake centered closer to the structure would have a very different effect on the building.

“You might get a totally different kind of shaking” than had been anticipated, he said.

What it all means is that there are so many variables, so many things that are not well understood, that no one can say with confidence exactly what Southern California will look like the day after the Big One finally strikes.

But one of these days, we’re going to find out.

Of the 42 surface faults in the Los Angeles Basin area, seismologists have paid closest attention to the San Andreas and the Newport-Inglewood. But many of the other faults are believed capable of delivering destructive earthquakes.

Some geologists believe many faults in the map above have long been inactive and do not pose a serious threat. But there are surprises. A number of sizable California earthquakes in recent years have come from faults that were either unknown or not believed capable of producing a large temblor.

Scientists are now aware that subterranean faults can be dangerous and these underground fissures have added a new dimension to the earthquake threat in the Los Angeles Basin. The Whittier quake, for example, occurred on a segment of the Elysian Park Fault.

Researchers now believe there are at least two other important subterranean fault zones underlying this area--the Santa Ynez-San Gabriel Fault and the Palo Verde Fault, which underlies the Palos Verdes peninsula.

SURFACE FAULT LINES AND ZONES

1 SAN ANDREAS FAULT 2 SAN JACINTO FAULT 3 SAN GABRIEL FAULT 4 HOLSER FAULT 5 SAN GAYETANO FAULT 6 OAK RIDGE FAULT 7 LIEBRE FAULT 8 CLEARWATER FAULT 9 BEE CYN. FAULT 10 SAN FRANCISQUITO FAULT 11 VASQUEZ CYN. FAULT 12 MINT CYN. FAULT 13 GREEN RANCH FAULT 14 SOLEDAD FAULT 15 POLE CYN. FAULT 16 MAGIC MOUNTAIN FAULT 17 ACTION FAULT 18 TRANSMISSION FAULT LINE 19 PACIFIC MOUNTAIN FAULT 20 SIERRA MADRE FAULT ZONE 21 CUCAMONGA FAULT ZONE 22 SANTA SUSANA THRUST 23 SANTA ROSA FAULT 24 NORTHRIDGE HILLS FAULT 25 CHATSWORTH FAULT 26 MALIBU COAST FAULT 27 SANTA MONICA FAULT 28 SEPULVEDA FAULT 29 TUJUNGA FAULT 30 VERDUGO FAULTS 31 RAYMOND HILL FAULT 32 NEWPORT INGLEWOOD FAULT ZONE 33 CHARNOCK FAULT 34 PALOS VERDES FAULT ZONE 35 CABRILLO FAULT 36 NORWALK FAULT 37 WORKMAN MILL FAULT EXTENSION 38 WALNUT CREEK FAULT 39 SAN JOSE FAULT 40 WHITTIER FAULT ZONE 41 CHINO FAULT ZONE 42 ELSINORE FAULT

SUBTERRANEAN FAULT ZONES

TOP: Santa Ynez-San Gabriel Fault Zone

MIDDLE: Elysian Park Fault Zone

BOTTOM: Palos Verdes Zone

Source: Don Clement (Los Angeles Times), Davis & Namson (consulting geologists)