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Lessons from L.A. quake. New data are causing geologists to reevaluate current forecasts of earthquake frequency. But the likelihood of another `great quake' is still high, they say, and more preparations are needed.

The earthquake and aftershocks that have rocked Los Angeles have driven home the value of almost 60 years of preparations. ``That's the important lesson,'' says Kerry Sieh, a professor of geology at the California Institute of Technology in Pasadena.

While noting that this may be of small comfort to the families of the seven people who died in Thursday's quake and Sunday's large aftershock, those preparations, largely changes in building codes, ``saved 10,000 lives,'' he says.

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Dr. Sieh bases his estimate on the 10,000 deaths attributed to a 1972 quake that hit Managua, Nicaragua, and registered 6.2 on the Richter scale. The Los Angeles quake registered 6.1 and the aftershock hit 5.5. Each whole-number increase represents 10 times the amount of motion and a release of 30 times as much energy.

The Los Angeles quake, which caused damage estimated at $125 million, emphasized the need for still more effort to make major structures in the region as earthquake-resistant as possible, as well as improving emergency response measures, Sieh says.

``This earthquake was produced by a fault moving right under downtown L.A. That means we have a source for magnitude 6 earthquakes right downtown. We may do well to shift some of our concerns from the San Andreas fault to the smaller faults in the L.A. area.''

The southern portion of the San Andreas fault is where geologists say the state's next ``great'' earthquake is likely to occur. The fault forms a boundary between two vast plates on the Earth's crust.

Current thinking holds that there is a 50 percent chance that a quake with a magnitude of at least 8 will occur within 30 to 50 years along some part of the fault that lies between the Tejon Pass, Wrightwood, and the Coachella Valley. Such a quake would release about 1,000 times as much energy as Thursday's main shock.

But the forecast for the southern San Andreas may change. William Ellsworth, with the US Geological Survey in Menlo Park, Calif., says the forecast is being reevaluated in a report that is due out early next year.

One factor prompting the review might be new data gleaned from tree rings suggesting there may have been a previously undiscovered rupture along the southern San Andreas 175 years ago that yielded a quake with a magnitude of at least 8. If additional analysis confirms that conclusion, it would throw more uncertainty into estimates of how frequently such major quakes occur.

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Sieh allows that the probability currently assigned for the next magnitude 8 quake is ``a little high.''

Still, the estimate is useful despite uncertainties, says Bruce Bolt, a geology professor at the University of California at Berkeley. ``It helps city and state officials know what they're dealing with'' as they prepare.

``There is a 50 percent chance of it not happening,'' adds Dr. Ellsworth about what many southern Californians call ``the big one.'' ``But many feel that the threat is high enough that you don't want to gamble on it.''

In theory, scientists say, the southern San Andreas could release its pent-up strain through a series of smaller, less-destructive quakes.

``You can't rule out that possibility. But it's implausible,'' Caltech's Sieh says. The problem, he says, is that the Pacific plate's inexorable creep to the northwest has ``probably stored up five meters [5 yards] or more of potential slip'' along the southern San Andreas. Based on the historical and geological record, the northern portions of the segment haven't seen major activity since 1857; the southern reaches have been fairly quiet since 1680. It would take 10 quakes with magnitudes like that of last Thursday's at each of 200 locations along the stressed segment to relieve the strain, he notes. Based on the evidence, ``That's just not in the cards.''

Nor are the 30 million or so magnitude 3 quakes that Ellsworth estimates would need to occur.

All of this, researchers say, puts the responsibility for controlling damage and casualties squarely on the shoulders of those living in seismically active regions.

When ground buckles, just think of Earth as being at a rolling boil

The earthquake that shook southern California was a byproduct of processes that keep the Earth from literally melting, says Max Wyss, a geology professor at the University of Colorado at Boulder.

``The heat in the center of the Earth is the source of energy for earthquakes,'' he says.

The heat is generated largely by the long-term radioactive decay of elements in the Earth's center.

The core of solid iron and the outer core of molten iron transfer their heat up to the mantle, made of semifluid rock. Material in the mantle circulates somewhat like water heating in a pot, where the hot water rises to the surface while the cooler surface water sinks to the bottom to be heated.

The Earth's crust is broken up into several large plates, which are pushed apart by this circulation as well as by the welling-up of material from the mantle, which then cools to form more crust.

The Pacific plate, which scrapes along the North American plate to form the San Andreas and its nearby faults, originates along the East Pacific Rise. This rise stretches from Antarctica north into the Gulf of California.

The Pacific plate is moving northwest at a rate of a few centimeters a year and is being shoved back beneath the crust along the eastern edge of Asia.

``The planet has found a form of equilibrium, a way to get rid of the heat generated by radioactive decay,'' Dr. Wyss says.

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