May not be a long wait for the real Big One

By Keay Davidson
EXAMINER SCIENCE WRITER
Sunday, October 17, 1999
©1999 San Francisco Examiner
URL: http://www.sfgate.com/cgi-bin/article.cgi?file=/examiner/hotnews/stories/17/quake_sun.dtl

Ten years have passed since the Loma Prieta earthquake, and since then, scientists and structural engineers have learned a great deal about Earth's shivers and shakes.

What they have learned by programming high-speed computers, mapping fault lines with satellites, lowering instruments into deep holes and even digging trenches across golf courses is that a far worse quake could strike at any time, killing 5,000 Bay Area residents, injuring tens of thousands more, and turning much of the region into a shattered, smoking ruin.

Bitter lessons from Loma Prieta and other quakes ranging from recent seisms to ancient ones that have been unearthed like the bones of Tutankhamen have sent shocking messages, among them:

***The 21st century is likely to bring a renewal of quake activity in California, as the coastal crust, straining from friction between the Pacific and North American geological ""plates,'' slips and buckles in order to unleash the tension. (The plates are thick, creeping slabs atop which the continents ride, like logs stuck in ice.)

***Steel-frame buildings, once thought to be near-invulnerable, might topple during a quake.

***The worst killers may be faults we can't see: "blind thrust faults," which lurk like sharks be neath the pastoral California land scape. A blind thrust fault caused the severe Northridge quake of 1994.

***Coastal California could be damaged by tsunamis generated by major quakes in the Pacific North west, recent geological research in dicates.

On the bright side, scientists have learned much about earth quakes in the last decade. They've dug trenches that allow them to estimate how often given faults quake; they've tracked the wayward crust's creepings, millimeter by millimeter, using satellite observations; and they've probed subtle ground motions using "strainmeters" lowered 1,000 feet into the Earth. With luck, they say, such new technology might better their ability to foresee future terrestrial rumblings.

Many Bay Area residents seem to think that because they survived the Loma Prieta quake of Oct. 17, 1989, with nary a scratch, they'll dance unscathed through the next temblor.

What they forget, seismologists warn, is that the Loma Prieta quake's epicenter was in the Santa Cruz Mountains, far to the south. If the epicenter had been directly under the Bay Area, the damage and death rate might have been unimaginably worse. Over the last decade, "we've learned how damaging "moderate' earthquakes can be," warns Lloyd Cluff, one of the most respected geoscientists in the Bay Area. The epicenter of the Loma Prieta quake "was 100 kilometers (about 60 miles) away from San Francisco and Oakland -- and look at all the damage it did!"

Predicting quakes

And -- here's the really scary news -- such a "local" quake is likely some time during the next 30 years, according to the USGS' long-awaited "probabilities" report, issued Thursday at the fall conference of the Association of Bay Area Governments.

Between now and 2030, the Bay Area faces a 70 percent probability of "one or more damaging earth quakes" akin to the Northridge quake of 1994, quake expert David Schwartz of USGS warned at the meeting.

"Once you get over 50 percent, that's high," says Cluff, who co-chaired the prior probabilities study in 1990 but wasn't directly involved with the latest one. He personally thinks the Bay Area figure is "somewhere between 70 and 80 percent" over the next 30 years.

Up to 5,000 people would die in a magnitude 7.0-quake whose epicenter is in San Francisco or the East Bay, Cluff estimates. "With a magnitude 7.0 or greater on the Hayward (fault), you can have over $100 billion damage," Cluff warns.

Some of the most unsettling lessons came from the Northridge quake of 1994, which was unexpected because it struck on a previ ously unknown fault -- a "blind thrust fault" -- in the Santa Monica Mountains. Since then, geologists have found a number of other blind thrust faults, including one under Mount Diablo, which show no surface "expression" such as valleys.

Even more shocking is what the Northridge quake did to presumably sturdy buildings. After that quake, structural engineers were stunned to discover that it had badly damaged buildings they assumed would have resisted a temblor: steel-frame sky scrapers. Fractures were discovered on the connection of the columns to the beams, recalls Bill Holmes, a structural engineer at the engineering firm of Rutherford and Chekene in San Francisco.

Tough lessons

This "was definitely a surprise. ..... I don't think anybody could have guessed that would occur," Holmes says. "There were hundreds of buildings (in the North ridge area) which had one or more of these failures."

Scientists have learned a lot about earthquake behavior since Loma Prieta. One of the most important in novations in quake science since the 1970s has been the digging of seismic "trenches." They are long and narrow, roughly 10 feet deep. Geologists scan the trench's dirt wall for geological signs (say, evidence of a sudden incursion of beach sand) of past quakes, then date them.

In this way, they estimate how frequently a given fault is likely to quake. USGS has funded numer ous quake trench digs in the Bay Area in the last few years, even in an El Cerrito golf course.

Seismologists also have received help from above -- in the aero space, rather than religious, sense. "One of the major develop ments has been the increased use of GPS satellites that have made it possible to more accurately and easily measure crustal motions," says Tom Yelin of the USGS office in Seattle. "These motions of the Earth's crust are what place stress on the major faults and eventually lead to earthquakes."

Scientists hope to forecast future shakes by checking for imper ceptible variations in fault motion registered by gizmos called down hole (or borehole) strainmeters. Since 1992, when the first device was lowered into the ground in Garin Park near Cal State Hay ward, six more have been inserted into the ground -- typically 600 to 1,000 feet -- around the Bay Area. Agency officials hope to install an other 10 in the next several years, says USGS research geophysicist Malcolm Johnston.

The strainmeters are so sensitive that they can detect vibrations from pounding surf on the coast and crustal changes caused by the movement of high- and low-air pressure weather cells above ground, says seismologist Al Lindh of USGS.

The strainmeters are also "the best hope for earthquake predic tion because ..... if there are (crust al) changes in the Earth in the days and weeks preceding earthquakes, these (instruments) are the only hope to see it," Lindh says.

Still, quake-casting has been complicated by the growing fear that a quake on one fault might trigger quakes on adjacent faults, just as shaking a twig on a Christ mas tree may make the whole tree shake. The potential result: a cata clysmic super-quake across an en tire region.

That was one of the biggest shocks of the Landers earthquake of 1992 in the Mojave Desert, Cluff recalls. "We saw not just multiple seg ments but different faults rupture all together. ..... That was a big surprise to us." Trouble up north

Even far-off quakes could en danger Northern California. Recent evidence indicates that far to the north, the Pacific Northwest could sustain a horrendous quake in the foreseeable future. The quake could occur along the Northwest coast's "subduction" zones, where geological plates are diving, or subducting, into Earth's crust. The discovery that the Pa cific Northwest faces such mega- quakes is especially unnerving be cause previously, Yelin says, "there were many geologists and seismol ogists who speculated that subduc tion may have come to a halt."

It hadn't, as geological research has revealed. A resulting mega- quake could unleash a tsunami down the Pacific coast, judging by geological and historical evidence gathered over the last decade. Dur ing such a tsunami, the Northern California coast could be pounded by waves an average of 30 feet high, Cluff says.

The Pacific Northwest Fault "is a very likely fault to go off in the future ..... most likely in the next 30 years, because the recurrence (in terval) on that (fault) is probably 300 to 350 years," Cluff says. The fault last ruptured in 1700, so con ceivably, it might shake again be tween 2000 and 2050.

All in all, it's a strange time for earthquake forecasters, who have both much to lament and to cheer. On one hand, the last decade has dealt a bitter blow to their old dream of predicting quakes with precise timing -- say, with an ac curacy of a few days. Their long quest for earthquake "precursors," ranging from crustal emissions of gas to subtle shifts along fault lines to weird variations in the local magnetic field, has been frustrated by inadequate evidence.

On the other hand, many ex perts feel that the recent cataclys mic earthquake in Turkey was foreseen by two scientists at the U.S. Geological Survey in Menlo Park and a Turkish colleague. Ross S. Stein, James H. Dieter ich and Aykut A. Barka did so by noting that since 1939, quakes have moved from east to west along the North Anatolian Fault -- like falling dominoes -- and the Izmit region looked like the next target.

Sure enough, in mid-August a catastrophic seism hit a few miles south of Izmit, killing more than 17,000 people. Also, quake scientists are begin ning to develop credible-looking three-dimensional models of fault behavior in the Bay Area, Lindh says. "It used to be the computer models weren't worth a damn," he said. "Now the computer models really are worth a damn -- and it's important to get more (geological) data to (improve) them."

©1999 San Francisco Examiner