SCIENTISTS FIND CHINKS IN EARTH'S CRUST MEANS ... LOWER QUAKE DANGER.

Byline: David M. Drucker Sacramento Bureau

SACRAMENTO - The San Fernando Valley likely will not suffer earthquakes stronger than those that were centered in Sylmar in 1971 or Northridge in 1994, according to a new research report published in the journal Geology.

Using advanced measurements of magnetic fields on sediments and other information, seismic researchers at Oregon State University found that the earth's crust in the Valley and the Santa Clarita Valley to the north is broken up into blocks rather than being in a single piece as it is in the San Gabriel Valley and areas of eastern Ventura County.

The significance is that when the plates move, causing a quake, the smaller blocks of crust don't unleash as much energy as a single large tectonic plate. So expectations of the potential for more severe quakes in the Valley and north county are unfounded, according to the Oregon State researchers, geology professor emeritus Robert Yeats and fellow seismic scientist Shaul Levi. ``If you're in the San Fernando Valley, you live in earthquake country, but you don't live in 7.2 to 7.5 country,'' said Yeats, also a consultant for Earthquakes International in Tustin and a member of the Southern California Earthquake Center.

Levi, a paleomagnetist and professor emeritus in OSU's College of Oceanic and Atmospheric Sciences, was lead author of the study and Yeats and John Nabelek, a seismologist in the OSU college, were co-authors.

Although there's no telling how many earthquakes might hit the San Fernando Valley in the coming years, none is likely to be much larger than the 1994 Northridge temblor that registered 6.7 on the Richter scale or the Sylmar one that measured 6.5, the study published this week concluded.

Yeats studied the aftermath of the Sylmar Quake and many other Southern California temblors, examined the history of earthquakes in the region and reviewed geological samples of fault lines, where separate blocks within the tectonic plates that make up the earth's crust rub together to create the friction that leads to shakers.

While Yeats said the Valley is unlikely to experience anything larger than a 6.9-magnitude quake in the future, other scientists warned that even if the study proves true, it doesn't mean damage won't be severe.

``If you're trying to reassure the individual,'' this won't do it, said Lucy Jones, the scientist in charge of the U.S. Geological Survey's Southern California sector and chairwoman of the state Seismic Safety Commission.

``If you're right on top of a 6.7, it's just as bad as being on top of a 7.5.''

Jones didn't quibble with Yeats' credentials, and has yet to examine his study. But she is skeptical of his conclusions.

She said an 8.0-magnitude quake in another part of Southern California - especially over the hill in downtown Los Angeles or in the San Gabriel Valley - is still going to cause severe shaking in the Valley. Fault lines in the Valley also could link up with fault lines outside the Valley that are in the midst of a temblor larger than 6.9, effectively causing an earthquake of 7.0 or above to roll underneath the Valley.

``I have a problem saying 'can't,''' Jones said. ``I don't think we know enough.''

Yeats' study said the earth's crust near Saugus in the north county is broken into four blocks each about 10 kilometers to 20 kilometers in length and width.

The blocks in Santa Clarita and at the Van Norman Dam in the North Valley have not rotated. But the block under Six Flags California-Magic Mountain rotated clockwise 30 degrees and another block farther east, north of the Foothill Freeway, recorded a clockwise rotation of 34 degrees. The fault lines along these blocks are not long enough to allow for an earthquake above a magnitude-6.9, Yeats said.

``We'll continue to have (earthquakes),'' Yeats said. ``But we don't think they'll be as big as those happening in the San Gabriel Valley area or in the eastern Ventura basin in Piru. Those are much bigger faults.''

A key element of the study was the researchers' ability to use measurements of the magnetic-field orientations of the sediments and how they changed after quakes caused by collisions between the Pacific and North American plates. From the ages of the sediments and their magnetic orientations, they concluded that the blocks in the north county region have acted independently for 800,000 years.

More detailed knowledge of the crust structure will help disaster management planners make more informed decisions, the researchers said.

``When there aren't enough data,'' Yeats said, ``managers tend to prepare for the worst-case scenario in a general sense rather than what the worst case may be for a particular region. Even though our study area sustained two damaging earthquakes in recent decades, it is not expected to endure earthquakes as large as those anticipated to the east and west.''

On Jan. 17, 1994, the magnitude-6.7 Northridge Earthquake shook residents from their beds at 4:31 a.m., rumbling from 11 miles beneath the San Fernando Valley for more than 20 seconds.

The quake leveled or damaged an estimated 12,500 homes and businesses, inflicting $40 billion in losses. Fires raged. Freeways collapsed. Power, telephone and water service died. More than 20,000 homes went without gas, 48,500 without water.

When the dust settled, 57 people had died, nearly 12,000 were injured and thousands found themselves homeless.

In 1971, the magnitude-6.5 Sylmar Quake killed 62 people. The dead included 49 people killed at the San Fernando Veterans Administration Hospital after two main buildings crumbled.

Daily News wire services contributed to this report.

David M. Drucker, [phone]

david.drucker(at)dailybulletin.com