Quakes Moved L.A. Basin--Literally : Seismology: Scientists say ground shifted northwest about half an inch. Land closer to the temblors in Landers and Big Bear was displaced as much as five feet.

The Los Angeles Basin moved about half an inch to the northwest as a result of the June 28 Landers and Big Bear earthquakes, according to calculations using satellite readings and sensitive ground-based instruments.

More dramatic shifts were detected closer to the earthquake epicenters by scientists from NASA’s Jet Propulsion Laboratory, Caltech and the Lawrence-Livermore National Laboratory. San Bernardino moved about two inches to the northwest and Palm Springs about four inches to the north-northeast.

Close to the surface rupture caused by the magnitude 7.5 Landers earthquake, the terrain shifted five feet or more, scientists say.

The calculations are believed to be highly accurate because they are based on satellite observations made through the Defense Department’s new Global Positioning System and the elaborate TERRAscope network of seismological digital-recording instruments operated by Caltech and the U.S. Geological Survey.

Ken Hudnut, a Caltech geologist who helped calculate the shifts, said observations of surface faulting and measurements at pre-established survey points have confirmed a computer model prepared by Hiroo Kanamori, director of the Caltech Seismological Laboratory.

In a separate calculation, Orange County Surveyor John Canas said his office resurveyed 25 architectural monuments within the county and will release its report soon. He said it will show that Orange County has moved about an inch to the north.

As befits the complicated nature of the twin earthquakes, the movements are complex. Generally, the land to the east of the 45-mile Landers surface rupture moved to the south or southeast, while the ground to the west of the rupture zone moved northwest.

But the movement on each side of the buried, unnamed fault involved in the magnitude 6.6 Big Bear earthquake contradicted the directions of movement in the Landers earthquake that occurred three hours earlier. As a result, the land to the immediate east of the Big Bear fault line moved to the northeast, while that to the west moved to the southwest.

In the Big Bear area, the shifts occurred over comparatively short distances from the fault line. The zone was far larger for the Landers quake, the third most powerful temblor in California this century.

Further confirmation of the findings by Hudnut’s team--which included Shawn Larsen of the Lawrence-Livermore lab--has come from scientists Frank H. Webb, Geoffrey Blewitt and several colleagues at the Jet Propulsion Laboratory in Pasadena.

Through a “constellation” of 17 Defense Department satellites capable of collecting data every 30 seconds, the JPL scientists have been able to pinpoint the pre- and post-earthquake locations of three Southern California signal stations operated by the National Aeronautics and Space Administration--Goldstone northeast of Barstow, Pinyon Flats south of Palm Springs and JPL.

Webb said as a result of the June 28 earthquakes, JPL moved nine millimeters to the north and six millimeters to the west. This is the equivalent of about half an inch of movement to the northwest. Shifts would be about the same at other locations in the Los Angeles Basin, he said.

The Pinyon Flat station moved 47 millimeters to the north and 21 to the east, mirroring the northeast movement of Palm Springs, Webb said.

Hudnut said that for most of Southern California, the land overlying the Pacific tectonic plate zone generally moved north, while land overlying the more interior North American plate zone generally moved south, as traditional tectonic theory for this region would indicate.

Tectonic plates are large land masses that float on the molten core of Earth. Their meeting grounds are often associated with volcanic or earthquake activity.

But Hudnut said it is a mistake to see the plate boundaries as crisp geographic lines. Instead, they are complicated zones that may be hundreds of miles wide.

“If you stood astride the San Andreas Fault, you wouldn’t have one foot on the North American plate and one on the Pacific plate,” the geologist said. “In order to stand on the two plates, you’d have to have one foot in Las Vegas and another in the Channel Islands. The boundary is a lot of active faults across a very broad zone.”

Over millions of years, Los Angeles, according to some tectonic theorists, will move north, while Reno, for example, will move south. Other theorists believe that the plate boundaries are changing and that the region’s main fault lines are shifting. Over time, those scientists say, the orientation of the San Andreas could change dramatically.

Webb said there was enough movement from the Landers and Big Bear earthquakes that it would be a good idea to resurvey the San Bernardino-Riverside county line, which runs in the quake area between the Coachella and Morongo valleys.

However, the idea has apparently been rejected.

Gerald Stayner, the Riverside County surveyor, said he and San Bernardino County Surveyor Larry Cotton decided that such a survey, which would cost $5,000 to $6,000 a mile, would be too expensive.

Stayner and Canas said it would be better to survey those areas close to the Landers faults that moved by feet rather than inches. Across the fault lines, properties may have slid by each other.

According to the state’s Cullen Earthquake Act, passed after the 1971 Sylmar-San Fernando earthquake, new boundaries resulting from earthquake-generated land movements are litigated on a case-by-case basis.

When the Earth Moved As a result of the magnitude 7.5 and 6.6 earthquakes June 28, parts of Southern California shifted as much as five feet, according to Caltech scientists studying the temblors. Even the Los Angeles Basin moved half an inch.

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(A) Big Bear quake: In the immediate vicinity of the unnamed fault, the east side moved northeast and the west side moved southwest.

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(B) Landers quake: In general, the west side of the Landers quake fault moved northwest while the east side moved southeast.

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Source: Ken Hudnut, Caltech and Shawn Larsen, Lawrence-Livermore National Laboratory.