Rocks Affect Quake Wave Patterns, Study Says : Seismology: Scientists believe that formations along faults determine why locations far from a temblor’s epicenter may shake more than the area of origin. They say research could apply in L.A.

Two UC Berkeley seismologists who studied why San Francisco and Oakland felt heavy shaking in the Loma Prieta earthquake despite considerable distance from the epicenter say that rock formations beneath the Los Angeles area could produce the same effect here.

In the Loma Prieta quake, seismic waves from the epicenter in Santa Cruz County bounced off underground rock formations and caused more severe shaking in San Francisco’s Marina District than in some areas much closer to the quake’s origin, the researchers said.

“Los Angeles has considerable variation in crustal formation that will swing waves in certain ways, depending on where earthquakes occur,” said Bruce Bolt, a past director of UC Berkeley’s seismographic station. “That can be calculated. It hasn’t been done yet, but it can be.”

Bolt said computers could determine in advance which parts of the metropolitan area would be shaken most heavily during quakes along distant faults, including the San Andreas, which passes north and east of the San Gabriel Mountains.

He said it might be discovered that waves from a quake at the foot of the Hollywood Hills could cause strong shaking in certain areas of the Los Angeles Basin.

In the Loma Prieta earthquake, San Francisco’s Marina District and areas across the bay in Oakland sustained severe damage, while many areas closer to the epicenter in the Santa Cruz Mountains received less damage.

Bolt and fellow seismologist Anthony J. Lomax were able to show in their study how the alignment of rocks on each side of the San Andreas Fault had caused severe shaking in a region between San Francisco and Oakland about six miles wide and nine miles long.

This area--which included the Nimitz Freeway section in Oakland that collapsed and killed 41 people--was shaken twice as long and hard as other areas nearby. Liquefaction of the ground contributed to the damage, especially in the Marina District and near the Oakland Airport, but the intensity of shaking was also responsible.

Bolt and Lomax’s study also indicated that in some cases the seismic waves bounced off the top of Earth’s mantle 60 miles beneath the surface, then converged with horizontally directed waves being channeled northward by rock formations on each side of the San Andreas Fault. This amplified the strength of the waves that reached some parts of San Francisco and Oakland.

Seismic energy waves that might otherwise have gone harmlessly out to sea or into sparsely populated farmland were diverted into heavily populated urban areas, the researchers said.

Bolt said other factors--such as the direction in which the fault rupture occurs--may influence where the strongest shaking occurs. But he said computer models can be constructed that will show a great deal about where damage will occur.

The information could influence building code requirements in areas where heavy shaking is expected.

Bolt said one of the most dangerous active faults is the Newport-Inglewood Fault, which sweeps through built-up areas from Newport Beach through Long Beach, Gardena and Inglewood to as far north as Culver City. This fault moved in the 1933 Long Beach quake, killing 120 people.

By examining rock structures, Bolt said, it could be determined what other areas might be shaken in a strong quake along that fault.