Geology May Have Been Tilted Against Westside : Faults: The location, coupled with outdated architecture, meant some buildings didn’t stand a chance in the earthquake.

Almost three weeks after the Northridge earthquake, nobody knows for sure why structural damage was highly concentrated in parts of Santa Monica, Hollywood and West Los Angeles.

But scientists, engineers and city officials cite a list of possible causes that can be summed up by a variation on the old real estate mantra: location, location, location. And, construction. Consider:

* Located 12 to 15 miles south of Northridge, the Westside areas are as close as anywhere in the Los Angeles Basin to the epicenter of the quake, which whipped through the ground in a north-south direction.

* The Westside communities are perched atop relatively thin, weakly compacted layers of sand and gravel at the junction of flatlands and the hills above Santa Monica and Hollywood, a zone more susceptible than most to geological trauma.

* They are crossed by the Santa Monica and Hollywood faults, linked to a system that stretches west to Malibu. These faults did not move during the recent quake, but they may have focused some of the energy from the Northridge temblor.

* Many of the area’s 30- to 40-year-old apartment buildings sit atop posts that rise from wide-open, ground-floor garages that offer little earthquake resistance. And a number of buildings dating from the early years of the century are made of unreinforced brick.

One suspect in the jolting--the Santa Monica Fault--was recently remapped by Caltech scientists, who traced its tracks from the greenswards of the Los Angeles Country Club to the beach at Potrero Canyon. One dramatic clue is the steep slope topped by the Los Angeles Mormon Temple in Westwood.

With traces no more than 100 feet wide at its eastern end, the fault generally follows Santa Monica Boulevard until the city boundary, then widens to 900 feet across as it angles northwest, according to a study by James F. Dolan and K.E. Sieh.

They classified it as an active fault, based on signs of movement within the last 11,000 years. Earlier studies, which described a set of parallel north and south branches bracketing the path suggested by the Caltech geologists, said the fault was no more than potentially active.

One of three underground formations that could threaten Santa Monica and surrounding areas, it is the only one that actually lies within city limits, underlying commercial streets and residential neighborhoods alike.

“Most of the city was built by the time they knew they had a fault,” said Eldon M. Gath, a geologist who prepared a seismic safety update for the city in 1992.

If the fault does move, it could wreak havoc with emergency evacuation routes, sever gas and water mains and threaten police, fire and medical care facilities, the report found.

“The thing we stressed big-time (in the seismic update) was public education,” Gath said.

He said that official adoption of a fault hazard zone could pose economic problems for property owners when they seek to sell or refinance their property.

Still, he said, “Let’s get people aware that they’re living in a hazardous zone, and let them be attuned to reinforcing older houses and apartment houses.”

Although many of the seismic report’s conclusions were included in a master environmental assessment released by the city in September, the call for a public information campaign was not.

“It’s still under review,” said Kate Vernez, a spokeswoman for the city.

The Hollywood Fault might have played a similar role in focusing the earthquake’s energy toward the surface in Hollywood, geologists said.

According to Thomas L. Henyey, executive director of the Southern California Earthquake Center, the geology between the earthquake’s epicenter and the damaged areas of Santa Monica and Hollywood was another likely factor.

“When you’re on the edge of a basin . . . a lot of damage is often on the periphery,” he said. “Transitional zones between basins and mountains are often areas where (seismic) waves are amplified.”

Liquefaction--a phenomenon in which seismic forces mix soil and water into an unstable soup--was probably not to blame for the damage in Santa Monica or Hollywood.

But the phenomenon was almost certainly to blame for the collapse of the freeway bridge across La Cienega Boulevard, Henyey said, noting that the street’s very name means “ the swamp “ in Spanish.

The ground under the damage zones is largely made up of sand and gravel deposited by ancient rivers, according to Caltech’s Dolan, whose findings on the Santa Monica Fault were published in 1991.

Similar deposits underlie the area where a medical building crumpled at Olympic Boulevard and Barrington Avenue, and parts of Hollywood, where the earthquake left 6,000 homeless and 257 buildings severely damaged.

Washed down from the Santa Monica Mountains and the Hollywood Hills, these so-called alluvial deposits form fan shapes that are thinner at the base of the hills. Given a good dose of seismic energy, they can transmit earthquake waves to the surface with disquieting speed.

According to readings from a sensor at Santa Monica City Hall, the Jan. 17 earthquake peaked with a horizontal jolt that amounted to 93% of the force of gravity. Hollywood reported 41%, while nearby Century City registered no more than 27%.

“It’s not as good as rock but not as bad as swamp,” Dolan said of the lightly compacted alluvial deposits that are common to the Hollywood and Santa Monica quake-damage zones. “We don’t know a lot about why damage is localized. This is going to be a very, very educational earthquake for us in those terms.”