Unique Geology Key to Quake’s Uneven Impact

The titanic energy of the magnitude 6.6 Northridge quake was reflected like sunlight through a prism of hidden rocks and soft sediments to violently shake some areas far from its epicenter in the San Fernando Valley, while leaving closer neighborhoods unscathed, scientists said.

In its path, the largest earthquake in the basin’s modern history left behind a puzzling seismic patchwork of shattered communities, often separated by areas where the sharp shudders Monday did no more than disturb sleep and fray nerves.

Geophysicists now believe that shock waves within the San Fernando Valley crisscrossed like ripples in the caldron created by the high, hard rock of the surrounding mountains. Where the crests of the waves intersected, the power of the ground shocks was intensified. Where the troughs of the waves met crests, their energy was canceled.

As the shock waves spread, the Los Angeles Basin also served as a lens to capture and focus the quake’s force, extending the duration of the shocks.

The combination was enough to make one side of Pasadena shake three times harder than another, strong enough at the interchange of the Santa Monica and San Diego freeways to hurl cars in the air, according to sensors maintained by the state Division of Mines and Geology.

The direction along which the fault ruptured also may have helped aim Monday’s tremors more directly toward the Westside of Los Angeles and Santa Monica, geologists said. There, dozens of buildings were severely damaged and a section of the Santa Monica Freeway was smashed, even though they were more than 15 miles from the epicenter.

“There was something happening in the West L.A. area which was causing an amplification of the ground motion higher than the adjacent areas,” said Wilfred D. Iwan, the Caltech earthquake specialist who is chairman of the California Seismic Safety Commission.

“There is a fairly wide variation in the intensity of the ground shaking even within a small area several miles around the epicenter in the Valley,” he said.

Scientists still are trying to determine precisely which thrust fault slipped. They do not know which way the rupture progressed either, from east to west, or from top to bottom--all of which could affect the direction and intensity of the temblor’s force.

Experts caution that there is no hard formula to calculate the immediate intensity of the ground shaking caused by a single earthquake. Each stamps the landscape with a unique imprint. Although a nuclear explosion radiates its deadly energy equally in all directions, an earthquake’s power can be dissipated safely or channeled into even fiercer intensity, depending on factors ranging from geophysics to the makeup of dirt.

“The fault plane is not complicated. What we do know, however, is that . . . thrust earthquakes radiate their energy in some unexpected ways,” said Jim Mori, head of the Pasadena field office of the U.S. Geological Survey.

Seismologists were caught off guard by the raw force of the ground motion generated by the Northridge quake, which yielded some of the highest ground-acceleration readings ever recorded in the area--motion strong enough to lift a building off its foundations and drop it like a handkerchief.

“It is like two pieces of bad news: Not only was it a large earthquake, it was an unusually strong one,” said USGS geophysicist John Boatwright in Menlo Park.

At some locations in the San Fernando Valley and on the Westside, the shaking intensities were even higher than those produced by the magnitude 7.6 Landers earthquake of 1992, sensors showed.

“I was very surprised that the ground motion was so big to the south of the epicenter and to the west,” said Hiroo Kanamori, director of the Caltech Seismological Laboratory.

“The earthquake’s magnitude is not a very good predictor of the actual ground motion we will feel,” he said.

In the desperate seconds of a major earthquake, the power of ground motion--the shudders that throw people to the floor and even topple buildings--is above all a matter of frequency, a low pitch to which people and the structures they inhabit naturally vibrate, like bells spontaneously responding to a tuning fork.

If the waves are short and fast--of high frequency--the earthquake’s energy may radiate harmlessly through buildings and other structures. But if the shock wave slows sufficiently--down to 10 hertz or less--its frequency dips into the ranges to which houses and high-rises will respond.

“What you really are feeling when you feel your house shake is the frequency of the building,” one seismologist said. “Other frequencies go through the building and don’t affect it.”

And when it comes to frequency, local soil conditions can mean the difference between safety on one side of the street and calamity across the way. That is what geologists call the “site effect.”

“The implication is that there may be something about a local site that amplifies ground shaking, independent of the source of the seismic shaking or how far away it is,” Iwan said.

In the Valley, sensors at a firehouse on Nordhoff Avenue recorded the ground accelerating during the earthquake at about one-third the force of gravity, while farther away from the epicenter at Sylmar Hospital, three miles east of the interchange of the Antelope Valley and Golden State freeways, sensors measured horizontal ground motion almost three times stronger.

The measurements taken in Tarzana, closest to the epicenter, showed even larger ground acceleration, almost twice the force of gravity--”enough to knock your socks off,” Iwan said.

“Anything in excess of one G would mean that something would actually hop off the ground,” Iwan said.

Iwan and other earthquake experts believe that part of the variation is in the difference between a house that sits on rock and one on fill dirt.

“Motion is amplified by soil,” said Boatwright of the Geological Survey.

As the shock waves move from hard, dense rock, such as granite, to more loosely compacted sedimentary rocks, the waves slow down. As they move from those rocks into gravel or dirt, they slow even more and their amplitude--the size of the waves--increases.

“When you have sedimentary basins . . . the energy gets trapped and bounces around for a longer period of time. The San Fernando Valley is a very good example of a place where the seismic energy becomes trapped,” Boatwright said.

Loosely compacted alluvial soil in the San Fernando Valley greatly enhanced the damage in the 1971 Sylmar quake, contributing to the collapse of the San Fernando veteran’s hospital.

Shock waves from the quake’s epicenter traveled through the hard rock of the San Gabriel Mountains at 35,000 feet per second. But when the waves hit the looser soil in the north Valley, they slowed to 12,000 feet per second and concentrated the energy, records show.

Where land is composed of loose fill, underlaid by a high water table--as in the Marina del Rey, Oxnard and Long Beach areas and San Francisco’s Marina District--shock waves can instantly transform solid land into quicksand.

The Marina District was devastated in the 1989 Loma Prieta earthquake, even though it was 70 miles or more from the epicenter, when its solid foundations became fluid. In the 1906 San Francisco earthquake, such soil liquefaction ruptured water lines, allowing fires to sweep through the city unchecked.

So far, no evidence has been found that soil liquefaction caused the damage on the Westside.

But even compacted sediments or sandy soil, such as that underlying some of the Westside and much of the Valley, can slow an earthquake’s shock waves to the frequency that will shake a building apart, earthquake experts said.

State seismic hazard maps indicate that the land under the collapsed Santa Monica Freeway interchange at La Cienega is former swampland and fill.

“I am sure the site effect was very important in the case of the highway,” Kanamori said.

As scientists better understand the way an earthquake’s energy is distributed block by block, the knowledge could have far-ranging implications for zoning, building codes and real estate values, earthquake experts said.

“The scientific understanding is so imperfect now that we are not able to differentiate one block from the next,” Iwan said. If they tried, he added, “we would create either a false sense of security or a false sense of panic.”

Times staff writer Kenneth Reich contributed to this story

* RELATED STORIES: A2-A5, A33, B1-B4, B6, B8, B15, D1, F1, F3

Liquid Earth

Earthquakes are typically measured by magnitude, which may not be the best indicator of how powerful local shaking may be. The violence of earth movement is determined by many factors, including soil type, local terrain and surface irregularities in the fault itself. These variables help explain why in Monday’s 6.6 quake, homes a few miles from the epicenter lost only their chimneys while massive sections of a freeway collapsed 20 miles away.

ON SHAKY GROUND

Shaded area shows how much of the metropolitan area could be subject to failure because of soil conditions, including liquefaction. Ground motion in such areas is magnified, so damage could be substantial even in a moderate quake.

Measuring stations around Southern California recorded the intensity of ground motion at key sites during the 4:31 a.m. quake. The sensors measure the force of the motion by recording the acceleration caused by the quake. The motion is measured in terms of the force of gravity. One ‘g’ is strong enough to make unsecured buildings or vehicles literally hop off the ground.

Woodland Hills: 1.82g

Sherman Oaks: 0.90g

Sherman Oaks: 0.59g

Santa Monica: 1.00g

Hollywood: 1.61g

Inglewood: 1.21g

Universal City: 0.66g

El Segundo: 0.25g

Burbank: 0.79g

Los Angeles: 0.19g

Liquefaction

Liquefaction occurs in areas of loosely packed, fine-grained soil that is saturated by ground water. The particles of soil move freely, lubricated by the water, and with repeated shock waves take on the characteristics of gelatin or liquid. Where ground water may not be a factor, the uncompacted soil tends to amplify shock waves and intensify local shaking.

Shock Waves

Trapped Energy: In a sedimentary basin like the San Fernando Valley, shock waves radiating from an earthquake’s epicenter can be trapped by harder surrounding rock and reflected back on themselves.

Amplification: Where the crests of the shock waves intersect, the power of the earthquake is effectively amplified; where the crests intersect a trough, the power is canceled.

Sources: “Earthquakes and Geological Discovery” by Bruce A. Bolt, 1993; Los Angeles Times

Researched by VICTORIA McCARGAR, ROBERT LEE HOTZ