New Quake Maps Show Heavy Shaking Far From Epicenter : Seismology: Mercalli scale, based on interviews, reveals a widespread, irregular pattern of the Northridge temblor’s effects.

The most definitive maps on the Northridge earthquake yet released by official sources show that comparatively heavy shaking was experienced far from the epicenter of the earthquake and in the same irregular patterns found closer in.

The maps from the National Earthquake Information Center in Golden, Colo., use the Mercalli scale--which is based on interviews and damage observations--to show how severely the quake was felt in Southern California and Nevada. The Mercalli scale uses Roman numerals from I to XII to express shaking intensity.

Some level V shaking--described as capable of causing minor, mostly non-structural damage--occurred in the Imperial Valley, Santa Maria, Fresno and Laughlin, Nev., as far as 200 miles from the Northridge epicenter.

The maps also show that the intensity of the shaking was lower in some places closer to the epicenter.

In the San Fernando Valley and other parts of Los Angeles and Ventura counties where heavy damage occurred, the intensity level reached IX. The worst shaking occurred in parts of Northridge, Reseda, the Newhall Pass, Sherman Oaks, Santa Monica and the West Adams district of Los Angeles, the maps show.

Other areas subject to higher shaking than nearby communities included Manhattan Beach, Hermosa Beach and Redondo Beach and, in Ventura County, Oxnard and Fillmore.

The maps are preliminary and subject to change, cautioned Jim Dewey, the research geophysicist who coordinated the project for the Earthquake Information Center, a branch of the U.S. Geological Survey.

They are based on 4,000 telephone interviews conducted by a polling team at Humboldt State University and 700 mail inquiries by the Geological Survey.

Unlike the single magnitude reading, which is used to characterize an entire earthquake--6.8 in the case of the Jan. 17 Northridge quake--Mercalli scales give a picture of a quake as it was felt in various areas.

The scale--used to describe how people felt a quake, how frightened they were and what damage was sustained--was developed by Italian seismologist Giuseppi Mercalli in 1902. Its definitions were modified by American seismologists Harry Wood and Frank Neumann in 1931.

In areas of level IX shaking, for instance, the definitions say there is general panic, spectacular partial collapses of modern buildings, widespread destruction of wood-frame apartments and collapses of elevated highways. All of these phenomena have been verified by the telephone and mail surveys.

Lori Dengler, the Humboldt State earthquake scientist who has been coordinating the telephone survey, said about 20 interviews averaging five minutes each were conducted in each Southland community sampled.

She released information on Friday showing that 100% of those surveyed in Los Angeles, Santa Monica, Palmdale, Long Beach, Ventura and Barstow said they were awakened by the Northridge quake at 4:31 a.m. Jan. 17.

In Santa Monica, 79% of residents sampled said they had suffered damage in their households. The damage figure was 19% in Long Beach, 20% in Ventura, 14% in Pomona and 20% in Santa Ana.

In Las Vegas, 36% of residents sampled said they had been awakened by the earthquake and 3% said that some items had fallen off shelves. In San Diego, 60% of the sample said they had been awakened and 2% reported damage.

Dengler said tables have yet to be developed for the San Fernando Valley, where telephone interviewing will continue this weekend.

Dewey, at the Earthquake Information Center, said that reactions to the telephone calls and the mail inquiries contain “a strong component of individual perception” and that the Mercalli scale is therefore subjective.

“But we ask a bunch of questions of the person who’s responding,” he added. “How they perceived the quake, whether they were awakened or frightened, whether they had difficulty in maintaining balance or walking and how they would qualitatively describe the shaking, whether light, moderate or strong.”

Shaking was higher near the epicenters of past earthquakes in the Yucca Valley, the Tehachapi Mountains and along the potentially dangerous San Jacinto fault near Hemet. “It wouldn’t be a single previous quake,” he explained. “It would be faults that have been active for millions of years.”

It is also possible that people living near the sites of big quakes, like the Landers earthquake, might be more sensitive to ground movement and inclined to perceive the shaking from Northridge in different terms, he added.

While Dewey said he considers what has been obtained from the survey “a pretty good sample,” he added that the interviews and questionnaires will go on for some time.

The U.S. government has been collecting information about significant earthquakes for Mercalli maps since 1928, “and, in fact, one of the values of the intensity data is that it provides a means of comparing earthquakes of the present day, for which we have instrumental data, with earthquakes that occurred before present instruments were developed,” Dewey said.

Patterns of Destruction

One of the intriguing characteristics of the Jan. 17 quake was the irregular pattern of damage. The Mercalli scale measures shaking intensity based on accounts of eyewitnesses who classify local damage according to levels detailed below. These preliminary scales, showing the damage it did throughout Southern California and parts of southern Nevada, demonstrate that such areas as the Imperial Valley, Laughlin, Nev., and towns close to Fresno and Santa Maria had considerably more shaking than other nearby places. There also appears to have been somewhat higher than average shaking in the vicinity of previous big quakes, such as the 1992 Landers temblor.

II-IV: Intensity II is felt indoors by a few people, especially on upper floors. III is felt by some people as a rapid vibration. IV is felt indoors by many, outdoors by few. Vibration like that of a heavy truck passing. Frightens no one.

V: Frightens a few. A few small objects overturned and fallen; a few items thrown from store shelves; hairline cracks in interior walls; a few cracked windows, respondents report difficulty in standing up, walking or maintaining balance.

VI: Frightens many. Some broken windows; a few instances of fallen plaster or damaged unreinforced masonry chimneys; many items fallen from shelves.

VII: Frightens everyone. Damage to unreinforced masonry buildings; unreinforced masonry chimneys fallen or broken at the roofline; some masonry walls partially or completely fallen.

VIII: Alarm approaches panic. Considerable damage to unreinforced masonry buildings with partial collapse; old wood-frame houses shifted off foundations; widespread damage to wood-frame apartment buildings with some destroyed; significant damage to reinforced, lined, masonry chimneys on houses; some structural damage to reinforced-concrete structures.

IX: General panic. Spectacular partial collapses of modern store building and office buildings; widespread instances of destroyed wood-frame apartment buildings; collapses of elevated freeway sections.

Fresno area: V

Santa Maria: V

Laughlin: V

Barstow: V

Yucca Valley: V

Imperial Valley: V

Such widely scattered communities as Sherman Oaks, Santa Monica and the West Adams district suffered shaking of the same severity as epicentral Northridge and Reseda. The readings also reveal unexpectedly intense shaking in areas as far away as Seal beach and Oxnard, about 50 miles from the epicenter.

Seismic Spread

Although California is renowned for seismic drama, its fractured geology actually tends to limit the scope of earth movement in a big quake, as crustal cracks dampen the shock waves. That is not the case in the ore solid eastern half of the United States, as these historic examples show. Shaking is indicated by Mercalli readings.

VII or greater: San Francisco, 1906

VII or greater: Northridge, 1994

VII or greater: New Madrid, Mo., 1811-12

VII or greater: Charleston, S.C., 1886

Researched by KENNETH REICH and VICTORIA McCARGAR / Los Angeles Times

Sources: Jim Dewey, National Earthquake Information Center; Lori Dengler, Humboldt State University; “Earthquakes and Geological Discovery,” by Bruce A. Bolt; “Earthquake,” by Bryce Walker.