Earthquake: The Road To Recovery : Rumors Enhance Actual Size of Quake, Experts Say : Seismology: Confusion among different scales fuels beliefs that temblor was larger than reported.

Confusion over different kinds of measurements of the size of the Northridge earthquake has led to rumors, growing into convictions in some quarters, that the quake was bigger than it was.

Part of the problem is that numbers used in some measurements--such as the Modified Mercalli scale, or gravity force readings from strong motion accelerographs--are frequently higher than those used by scientists in news reports of the magnitude of the quake.

A 9 or 10 reading on the Mercalli scale, normally stated in Roman numerals as IX or X, is not a magnitude reading, does not measure the same phenomena, and does not alter the 6.8-magnitude now used by the National Earthquake Information Center in Golden, Colo., to describe the Northridge earthquake.

Meanwhile, a chart that has been circulating in the San Fernando Valley by fax, or passed hand to hand, seems to say that the quake reached an unheard-of magnitude 9.8 in San Fernando and 9.5 in Northridge, among other spots. But these are not magnitude readings, seismologists and Caltech researchers pointed out Friday.

Some callers to The Times have said the chart comes from Caltech. But Caltech spokesmen say the school put out no such chart, noting that, in any case, there is only one magnitude reading per quake, which does not vary from one community to another.

“The magnitude of an earthquake does not change with location,” a Caltech spokesman said. “The Northridge earthquake was (the same) whether you live in Northridge or in San Diego.”

The commonly used magnitude scales are based on the power of the earthquake at the spot where it started, although they also reflect the length of the fault rupture and the duration of the shaking. The Northridge earthquake could not have been an 8, scientists say, because the fault rupture was not long enough, nor did the shaking last long enough.

The 6.8-magnitude quake lasted, including reverberations of quake waves, no more than 30 seconds. By contrast, the shaking from the great Alaska earthquake of 1964, whose magnitude was put by some scientists at 8.9, lasted as long as seven minutes.

The intensity of the shaking, moment by moment, might not have been much stronger than the worst moments of the Northridge earthquake, but the Alaska quake is rated as more powerful because it lasted longer and affected a bigger area, seismologists say.

The Northridge magnitude, first put at 6.6, was later officially upgraded to 6.8 by the scientists at Golden, using a surface wave scale, and to 6.7 by Caltech, the U.S. Geological Survey and UC Berkeley, using a moment magnitude scale of quake waves that have passed through the Earth.

None of these groups used the old Richter scale, which has fallen into disuse because it distorts readings of quakes larger than magnitude 5. Caltech scientist Egill Hauksson noted recently that the Richter reading of the Northridge quake would have been only 6.4.

The Mercalli scale, developed by the Italian seismologist Giuseppi Mercalli in 1902, and modified by American seismologists Harry Wood and Frank Neumann in 1931, is a set of criteria, numbered from I to XII, that is used to describe a quake as it is variably experienced at many different locations.

Such a quake may rate a XII (“Damage is total, and practically all works of construction are damaged greatly or destroyed”) near the epicenter. But the same quake would rate only a I (“Not felt by people, except under especially favorable circumstances”) at another place, several hundred miles away.

Because the scale is based on subjectively perceived affects, rather than objectively measured ground movement, it has fallen out of use by seismologists.

The authorized descriptions are fairly lengthy, and most anyone who has felt a quake, by reading through the criteria under each Roman numeral, can form a conclusion as to what the Mercalli scale reading was where he or she felt the quake.

Last week, at a hearing of the California Seismic Safety Commission, Richard Andrews, the director of the state Office of Emergency Services, released a map of “estimated Modified Mercalli intensities” for the Northridge quake, developed by EQE, a firm in San Francisco. Andrews hoped the map would show the quake in a different perspective, with its widely varying affects.

The map showed various Mercalli intensities, ranging from X in Northridge and Reseda, down to VI in Malibu, Torrance, Carson, Long Beach and other points, and less than VI farther away.

The Mercalli definition for X shown on the map was: “Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides . . . Rails bent slightly.”

The definition for VI was: “Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Knick knacks, books, etc. off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry cracked.”

Some people from Van Nuys, which was mostly assigned a IX, felt that the shaking in their neighborhood was, instead, a VI, and they also questioned why nearby Sherman Oaks, where damage was greater, was assigned only an VIII, less than Van Nuys.

Andrews, when asked about such matters, noted that the map reflected a computer model and not interviews with residents of specific neighborhoods coming to a consensus.

Charles Scawthorne, EQE’s vice president for research and development, readily agreed Friday that the map released last week “is based on certain approximations,” and where more than one type of shaking occurred within a neighborhood, he acknowledged that the Mercalli measurement had split the difference.

“I think the main message is, this map is a simplified picture of the effects of the earthquake,” he concluded. “In general, it’s accurate. In detail, it may not be.”

In some areas, the chart being circulated by unknown parties in the Valley appears to list EQE’s assigned Mercalli levels as if they were magnitudes. Changing Roman numerals to Arabic ones, it conveys the false impression that the magnitude for the quake was higher than announced.

Scawthorne said he believes that another type of quake measurement--accelerographic instruments measuring gravity forces, which has come more into vogue in the aftermath of the Northridge earthquake--should be a preferred form of measurement.

These numbers, such as the extraordinarily high 1.82g Tarzana reading reported in a Times story Friday, show shaking intensities at various points, and may be a better way of gauging the strength of an earthquake at various points than either magnitude or Mercalli scales.

But accelerographs must be there before the quake occurs, and in most recent quakes, they have been too widely scattered to provide a comprehensive picture.