Magnitude of Quake May Be Raised to 6.8

The National Earthquake Information Center in Golden, Colo., expects to upgrade the magnitude of the Northridge earthquake next week from 6.6, probably to 6.8, based on higher readings from European seismographic stations, a spokesman for the center said Friday.

This would indicate that the deadly earthquake may have been about 1 1/2 times stronger than scientists first thought, said Jim Mori, who is in charge of the Pasadena field station of the U.S. Geological Survey.

Magnitude figures often change as more distant stations report, said geophysicist John Minsch, duty officer at the national center, which is operated by the Geological Survey.

He said magnitudes represent an average of all authorized readings worldwide, and a European measurement has as much validity as a local measurement.

Minsch said a Caltech report that at least one station, in Sweden, has put the quake’s magnitude as high as 7.2 coincided with information the center had received. Mori, however, said that 7.2 is unreasonably high.

Different stations often get different readings depending on how earthquake waves travel from the epicenter to the station. Sometimes, the final averaging is not made for weeks or months.

No matter how distant a seismographic station is, it reads the waves by the same method. Tiny earthquakes are not detectable at great distances, but all sizable quakes trigger the movement of seismographic needles, just as they do at facilities close to the epicenter.

Mori said an earthquake radiates energy preferentially in certain directions. Because scientists get different readings in different directions, it is important to take a global average.

Because violent shaking locally can push a needle completely off the seismographic scale--as happened at Caltech with one of its instruments on the morning of Jan. 17--scientists may need more distant readings to help them reach as accurate a magnitude figure as possible, Mori said.

Reports of an impending higher magnitude began to circulate after release of a preliminary report on the Northridge earthquake by a team of UC Berkeley scientists who contended that the quake was stronger, possibly longer in duration and on a different fault than had been initially concluded by scientists from Caltech and the U.S. Geological Survey.

Barbara Romanowicz, director of the UC Berkeley Seismographic Laboratory, said its instruments measured the magnitude at 6.7, rather than the Caltech and Geological Survey figure of 6.6. The new report also said the initial shaking lasted 25 to 30 seconds, somewhat longer than reported by others.

The primary difference between the Berkeley team and the others is that the Berkeley group continues to hold that the earthquake occurred on an easterly extension of the Oak Ridge Fault system, a belief Caltech scientists espoused and then abandoned.

Recent Caltech and Geological Survey statements have suggested that the Northridge earthquake was on an unnamed fault related to the Elysian Park thrust fault system that sweeps from the San Gabriel Valley through Downtown Los Angeles, the Hollywood Hills and the Santa Monica Mountains.

But Egill Hauksson of Caltech said Friday that it is “still a matter of research how this particular fault (that caused the Northridge earthquake) relates to other faults. Not enough is known at this time to reach a firm conclusion.”

Romanowicz said: “I would agree with Hauksson that this is still to be concluded. When we do more mapping perhaps this will be settled, but it will take some time.”

Pat Williams, a geologist at the Lawrence-Berkeley Laboratory, said the Berkeley scientists reject involvement of the Elysian Park Fault because it is on a northerly dipping plane, while the Northridge earthquake occurred on a southerly dipping plane.

The National Earthquake Information Center is charged with assembling all the data, averaging the magnitudes and determining the official numbers for all earthquakes.

A change in magnitude could also have economic aftershocks. A spokesman for Caltech said earlier Friday that he had heard from an insurance industry representative seeking information about an upgrade. The spokesman, Max Benavidez, said the industry representative told him that some insurance payouts would be affected if the magnitude was higher.