U.S. Center Plans to Upgrade Quake to 6.8

The director of the National Earthquake Information Center in Golden, Colo., confirmed Tuesday that the center has decided to formally upgrade the magnitude of the Northridge earthquake to 6.8.

The step will be taken Friday in the center’s weekly publication, “Preliminary Determination of Epicenters,” Waverly Person said.

Geophysicist John Minsch at the national center announced five days ago that 6.8 “probably” would be the figure used. Monday, Caltech and the U.S. Geological Survey announced that they had upgraded the magnitude from 6.6 to 6.7.

But it is the generally recognized prerogative of the national center to set the official magnitude.

The center’s magnitude figure could still change as the readings of more seismographic stations from around the world are figured into the average. And the evolving understanding of seismology often results in disputes years later on how strong an earthquake was.

The great San Francisco earthquake of 1906 has been variously estimated in recent years at anywhere from magnitude 7.8 to 8.4, with 8.3 the most commonly used.

A 6.8 quake is considered about 1 1/2 times more powerful than a 6.6, according to Jim Mori, the scientist in charge of the Geological Survey’s Pasadena field station. The expected upgrade would represent a substantial change in the official concept of how powerful the Jan. 17 earthquake was.

Even before the upgrade, the quake was the most powerful in the Los Angeles Basin in the two centuries of available records, and it also stands as the most powerful quake in U.S. history to affect such a large population.

The three weeks since the earthquake have been marked by repeated changes in thinking and disagreements among scientists about what fault was primarily involved, what the fault angle was, where the epicenter was and, to a lesser extent, what the name of the earthquake should be.

The Northridge quake name will probably stick, even though the latest calculations show that the epicenter was in the neighboring community of Reseda.