Quake magnitude upgraded to 7.1

PASADENA, Calif. - The major earthquake that ripped a nearly 25-mile-long gash across the surface of the Mojave Desert as it rocked the Southwest during the weekend was upgraded from magnitude 7.0 to 7.1 today.

The 2:46 a.m. Saturday quake on the Lavic Lake fault, which seismologists thought was inactive, was upgraded after a review of seismic data recorded worldwide.

The increase in magnitude represents about 25 percent more ground motion, U.S. Geological Survey seismologist Lucy Jones told a press conference at the California Institute of Technology.

The temblor on the vast Marine Corps Air Ground Combat Center near the remote town of Ludlow caused a passenger train derailment but only minor injuries and light damage. Scientists have dubbed it the Hector Mine Earthquake, after a local landmark.

Preliminary analysis indicated the quake occurred in a region where stress was increased by the magnitude-7.3 Landers earthquake seve years ago.

''It's clear that the faults in this area are talking to each other,'' Jones said.

The Landers quake struck in the desert on June 28, 1992, and was followed within hours by a 6.5 jolt near Big Bear Lake in the San Bernardino Mountains. The Landers quake killed a boy, and both quakes caused 400 injuries and $100 million in damage.

Saturday's quake is not considered an aftershock of Landers but scientists will be exploring the relationship, the USGS said.

The biggest offset measured along the rupture was 12 feet to 15 feet. An offset is the distance a point on one side moved relative to where it had been touching the other side before the rupture. Stream beds or roads typically show offsets dramatically.

No damage was reported from the hundreds of aftershocks - including three of magnitude-5.0 or greater - that continued to rock the region.

The rate of aftershocks was close to the average expected for such a main shock, the USGS said. A total of seven quakes 5.0 or greater aftershocks are likely within a week. The possibility that the big quake was a foreshock to a larger quake rapidly decreases and is unlikely after three days, Jones said.

The main temblor occurred along a 25-mile-long fault geologists that was considered inactive because there was no evidence it had ruptured in the past 10,000 years.

''The Hector Mine earthquake may therefore be a 'rare' event, one whose occurrence could not have been anticipated based on standard probabilistic assessment of earthquake rates,'' a USGS statement said.

The fault was only partly mapped and hadn't been named, USGS geologist Ken Hudnut said Sunday.

''We weren't going to do a lot of research along a fault that would only bother a rattlesnake,'' Jones said Sunday.

It will now become one of the most studied faults.

''We got a lot of information about this quake, a ton,'' said Hudnut.

Hudnut and two colleagues took a helicopter over the area Saturday to inspect the fault. The Marines halted live-fire training for three hours and two Marines accompanied the researchers as they periodically landed to measure the cracked earth.

Hudnut said colleagues marveled when they saw the giant fissure that moved a dry river bed 12 feet to the side.

''It had wonderful surface rupture,'' he said. ''It was exciting for us. Most geologists study things that happened thousands of years ago. This is something that happened yesterday.''

The more geologists can learn about the physics of earthquakes, he said, the more they can do to advise planners to avoid building on active faults and keep buildings already on them safer during temblors.

The quake was the first major event recorded on the TriNet quake data system, a computer-linked network of 200 seismographic stations that measure quake intensity and other properties.

''We had all this equipment in place,'' Jones said. ''We were looking for a model to test it. This was perfect.''

Some equipment, however, ended up with the Los Angeles Police Department.

Detectives recovered parts of a seismometer found partly buried Sunday in a wash near Reseda in the San Fernando Valley. Officers called in the bomb squad when they saw the unfamiliar blue boxes. Squad members dismantled one box to determine whether it was suspicious. They left the other seismometer in the ground.