TWIN TEMBLORS: THE LANDERS AND BIG BEAR QUAKES : Questions and Answers

Scientists, like most people in Southern California, were surprised Sunday morning when two powerful earthquakes struck areas about 20 miles apart in San Bernardino County.

Within hours, eight seismologists and geophysicists at Caltech and the U.S. Geological Survey’s Pasadena office confirmed their belief that the quakes were separate events, on separate fault lines. They said there were two main shocks: the first, the Landers earthquake, and the second, the Big Bear earthquake.

There are some things about Sunday’s quakes that scientists can tell us right now, but in other cases they will be studying the temblors for weeks to come. Here are some questions and answers about the quakes:

Q: What happened?

A: At 4:58 a.m., a magnitude 7.4 earthquake struck six miles north of Yucca Valley, at the northern end of a zone affected by the 6.1 earthquake that occurred April 22. Three hours and six minutes later, about 20 miles west, a 6.5 quake hit six miles southeast of Big Bear Lake.

Q: How unusual is it to have two such powerful separate quakes in such proximity?

A: It is uncommon, but not unheard of. Just two months ago, in the April 25-26 Humboldt County quakes, two powerful shocks, magnitudes 6.6 and 6.7, followed a 7.1 quake and apparently were centered on a different fault 25 miles away.

Q: If there were two separate quakes, does that make it more or less likely that there will be a third quake in days to come?

A: Scientists are not sure. But in drafting an advisory with state authorities Sunday, they did not refer, as they had in some past advisories, to the possibility of a larger earthquake. They believe, however, that there is a 50% chance of aftershocks of magnitude 6 or greater in the next week in the same general area.

Q: Do they know why there were separate quakes at two locations?

A: Some scientists said the first quake had triggered the second, in a possible transfer of strain from one area to the other. But they called the matter complicated and said they did not understand the mechanics.

Q: How do the scientists know Sunday’s events were separate quakes instead of two seismic events on the same fault?

A: The northwest-trending Johnson Valley Fault, which the eight scientists said may be involved in the 7.4 quake, is located well away from the unnamed northeast-trending fault believed involved near Big Bear.

Q: What instruments do the scientists use to measure the quakes?

A: Caltech and the Geological Survey have placed 220 seismographic stations all over Southern California to measure quake waves and intensities. Hundreds of other stations will eventually report their assessments from around the world. Magnitudes assigned Sunday are preliminary and are likely to be revised.

Q: Do scientists go out in field too? What did they find?

A: Within hours, Caltech had dispatched teams to the earthquake areas. By noon, Kerry Sieh, a member of one Caltech team, reported finding a surface rupture near Landers that is 13 miles long and showed a horizontal offset of nearly 10 feet. No surface rupture was immediately reported near Big Bear.

Q: What role does the San Andreas Fault play in this quake scenario?

A: The Big Bear earthquake was only 10 miles from the San Andreas Fault, and a few scientists theorize that there could be a connection. This possibility will be explored.

Q: Are scientists concerned that this quake--the strongest in Southern California in 40 years--means the Big One might be near?

A: They are concerned. The Geological Survey’s Lucile Jones noted that there have been six earthquakes greater than magnitude 6 in Southern California since 1986 and several others close to magnitude 6. This is a much higher rate of frequency than during other parts of the century and may indicate a greater likelihood of the Big One occurring soon, but scientists cannot be sure.