L.A. in ‘Quiet’ Seismic Period : Earthquakes: In last year, basin has felt only two temblors as strong as magnitude 3, U.S. Geological Survey reports.

The number of earthquakes centered within the Los Angeles Basin has dropped substantially in the last two years, and the area is in a seismic “quiet period” similar to one about 10 years ago, a U.S. Geological Survey seismologist reported Friday.

Speaking to a meeting of the California Earthquake Prediction Evaluation Council in Pasadena, Lucile M. Jones said that only two mild temblors as strong as magnitude 3 have hit anywhere in the Los Angeles area in the past year, on June 28 and 29. Those may have been related to the 7.6 Landers quake and its 6.7 Big Bear aftershock, both on June 28, Jones said.

The Landers and Big Bear quakes were felt strongly in Los Angeles and Orange counties, but they were centered outside the metropolitan counties, well to the northeast, Jones said.

She also reported a distinct falloff in small earthquakes in the magnitude 2 range in the Los Angeles Basin. Elsewhere in Southern California, there have been no earthquakes as strong as a moderate magnitude 4 in the last year outside the Landers and Big Bear aftershock zone, Jones said.

The director of the state Office of Emergency Services, Richard Andrews, chairing Friday’s meeting, sounded a note of caution. He said that the falloff in earthquakes does not necessarily invalidate a finding by a panel of scientists last November that there is up to a 47% probability of a major quake, magnitude 7 or larger, occurring somewhere in Southern California in the next five years.

State Geologist James F. Davis said that over the past seven years, there has still been an unusually high number of sizable earthquakes in the Southland, and it is too early to say that the whole region has become seismically quiet.

Jones too was cautious as to the meaning for the future of her report, even for the Los Angeles Basin.

She noted that the last of a series of four moderately strong earthquakes--magnitude 5.0 to 5.9--from 1987 to 1991, came nearly two years ago, doing substantial damage in Sierra Madre and nearby San Gabriel Valley communities on June 28, 1991.

While there has been nothing of that magnitude since in the Los Angeles Basin, Jones said that the same 40-mile segment of the Sierra Madre fault system, between Mt. Baldy and San Fernando, has had very few small quakes for many years and can be considered effectively locked, meaning that strain may be accumulating and eventually there could be a big quake somewhere along the system.

Jones called the Sierra Madre fault system “the primary hazard in this region,” meaning that over a long period of time it constitutes a bigger seismic threat than the Newport-Inglewood fault system, which extends from Orange County into the South Bay and Westside sections of Los Angeles County.

Later Friday, the prediction council approved a program under which imminent public alerts of major quakes will be automatically issued for 72 hours if a magnitude 6.5 earthquake should strike within six miles of the San Andreas Fault anywhere between Ft. Tejon and the Salton Sea.

In areas where scientists know the exact location of the San Andreas fault line, such an alert would also be issued if a magnitude 6 earthquake were to strike within two miles of the line. Over much of the fault, scientists have only an approximate idea of the location of the fault line.