Higher Likelihood of Major Quake Indicated : Seismology: Scientists report excavations show Wrightwood area has been hit by powerful temblors more frequently than thought. They say region is overdue for another one.

Excavations in the San Gabriel Mountains 45 miles northeast of Los Angeles indicate that large earthquakes on the San Andreas Fault have affected the Wrightwood area 12 times in the last 1,300 years, a higher frequency than was thought, scientists report.

The implication of the findings outlined in today’s Science magazine is that there is a greater likelihood of a damaging earthquake soon in the Los Angeles area than many scientists believe.

Based on the new findings, a big quake occurs on average about every 100 years along the Wrightwood portion of the San Andreas. On Saturday, it will be 136 years since the last magnitude 7 earthquake, the Ft. Tejon quake of Jan. 9, 1857, caused a ground rupture in the Wrightwood area.

But one of the four scientists who authored the article, Thomas E. Fumal of the U.S. Geological Survey, cautioned Thursday that in two cases during the past 1,300 years, the interval between quakes has been 170 years or longer.

The new study deals only with earthquake frequency, not with the severity of the shaking that would be felt in Los Angeles. Although Wrightwood is closer to Los Angeles than areas along the San Andreas east of San Bernardino, where experts recently said a catastrophic quake is most probable, many of the quakes measured in Wrightwood were centered elsewhere along the San Andreas.

In cases where the temblor occurred some distance away, the researchers from the Geological Survey and University of Oregon found lesser ground ruptures in the Wrightwood area.

For this study, the scientists dug 16 trenches in the Swarthout Creek basin at Wrightwood and studied how seismic activity had disrupted sediments.

They estimated that the last large quakes, probably magnitude 7 or larger, had occurred in 1857, 1812, 1700, 1610 and 1470. The 1857 and 1812 quakes are a matter of record, but the earlier ones could have occurred sooner or later.

The 1700 quake may be an indication of a magnitude 8 temblor that caused a rupture from the Coachella Valley near the Salton Sea all the way to Wrightwood, researchers said. Earlier, scientists had said that no such huge quake, rupturing through the San Gorgonio Pass, had occurred since the 15th Century.

Last fall, scientists examining stress changes resulting from the damaging Landers-Big Bear earthquakes said stress had been reduced along the so-called Mojave segment of the San Andreas. But Fumal said that it is unclear whether the Wrightwood area belongs to the Mojave segment or to the San Bernardino Mountains segment where stresses have increased.