End of Seismic Lull Is Possible

Earthquakes of the last two months in Southern California, including a 5.1-magnitude quake centered in Riverside County late Tuesday night, may mark the end of a period of seismic quiescence that followed the 1992 Landers and 1994 Northridge earthquakes, scientists said Wednesday.

Tuesday’s quake was centered near the town of Anza, 21 miles south of Palm Springs, at 11:56 p.m. and was followed by scores of small aftershocks. Caltech and U.S. Geological Survey scientists said the quake suggests that the “stress relief shadow”--a period of reduced seismic activity--following those big quakes of the last decade has worn off.

Seismologist Egill Hauksson of Caltech said, “We are most likely to go back to the kind of earthquake frequency we saw in the late 1980s.”

The latest quake followed 4.2 and 3.7 (now revised to 4.0) temblors in recent weeks in the Los Angeles Basin near West Hollywood and Compton. A 3.0 quake was centered in Silver Lake.

A magnitude 5.1 quake typically would have caused some light damage, but Tuesday night’s jolt was centered in such a sparsely populated area of the Santa Rosa Mountains that no damage was reported.

Jane Melser, who lives on a ranch near the epicenter, said the quake woke her up. “It was just a boom and everything shaking. Nothing fell off the walls or anything. Some of our pictures ended up hanging at an angle.”

Palm Springs police reported that some callers asked whether it was a terrorist attack, and in San Diego, police cautioned that using 911 emergency lines for such inquiries could prevent people with life-threatening emergencies from getting through.

The quake was felt lightly in the Los Angeles area, and more strongly in parts of Orange County.

The scientific report on the temblor, issued Wednesday, was released by Hauksson and Kate Hutton from Caltech, and Lucy Jones and Doug Given of the Geological Survey’s Pasadena office.

They said it was within the 3-mile-wide San Jacinto fault zone.

The San Jacinto fault branches off from the San Andreas fault near the Cajon Pass and veers off to the southeast, past San Bernardino, Riverside and Hemet all the way to the Imperial Valley.

The San Jacinto fault is historically the most active in Southern California, the scientists said. Tuesday’s quake occurred in the so-called Anza gap, a 40-mile segment that has not experienced a major quake since at least 1895 and most likely since 1850.

The scientists said the location of this week’s quake was just south of the 5.5-magnitude Whitewash earthquake of Feb. 25, 1980, and just north of a 5.9 quake in 1937.

The “stress relief shadow” that may be ending is a well-known phenomenon that is most pronounced after large earthquakes.

For instance, the great San Francisco quake of 1906, estimated as ranging in magnitude from 7.8 to 8.3, caused a period of seismic quiescence in parts of the Bay Area that lasted more than 60 years.

The 7.1-magnitude Loma Prieta quake in the Santa Cruz Mountains in 1989 caused a period of seismic quiescence that Tousson R. Toppazada of the state Division of Mines and Geology has estimated may end by 2004.

The prospects of an end to stress shadows in both Northern and Southern California should not cause undue alarm, officials said.