Joshua Tree Area Jolted by Strong Aftershock

A strong aftershock, magnitude 4.8, struck an area near the epicenter of the Joshua Tree earthquake near Desert Hot Springs on Monday morning, continuing what a Caltech scientist called “a rich aftershock sequence” that has followed the 6.1 main shock nearly four weeks ago.

Felt strongly in Palm Springs, mildly in parts of Los Angeles and as far away as Santa Catalina Island, the 8:44 a.m. shaker was as powerful as any aftershock of the April 22 temblor. No new damage or injuries were reported.

Caltech seismologist Egil Hauksson said that through Monday about 4,000 aftershocks had been reported from the original earthquake on an unnamed fault in the Little San Bernardino Mountains, nine miles east of Desert Hot Springs. Of these, 15 have been magnitude 4.0 or stronger and 128 have been 3.0 or stronger.

Quakes are called aftershocks when they occur at or near the site of large quakes. Although as time passes, they usually occur less frequently and with less average intensity, they may continue for years. Monday’s was about a mile from the original epicenter.

Although the U.S. Geological Survey and the California Office of Emergency Services had issued an advisory warning of an imminent, possibly larger, earthquake on the nearby San Andreas Fault after the main shock last month, Hauksson said the chance of such an earthquake is now no greater than at any time.

In 1990, a task force chaired by Lucile M. Jones of the Geological Survey and Caltech’s Kerry Sieh estimated that there was a 40% chance of a quake stronger than magnitude 7.5 in the San Gorgonio-Salton Sea portion of the San Andreas Fault before 2020.

On Monday, Hauksson noted that there is “quite a bit of variability in aftershock sequences and this is one of the richer ones.” But, he added, “it’s not anomalous.”

A spokeswoman for the Geological Survey’s western regional headquarters in Menlo Park said there have also been thousands of aftershocks of the April 25 Cape Mendocino earthquake in Humboldt County. The 6.9 quake was followed by 6.5 and 6.2 aftershocks within a 24-hour period.

Pat Jorgenson said the aftershocks have included more than 25 of magnitude 4.0 or larger, but have tailed off in recent days. The most substantial in the last week was a magnitude 3.8 on Saturday. Jorgenson said no aftershocks stronger than magnitude 5.0 have occurred since April 26.