Seismologists Dismiss Concerns About Recent Swarm of Quakes

A swarm of about 200 small earthquakes, the largest a magnitude 3.4, have shaken an area southwest of the San Bernardino Valley town of Fontana in the last two weeks, but scientists at Caltech and the U.S. Geological Survey doubt that they are omens of a large quake.

“Because these earthquakes are so shallow and most larger earthquakes start on deeper parts of their faults, we see no particular connection between these swarms and larger earthquakes,” said a statement written by seismologists Lucile M. Jones of the U.S. Geological Survey and Egill Hauksson of Caltech.

They also noted that this is the fourth quake swarm to strike a two-mile area centered five to seven miles southwest of Fontana in the last three years but that so far there have been no serious seismic consequences from any of them.

A quake swarm is a large number of quakes occurring in the same narrow area over a fairly short period of time. Many swarms last only hours, but some--like the current one--can last much longer.

Jones and Hauksson said only 20 of the 200 quakes since July 13 in the Fontana area have registered magnitude 2.0 or above. A quake usually must be 2.5 or stronger to be felt by human beings.

The quakes have all been very shallow, occurring in the top three miles of the earth’s crust, and scientists said they are certain that none has been an aftershock of either the 1992 Landers-Big Bear quakes, or January’s Northridge quake.