A ‘Quiet’ Period--3,818 Quakes in 1st Quarter

Earthquake scientists at Caltech and the U.S. Geological Survey said Thursday that there were 3,818 recorded earthquakes in Southern California in the first quarter of 1995, the smallest number in any quarter since before the Joshua Tree-Landers-Big Bear earthquake sequence began three years ago.

The quakes--which include tiny temblors less than magnitude 1.0 that cannot be felt--have been numerous enough so that scientists have not finished processing them into their official records.

Scientists said the apparent lull this year is partly a function of time passing since the Jan. 17, 1994, Northridge quake and the Landers quake in 1992.

From Jan. 1 through March 31, Southern California had 29 earthquakes of magnitude 3.0 or greater, which is just above the long-term quarterly average of 25, said Lucy Jones of the U.S. Geological Survey.

During the first quarter, 399 more Northridge aftershocks--most of small magnitudes--were reported, bringing the total number of aftershocks from the 6.7 earthquake to 11,426, said Jones and Caltech seismologist Kate Hutton.

There have been more than 45,000 aftershocks to the 1992 Landers quake, including 1,516 during the first quarter of 1995. The Landers quake of June 28, 1992, is now assessed by state geologists at magnitude 7.5.

Just after the main Landers shock, scientists recorded more than 25,000 quakes in the third quarter of 1992, including 781 quakes of magnitude 3.0 or greater. That was the most active quarter since 1990.

In the first quarter of 1994, including the Northridge main shock, there were 10,915 recorded earthquakes, including 417 at or greater than magnitude 3.0, the level at which earthquakes can be felt in the local epicentral area.

Jones said Thursday that once again Southern California “seems to be in a relatively quiet period seismically,” but that this does not necessarily indicate that the quiet will continue.

In the two years before the Northridge earthquake, the Los Angeles Basin had very few earthquakes as strong as magnitude 3.0. The lull was broken by a series of mild earthquakes in the Santa Monica Bay and, then, the following week, the destructive Northridge temblor.

The scientists said the most significant earthquakes this week in Southern California have been magnitude 3.7 and 3.5 temblors Monday night in San Bernardino County, the first striking in the Cajon Pass and the second near Loma Linda.