More Than 121,000 Quakes Since 1987 : Geology: About 2,700 Southland temblors have been magnitude 3.0 or greater, scientists say.

In one of the most comprehensive earthquake inventories to date, scientists from Caltech and the U.S. Geological Survey reported Monday that more than 121,000 quakes have been recorded in Southern California since the beginning of 1987.

A little more than 2,700 have been magnitude 3.0 or greater and thus could be felt at least by people living in the vicinity where they occurred. This works out to an average of about one earthquake felt somewhere in the region every day.

But some areas are more active than others. There have been more than 53,000 aftershocks from the 1992 Landers-Big Bear earthquakes and nearly 9,000 aftershocks from the Jan. 17 Northridge quake.

In the last eight years, the smallest number of quakes recorded in any one year was 7,961 in 1991.

When big earthquakes occur, the number rockets upward as thousands of aftershocks occur. In 1992, 49,981 earthquakes, including aftershocks, were recorded in Southern California--in an area bounded by the Colorado River on the east, the Mexican border on the south, the offshore islands on the west and Lone Pine and Coalinga on the north.

Most of those earthquakes were too weak to feel. But 1,707 earthquakes that were at least magnitude 3.0 and could be felt by people living in their vicinity were recorded in 1992.

The figures released Monday were compiled from instruments of the Southern California Seismographic Network by scientists Kate Hutton and Egill Hauksson of Caltech and Lucile M. Jones of the U.S. Geological Survey.

They said that virtually all detectable earthquakes were included in the data. However, no matter how refined the seismological network, some quakes are too small to be registered by any station on the network.

The scientists reported that more than 8,750 jolts have stemmed from the Northridge quake, including 381 aftershocks registering between 3.0 and 3.9, 48 between 4.0 and 4.9 and eight--including the main shock--5.0 or stronger.

As for Landers, now calculated by Hutton, Hauksson and Jones as a magnitude 7.3 main shock, there have been 53,040 jolts, including 1,458 aftershocks registering between 3.0 and 3.9, 156 between 4.0 and 4.9, and 21, including the main shock, 5.0 or stronger. A 4.8 Landers aftershock was recorded 28 miles northeast of Lucerne Valley at 2:34 p.m. Monday.

The scientists reported 11,684 earthquakes in 1987, 11,295 in 1988, 9,538 in 1989, 9,311 in 1990, 7,961 in 1991, 49,981 in 1992, including the 1,707 of at least 3.0, 19,404 in 1993 and 18,110 thus far in 1994, including 523 of at least 3.0.

“The overall rate at which aftershocks occur depends strongly on the magnitude of the main shock, and the Landers sequence is almost exactly average for its size,” the scientists said. “The Northridge sequence has more aftershocks than the average for a main shock of (its size) . . . but still far less than Landers because the Northridge main shock was much smaller.”