A series of medium-size tremors may presage big quakes in California, and the telltale pattern might be used to improve predictions of when devastating quakes will strike, scientists say.

A historical study of tremor activity surrounding the major San Francisco Bay quakes of 1868, 1906 and 1989 found the number of moderate earthquakes increased rapidly on nearby faults one to two decades before the serious quakes.After the 1868 and 1906 quakes struck, seismic activity dropped off sharply and there were relatively few quakes for periods of 13 years and 42 years respectively, the study showed.

A comparable pattern was also found for the sizable 1948 shock on the San Andreas fault in Southern California.

"The similarity of the sequences in time and space before the three large earthquakes in the Bay area and the 1949 shock argues that these patterns are not fortuitous (chance) occurrences but instead are causally related to those four main shocks," Lynn Sykes and Steven Jaume wrote in their study published in the journal Nature.

In addition, the researchers noted high rates of moderate earthquakes occurred in the decades before the large 1703 and 1923 quakes in Japan and the 1957 temblor in the central Aleutian Islands.

Sykes and Jaume, seismologists at Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y., conceded the number of examples in their California study is small.

However, the scientists said they were optimistic the pattern would lead to more precise predictions of earthquakes measuring more than 6.8 on the Richter scale in the Bay area. The information may be able to narrow quake forecasts within a few years to a decade, instead of the 20- to 30-year periods typical of current forecasts.

Based on their findings, Sykes and Jaume said there appears to be a low probability of large quakes on the San Andreas fault north of San Francisco or the northern half of the Hayward fault in the next few decades.

However, the seismologists said it is too soon to tell if people living along the San Andreas fault south of San Francisco and the southern Hayward fault can breathe easy.

That is because it is too soon to tell how the 1989 Loma Prieta quake, which measured 7.1 on the Richter scale and occurred along the San Andreas fault south of San Francisco, fits into the newly discovered cycle. That temblor killed more than 60 people and caused more than $8 billion in damage.

If the number of smaller quakes remains high, the scientists said a second large quake should be expected soon.