2008 saw notable increase in moderate Southern California earthquakes

Do you think the ground feels a little shakier these days? It's not your imagination.

Last year saw a significant increase in the number of temblors of magnitude 3.0 or greater in Southern California and the northern portion of Baja California, according to data from Caltech and the U.S. Geological Survey.The region recorded 267 shakers with magnitudes of 3.0 and above last year, compared with 125 in 2007. Seismologists said 2008 had the highest number of such quakes of any year since 1999.

What experts don't know is whether the quake cluster is a harbinger of bigger quakes to come. The 1990s was considered a seismically active decade in Southern California, producing the magnitude 7.3 Landers quake in 1992 and the destructive Northridge temblor in 1994. During the quake cluster of 1999, the region was hit by the magnitude 7.1 Hector Mine temblor in the desert and several sizable aftershocks. There were 828 quakes with magnitudes of 3.0 and above that year.

Lucile Jones, a seismologist with the U.S. Geological Survey, said that although experts can't predict future quake activity, it appears Southern California is waking up from a steep drop-off in seismic activity so far this decade."It looks more like we're returning to a more normal rate," she said. "The last 15 years was one of the quietest times we've had in terms of [magnitude] 3's, 4's and 5's."

But the shift underscores one of the more perplexing elements of seismology: That quakes tend to happen in clusters, but not in any patterns that are easy to understand.

The clusters often come and go cyclically, but it's not clear whether they are laying the groundwork for a major quake.

via 2008 saw notable increase in moderate Southern California earthquakes - Los Angeles Times.