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.
"The analogy is the weather in California," said Caltech seismologist Kate Hutton. "Some years are rainy and some years are completely dry. With earthquakes, they never go away completely, but they do clump together in time, and we don't know why."
Since the end of November, clusters of earthquakes ranging from magnitude 3.0 to 5.1 have bloomed in areas near Barstow, Trona and two areas of Baja California not far from Calexico. Four such quakes, which are considered light to moderate, have occurred so far this year.
The current uptick has not included any major quakes, but a number of the temblors have been felt across the region, including Thursday's magnitude 4.5 San Bernardino shaker.
