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.
There is some evidence that rising seismic activity can be a precusor to larger temblors, earthquake scientists say. A classic example is the earthquake that devastated San Francisco in 1906, judged to have a magnitude of about 7.8.
There was a crescendo of quakes in the Bay Area of magnitudes 4, 5 and 6 in the decades leading up to the 1906 earthquake on the San Andreas fault, said James Dolan, a professor of earth sciences at USC. Another crescendo in the Bay Area began in the 1950s, peaking with 1989's magnitude 6.9 earthquake at Loma Prieta.
"If we saw this pattern repeating here over the next five or 10 years with a gradual crescendo and increase in small quakes, that would be extremely interesting," Dolan said. "There's certainly nothing to be alarmed about in the short term."
The Bay Area's major faults tend to run parallel to each other, but Los Angeles has so many faults of different sizes and orientations that this crescendo model may not apply very well, he cautioned.
"This is a very structurally complicated part of the world," he said. "There are dozens of big faults and hundreds of little ones capable of generating a magnitude 3 quake."
Thursday's quake was particularly notable for scientists because it occurred so close to the San Jacinto and San Andreas faults. Although quakes of that size do not tend to ripple out far from the epicenter, the San Bernardino quake could have changed stress patterns on two of the faults capable of producing a large earthquake, Dolan said.
Of last year's quake cluster, the largest occurred July 29 in Chino Hills, with a magnitude of 5.4. Only minor injuries and damage occurred, but it was the largest quake in Southern California since the Hector Mine quake and its aftershocks.
With the lull in seismic activity, earthquake experts worried that preparedness efforts had dragged, and Hutton and Jones used the interest in the Chino Hills quake to remind locals about earthquake hazards.
Residents of the seismic hotspots have been trying to take the latest temblors in stride. Mirna Velasquez, a 45-year-old receptionist at a doctor's office in Calexico, said she has felt many of the 16 jolts of magnitude 3 or greater since November.
"There's usually a rumbling and then a shaking," Velasquez said Thursday. "If we're at work, we just keep on working and make sure the patients are OK. If I'm at home, I make sure the big-screen TV doesn't fall."
She remembers being awakened by a quake on Christmas morning and hearing her husband exclaim, "Oh my God!" He is usually home only every other weekend because he works in Los Angeles. Velasquez said she was surprised, but immediately turned to soothing her husband.
"He doesn't experience them as much as we do," she said. "I tried to calm him down."