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California sees uptick in sizable earthquakes since the Mexicali temblor

April 12, 2010 |  3:06 pm

http://earthquake.usgs.gov/earthquakes/pager/events/ci/14607652/shakemap.png

If you’ve been feeling more shaking this year, it’s not your imagination. The number of quakes greater than magnitude 4.0 in Southern California and Baja California has increased significantly in 2010. There have been 70 such quakes so far this year, the most of any year in the last decade. And it’s only April.

There were 30 in 2009 and 29 in 2008. Seismologists said they are studying the uptick but cannot fully explain it.

Major earthquakes tend to occur in cycles, and experts have said the region in recent years has been in a quiet cycle when it comes to sizable temblors. The string of quakes this year raises the possibility that Southern California might once again be entering a more active seismic period.

Scientists said the uptick does not mean that the Big One is any more imminent, but it could mean that more significant quakes are on the way.

Egill Hauksson, a geophysicist at Caltech, said the rate of quakes in the region is “probably ... picking up again” after a relative lull that lasted more than a decade.

“What it means is that we are going to have more earthquakes than in the average year,” said Hauksson. “With more earthquakes, we’re bound to have more bigger ones. But there are always fewer of those than the smaller ones.”

Scientists, however, have not been able to fully explain the increase. “We would like to be able to explain it,” said Kate Hutton, a seismologist at Caltech. “But there’s no real correlation with any cause.”

Many of the quakes this year have been aftershocks to the magnitude 7.2 temblor that rattled the Mexicali area earlier this month. The border area had experienced a swarm of smaller quakes before that one.

And there have been more than 1,000 aftershocks, including more than a dozen that registered more than magnitude 5.0. The Mexicali quake was the region’s largest in nearly two decades -- since the 7.3 Landers quake in the Mojave Desert in 1992.

Despite their size, neither quake did catastrophic damage because they occurred in relatively remote areas far from major population centers.

The Landers quake occurred during a particularly active seismic period in the L.A. area. Between 1987 and 1994, the region experienced five major quakes. In addition to Landers, there were the Whittier Narrows quake, which killed eight people, quakes in Big Bear and Joshua Tree, and the Northridge quake, which killed 57 people, injured 4,500 and caused about $40 billion in damage.

Beginning in the late 1990s, however, the number of these big, memorable quakes subsided. Experts are not sure of the reason for the cycles.

Experts said one possibility is that the ups and downs are random. Another possibility is a “cascade effect” in which a quake on one fault changes the stresses on another. “If that fault is ready to produce an earthquake anyway, it might do something. But it would have to be pretty close” for that to happen, Hutton said.

Earthquakes have been in the forefront of public consciousness this year after January’s devastating temblor in Haiti, which killed tens of thousands. It was followed weeks later by a destructive temblor in Chile.

And then came the Mexicali quake, which was stronger than Haiti's although much less destructive.

Hauksson said it’s easy to read too much into this year's quake uptick.

Although it comes after several relatively quiet years, he noted that it’s not uncommon for one large quake to produce months if not years of increased seismic activity. So in that sense, the quake pattern this year is fairly typical.

-- Cara Mia DiMassa

Map: Intensity shaking map for Mexicali quake. USGS
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