Is the Earthquake Activity in Southern California Unusual?

Karen Rowan

Date: 08 July 2010 Time: 05:58 AM ET

Small aftershocks are continuing to rumble through Southern California today, in the wake of the 5.4-magnitude earthquake that struck about 30 miles (45 km) south of Palm Springs yesterday. Although it may seem there has been increased seismic activity in the region lately, scientists say the latest series of temblors is nothing unusual.

"The reason it seems like there's more activity lately is that you don't have an earthquake until you do," said Don Blakeman, a geophysicist at the U.S. Geological Survey National Earthquake Information Center. "But it really all depends on what time frame you're looking at."

In any given year, there are about 16 to 18 large earthquakes that reach magnitude 7 or greater in the world, Blakeman said. But earthquakes don't necessarily come on a neat schedule that follows the annual calendar. So to arrive at the average, geologists had to consider the number of earthquakes that occur over longer lengths of time – five, 10 or 30 years.

Scientists generally focus on large earthquakes because, although today's technology can detect tiny tremors in remote areas, historical data is only reliable for larger quakes.

A 5.7-magnitude earthquake struck along the U.S.-Mexico border on June 15, and a 7.2-magnitude temblor struck on April 4. It is likely that both the June 15 quake and a smaller, 4.9-magnitude quake that struck on June 12 were aftershocks of the April earthquake, according to the USGS.

When earthquakes as small as magnitude 2 are considered, there have been hundreds of aftershocks from the April quake, Blakeman said.

Although yesterday's quake was probably not an aftershock from the April quake, it may have been triggered by the other earthquakes, Blakeman said, and the USGS may decide to look into this. But the question of whether one earthquake actually led to another is very tricky for geologists to answer, and it is thought this only happens over short distances, such as tens of miles.

Some broad conclusions are easy to make, in terms of potential connections between earthquakes that have struck this year:

"The Haiti earthquake didn't cause the Chile earthquake," Blakeman said, referring to the magnitude-7.0 earthquake that struck Haiti on Jan. 12 and the 8.8-magnitude quake that struck in Chile on Feb. 27. "We only occasionally see a triggered quake. Most of the time, that's not what's at work."

Over short time frames, such as one year or a few years, there can be a lot of variation in the number of quakes, but that doesn't mean that anything unusual is happening, Blakeman said.

"If there's magnitude-8 that strikes California this afternoon, that would still not be unusual," he said.