California earthquake: Should we expect more aftershocks?

A long, rolling, 6.8-magnitude earthquake rattled northern California on Sunday at 10:18 p.m. local time, followed by at least 24 aftershocks in the following eight hours, including one with a magnitude of 4.6.

No injuries or damages have been reported, yet. But how sure can we be that there won't be any more aftershocks?

According to the Northern California Earthquake Data Center, northern Californians are not out of the woods yet. In its aftershock probability report published Monday afternoon, the center says there is a 90 percent chance of a magnitude-5-or-more aftershock in the next seven days. The probability of another shock larger than the original quake are about 5 to 10 percent.

Shallower quakes tend to produce more aftershocks, and this last one struck about 10 miles deep, relatively close to the surface.

The odds of aftershocks decrease rapidly as more time passes. According to Omori's law, first formulated by the Japanese seismologist Fusakichi Omori in 1894, the frequency of aftershocks decreases by roughly the inverse proportion of the time after the original quake. In other words, whatever the odds of an aftershock are on the first day, they will be half of that on the second day, a 10th of that on the 10th day, a 100th of that on the 100th day and so on. In 2000, researchers found that aftershocks follow Omori's law for hundreds of years.

"It kind of goes in spurts," National Earthquake Information Center geophysicist Don Blakeman told the San Francisco Chronicle. "You can't say when it will end, but as time passes, you get fewer and fewer aftershocks and in general, they get smaller and smaller."

Could an aftershock be more powerful than the original? Not technically, thanks to a semantic trick. When a subsequent quake is larger than one that preceded it, the US Geological Survey renames it the "mainshock," and the earlier one becomes a "foreshock."  This is, of course, cold comfort to anyone currently being jolted by newly reclassified seismic activity.

This particular quake measured at 6.8 on the moment magnitude scale, which was developed in the 1970s and replaced the Richter scale for medium-to-large quakes. Like the Richter scale, the MMS is logarithmic. An increase of one integer represents an increase of about 32 times in the release of energy of a quake. At 6.8, Sunday's quake is comparable in energy output to the 1995 Kobe, Japan, earthquake, which killed almost 6,500 people and caused about $100 billion in damage.

But the magnitude of an earthquake does not necessarily correlate with how many buildings it topples. For severe quakes, the best measure of how much damage it will cause is "peak ground velocity," which is the speed at which the ground is moving. For moderate quakes, the related concept of "peak ground acceleration" correlates most closely with damage. 

The velocity and acceleration of the ground varies from location to location, generally decreasing as one moves away from a quake's epicenter. Sunday night's quake occurred about 50 miles offshore, and a monitor in Eureka, Calif., measured the ground as moving at just over 2 centimeters per second. By comparison, monitors during the Kobe quake, whose epicenter was close to the city, measured the peak ground velocity at over 80 centimeters per second. 

”We had some alarms go off and other than that we dodged a bullet,” a lieutenant with the Humbolt County sheriff's office told the Eureka Times-Standard. “This very easily could have been a catastrophe that could have caused a lot of damage.”