Last Thursday, millions of people in California took shelter under tables and desks, protecting themselves from an earthquake . . . an imaginary one.
In this annual drill called the Great Shakeout, Californians practice responding to the injuries and destruction that are certain when the Big One hits.
Officials like San Francisco's Mayor Gavin Newsom say it's a civic duty to be prepared.
"Look, if you're going to move out to California and the West Coast, you're going live in not one earthquake zone but three earthquake zones. You've got to take that seriously. You have an obligation to take that seriously," Newsom said.
The seriousness of earthquakes has been obvious this year … tragically obvious.
The magnitude 7 quake that destroyed so much of Haiti in January was one of the deadliest ever, killing more than 220,000 people.
A month later, one of the most powerful earthquakes in a century . . . magnitude 8.8 . . . hit Chile. Better building standards limited the damage. Still, 370,000 homes were destroyed, and at least 520 people were killed.
In April, a magnitude 6.9 damaged 15,000 buildings in southern China, killing 2,700 people.
"This has been a busy year by any standard," said Tom Heaton, professor of Geophysics at Cal Tech. Heaton says where a quake hits can be at least as significant as how big it is.
The closest California has got to a notable quake this year is the 7.2 that rocked Mexico's Baja peninsula in April.
It was in the middle of the dessert, said Heaton, but if the epicenter were closer to a n urban center, "It would have been a very damaging event."
For those concerned about keeping Californians earthquake-ready, you could say that "nothing is finer than an earthquake that is minor." A little rattle now and then can keep people on their toes. Trouble is, California has been unusually stable recently.
Experts are certain that won't last.
"Basically, the San Andreas fault is locked and loaded and ready to rumble," said Thomas Jordan, director of the Southern California Earthquake Center.
"As a seismologist, of course, the longer we go without a big earthquake, the more nervous I get," Jordan said. "Because I know that, over time, we have to have earthquakes to relieve all of that energy that's being built up along the plate boundary. So the fact we haven't had an earthquake in this region for so long means that our time is getting due."
New calculations on the San Andreas near Los Angeles show major quakes happen there more frequently than previously believed, and the next one could be as big as magnitude 8.
"An earthquake of magnitude eight would affect the entire Los Angeles region, plus other communities along the fault," said Jordan.
The most recent big earthquake in L.A. was the 1994 Northridge quake. Fifty-seven died.
"It was the largest natural disaster before Katrina that the country had ever seen - $40 billion in direct economic damage," said Jordan.
It was magnitude 6.7. In the kind of math that only seismologists fully understand, a magnitude 8 quake would be many times more powerful.
David L. Ulin, who wrote about how Californians cope with earthquakes in "The Myth of Solid Ground," said the "Big One" could be 40-50 times as destructive as the Northridge quake. "For anyone who's lived through the Northridge quake, that's an unimaginable kind of circumstance," he said.
"People probably tend to kind of either get very scared or just sort of write it off, which seem to be the two poles of psychological response to these things - either they terrify you or you live in denial," Ulin said.
In a magnitude 8 earthquake, the shaking could last for a full minute. To appreciate what that could mean, consider the damage caused in the 15 seconds during which the ground shook in the San Francisco Bay area in 1989, in just 15 seconds.
At an Oakland park built as a memorial to the '89 quake, David Schwartz, of the U.S. Geological Survey, remembers that for all the destruction the epicenter of that quake was far away - 70 miles south of the Bay Area.
"We think our next big earthquake will occur right in the middle of the urban center," Schwartz said. "If you look out that way, there are hills, those are the East Bay hills, and the Hayward fault runs at the base of hills."
The Hayward fault is overdue. Add the San Andreas, and a startling number of lesser-known faults, and the San Francisco region is ripe for quakes.
"We're surrounded by faults, anywhere you go in the Bay Area," Schwartz said. "We could jokingly say, 'You can run, but you can't hide.' And that's sort of the truth."
The San Andreas fault brought on the Great San Francisco Earthquake of 1906. The violent shaking knocked down almost everything standing. The fire that followed ruined much of what was left. It's been estimated that 3,000 people died.
That's the kind of quake - much bigger, much more destructive than the one in 1989 - that seismologists figure could be on its way.
"So we've estimated in the next 30 years, a 63 percent chance - two out of three - that we'll have one or more magnitude six, seven or larger earthquakes," said Schwartz.
While earthquake scientists can make estimates, what they CAN'T do is make predictions.
"At one point people were quite optimistic that we'd come up with some systems to predict the earthquakes," said Heaton. "And the more we've studied the problem, the more we realize that that's kind of a false hope."
But once the earth has started moving, scientists may be able to send an alert that shock waves are on the way.
"We call it earthquake early warning," said Heaton. "Basically, it's based on the fact that earthquakes take a certain time to occur. They propagate along the fault. Here in California, we could get up to one minute of warning of a major earthquake on the San Andreas fault before it was to shake downtown Los Angeles."
Heaton also explained that "almost all of our smart phones have actually got seismometers in them."
Scientists working on an early warning system hope one day to say: "There's an app for that."
"So if we can take the information from the smart phones and assemble it and send it back to some central place, then not only is your smart phone telling you what's coming, but it's telling us what's happening where you are," said Heaton.
A few seconds of warning is good. Years of preparation is better. In the past two decades California has taken the lessons learned in the San Francisco and Northridge quakes to prepare for what could come next.
"The transportation system, the water pipelines crossing the faults, [and] the bridges have been retrofitted, except for the Bay Bridge, not quite finished yet, " said Schwartz.
The collapse of part of the Bay Bridge in the1989 quake made it a symbol of the state's vulnerable infrastructure. The replacement bridge scheduled for completion in 2013 includes innovations designed to let it bend and swing and rock in a major earthquake.
But its earthquake readiness comes at a cost: more than $6 billion.
But as Californians practice for the next Big One, it might be worth considering not just what earthquakes cost California, but what they have given California . . . the promise that it's a place where the past can be wiped away, and everything can start anew.