Almost all earthquakes are small. A small segment of a fault, miles underground, jerks a little, the rumble imperceptible at the surface. But with a few quakes, the fault continues breaking, the ground jumps several feet and the world shakes in cataclysm.

“How does a rupture go from an inch a year to 3,000 miles per hour in a few seconds?” asked Ross S. Stein, a geophysicist at the United States Geological Survey.

No one knows.

This gap in knowledge makes earthquake prediction a frustrating and chancy exercise, and complicates the effort to calculate the risk that a human construction like a water reservoir or a geothermal power plant could inadvertently set off a deadly quake.

Last month, Giampaolo Giuliani, a technician who works on a neutrino experiment at the Gran Sasso National Laboratory in Italy, issued an urgent warning that a large earthquake was about to strike the Abruzzo region. The prediction was based on measurements he had made of high levels of radon gas, presumably released from rocks that were being ground up by the stresses of an incipient quake.