The San Andreas fault, which produced the disastrous 1906 San Francisco earthquake, is shifting at a faster pace than expected, say scientists at Goddard Space Flight Center in Greenbelt, Md.
The ground shifts, measured by laser beams and an orbiting satellite, have reached about 3 inches a year and are "about 50 percent larger than we would have guessed from geological history," said geophysicist David E. Smith.
He said there was no indication that San Francisco may be in for a major earthquake soon, but he said the calculations suggest the next earthquake may be larger and might occur sooner than expected.
Scientists have predicted a second major earthquake for the area sometime between now and 2025.
The San Andreas fault begins in the Gulf of California and nearly parallels the coast as it moves northwesterly to a point north of San Francisco. Then it heads out to sea toward Alaska.
Smith said that as the two massive subterranean slabs that make up the fault continue to rub together as they shift, there is a buildup of "stored energy." When that energy exceeds the power of friction, which keeps the two plates from sliding past each other easily, an earthquake results.
Smith said the rate of movement of the plates "may be very spasmodic" and that after 10 years or more, the rate of movement may return to 2 inches a year.