It wasn't on the main strand of the San Andreas, but on a previously unknown offshoot. It moved in a way that pushes up mountains, rather than scooting two blocks of earth past each other like the classic San Andreas quake. Further study revealed that a third type of fault, another mountain-lifter, must exist in the shattered mass of rock beneath the Santa Cruz Mountains where Loma Prieta began. It may not happen very often. It seems the more scientists learn about earthquakes, the more questions they have.
Quakes turn out to be more capricious than scientists had thought.
They can leap from one fault to another, vaulting three-mile gaps and ignoring geographical barriers that were once thought to confine them and limit their size. By 1989, many scientists already were losing hope of ever predicting earthquakes, said Paul Somerville, a seismologist with URS Greiner Woodward-Clyde Federal Services, a consulting firm in Pasadena. None of the prediction methods people had tried really panned out, he said. These are very basic questions. The way experts see it now, there is no single Big One threatening the Bay Area.