The Bay Area quake was only about one-tenth as powerful as the great quake expected to hit Southern California.
The magnitude of an earthquake is primarily based on the amount of energy released by the temblor. It is a logarithmic scale, and each point represents a tenfold increase, so an 8.1 quake releases 10 times the energy of a 7.1 quake--the magnitude of the Bay-Area quake. A 9.1 quake, which would be among the greatest quakes ever measured, would be 100 times more powerful.
But the magnitude is a measurement of energy at the epicenter of the quake, and that is only one component of the destructive process. Equally important is the intensity of ground shaking and that depends on the distance from the quake, the type of rupture and local geology. Generally, the closer to the quake the greater the danger, but loose, saturated soils many miles farther away may experience much greater ground shaking than closer rock formations.