Frisco's "Big One" 100 years later.

A century after the famous 1906 earthquake, geophysicists at Stanford (Calif.) University have revisited San Francisco's "Big One" and paint a new picture of a fault that ruptured faster, farther, and further than previously supposed.

"Our understanding of seismic hazard in Northern California, including the Bay Area relies on a thorough understanding of this earthquake and the San Andreas Fault," notes Gregory C. Beroza, professor of geophysics.

Beroza maintains that the 1906 earthquake was a "watershed event" because it convinced geologists that the fundamental theory of how earthquakes work was, in fact, correct.

"That fundamental theory is the elastic rebound hypothesis," he explains. "It says that, before an earthquake, you get the Earth's crust straining and that, suddenly, the strain is released. Prior to this, people had hypothesized about elastic rebound but. for this earthquake, there were measurements showing that this was completely consistent with that idea."

Huge plates of crust on both sides of the San Andreas Fault move in parallel but opposite directions. The Pacific Plate slides to the northwest and the North American Plate slides to the southeast.

On April 18, 1906, at 5:12 a.m., the fault ruptured, probably starting offshore near Daly City, Beroza speculates. The magnitude was 7.7 based on seismic waves (vibrations produced by earthquakes--typically recorded on the other side of the Earth) but 7.9 based on geodetic measurements (of the angles on the Earth's surface before and after an earthquake). Due to the characteristics of logarithmic scales, that means the geodetic measurements indicate an earthquake that was twice as big as the one shown by seismic measurements.

Beroza hypothesizes that the discrepancy might be because the fault rupture proceeded at...