By studying the frequency of aftershocks following the 1992 earthquake near the town of Landers, Calif., in the Mojave Desert, they found a clue. Fluids resist the stresses that push the two sides of a fault together and may reduce the friction that keeps faults from moving. David Schaff, another geophysics graduate student, has been working with Beroza on the San Andreas and Calaveras faults. Schaff has studied the signature of hundreds of tiny earthquakes recorded by seismic stations in Northern California. Their work shows that damage to the Earth's crust caused during an earthquake actually may reduce the strength of the shaking.
Stronger rocks transmit waves generated by an earthquake much better than weaker rocks.
During an earthquake, rock in the vicinity of the fault may be weakened by cracks, reducing amplitude of strong shaking.