The Diablo Canyon Nuclear Power Plant, located on the central California coast, is nearing completion. Recent geologic and seismological investigations have indicated that the nearby Hosgri fault may be part of a major fault system. If so, the original Design Basis Earthquake (DBE) may be inadequate for Diablo Canyon. Therefore, several factors that could significantly affect the design response spectra for the site were examined. It was found that, because of the area's geology, significant site effects could occur that would reduce ground motion; possible soil-structure interaction would also reduce the seismic motion at the basemat of the main structure as compared to the free-field motion. Studies of wave-passage effects have shown that they are complicated and cannot be easily predicted. It is concluded that an increased-magnitude DBE should have little effect on the reactor design if the increase is caused by increased stress drop rather than greater fault rupture length.