We processed the complete digital seismogram database for northern California to measure accurate differential travel times for correlated earthquakes observed at common stations. Correlated earthquakes are earthquakes that occur within a few kilometers of one another and have similar focal mechanisms, thus generating similar waveforms, allowing measurements to be made via cross-correlation analysis. The waveform database was obtained from the Northern California Earthquake Data Center and includes about 15 million seismograms from 225,000 local earthquakes between 1984 and 2003. A total of 26 billion cross-correlation measurements were performed on a 32-node (64 processor) Linux cluster, using improved analysis tools. All event pairs with separation distances of 5 km or less were processed at all stations that recorded the pair. We computed a total of about 1.7 billion P-wave differential times from pairs of waveforms that had cross-correlation coefficients (cc) of 0.6 or larger. The P-wave differential times are often on the order of a factor of ten to a hundred times more accurate than those obtained from routinely picked phase onsets. 1.2 billion S-wave differential times were measured with cc ≥ 0.6, a phase not routinely picked at the Northern California Seismic Network because of the noise level of remaining P coda. We found that approximately 95% of the seismicity includes events that have cross-correlation coefficients of cc ≥ 0.7 with at least one other event recorded at four or more stations. At some stations more than 40% of the recorded events are similar at the cc ≥ 0.9 level, indicating the potential existence of large numbers of repeating earthquakes. Large numbers of correlated events occur in different tectonic regions, including the San Andreas Fault, Long Valley caldera, Geysers geothermal field and Mendocino triple junction. Future research using these data may substantially improve earthquake locations and add insight into the velocity structure in the crust.