An offshore zone of faulting approximately 10 km from the southern California coast connects the seismically active strike-slip Newport-Inglewood fault zone in the Los Angeles metropolitan region with the active Rose Canyon fault zone in the San Diego area. Relatively little seismicity has been recorded along the offshore Newport-Inglewood Rose Canyon fault zone, although it has long been suspected of being seismogenic. Active low-angle thrust faults and Quaternary folds have been imaged by seismic reflection profiling along the offshore fault zone, raising the question of whether a through-going, active strike-slip fault zone exists. We applied a waveform cross-correlation algorithm to identify clusters of microseismicity consisting of similar events. Analysis of two clusters along the offshore fault zone shows that they are associated with nearly vertical, north-northwest-striking faults, consistent with an offshore extension of the Newport-Inglewood and Rose Canyon strike-slip fault zones. P-wave polarities from a 1981 event cluster are consistent with a right-lateral strike-slip focal mechanism solution.