Late Holocene marsh deposits and a shoreline along the coast of the San Joaquin Hills, southern Los Angeles basin, range from 1 to 3.6 m above the active shoreline. Radiocarbon dating of ancient marsh shows that emergence occurred after A.D. 1635. The age, distribution, and geomorphic expression of elevated marsh and shoreline are best explained by tectonic uplift due to a M >7 earthquake. Radiocarbon dates and the historic record of seismicity suggest the earthquake occurred between A.D. 1635 and 1855, possibly in 1769. The historic record of earthquakes in the Los Angeles basin begins in A.D. 1769 and contains no other earthquakes greater than M 6.7. Therefore, the San Joaquin Hills earthquake may be the largest historic earthquake in the Los Angeles basin.