The seismic moment (M0) of an earthquake is a more consistent and more physical measure of source strength than magnitude (M) or strain release (see pdf for formula), and this measure of source strength is determined for 47 of the larger earthquakes occurring in the Southern California region since 1857. Most of the seismic moments are obtained by conventional seismological means, but a relationship between M0 and the areal distribution of Intensity VI (AVI) is developed and scaled to estimate M0 when intensity data are available but instrumental data are not. This relationship is log M0 = 1.97 log AVI − 2.55. For the region as a whole, earthquakes at the threshold of M0 ≥ 1025, ≥1026, and ≥1027 dyne-cm have occurred once every 3, 8, and 25 yr, respectively. The spatial occurrence of the five largest earthquakes (M0 ≥ 1 × 1027 dyne-cm) is not limited to a particular geologic province, mode of tectonic accommodation, or geographic locality. It is unlikely that this data set can reliably predict long-term spatial and temporal patterns of the M0 ≥ 1025 dyne-cm seismicity of the Southern California region.