Significant Earthquakes and Faults
Chronological Earthquake Index
Often referred to as the San Juan Capistrano earthquake, due to the death toll from the quake at that famous mission, the exact location and size of this earthquake are unknown, but based upon evidence from sediments along and tree-rings of pines growing near the San Andreas fault, this quake has been identified as one along the Mojave segment of the San Andreas, possibly resulting in as much as 170 km (106 miles) of surface rupture -- roughly, that length of the fault between Tejon Pass and Cajon Pass -- with a theorized epicenter near Wrightwood.
Using only the sparse damage reports, an epicenter in the vicinity of Mission San Juan Capistrano, and a magnitude of about 6, seemed reasonable estimates.
As mentioned above, this quake is remembered for its death-toll: 40 people, all Native Americans, attending mass at San Juan Capistrano were killed when the mortar in the church walls failed and the church collapsed. That even a magnitude 7.5 on the San Andreas fault could have such dire consequences on a structure as far away from the fault as the mission church seems unusual, but it was reported that the construction of the church was of dubious quality.
Some damage associated with this earthquake may have also been reported at Mission San Gabriel, and even in San Diego, but records from this time are poor, and accounts uncertain, so this damage may actually have been caused by the December 21 earthquake, mentioned below.
Another damaging earthquake occurred in southern California in the month of December, 1812, potentially triggered by the Wrightwood quake two weeks earlier that month. The epicenter of this one, too, is of uncertain location. Because of the widespread damage it caused, it was probably as large as magnitude 7. It is probable that the epicenter was located offshore, possibly in the Santa Barbara channel, but an inland epicenter, somewhere in present-day Santa Barbara County, or even Ventura County, cannot be ruled out.
This earthquake destroyed the church at Mission Santa Barbara, and caused near-total destruction at Mission Purisma Concepcion, near present-day Lompoc, causing that site to be abandoned, and a new Mission Purisma built several miles north. At Mission Santa Inez, damage was considerable, but not as severe as that at Santa Barbara or Mission Purisma. A seismic sea wave connected with this earthquake may have been responsible for damaging a Spanish ship at anchor 61 km (38 miles) from Santa Barbara.
Despite the extent of destruction this quake caused, no deaths were reported in connection with the December 21 earthquake. More information about this earthquake can be found at this UC Santa Barbara site.