DECEMBER 8, 1812 (M=7) [c6, p157]

The first of two significant earthquakes to occur in southern California in 1812 occurred on December 8 and destroyed the church at Mission San Juan Capistrano, killing 40 neophytes ( Figure 6.4); damage was also sustained at San Gabriel. The accounts of this earthquake and the later one on December 21 cannot be readily disentangled at San Fernando Rey and at San Buena Ventura, considerably complicating the interpretation of this event.

Analyses of these scanty data by Toppozada and others (1981) and Evernden and Thompson (1985) place the epicenter along the south half of the Newport-Inglewood fault zone ( Figure 6.3). This location is somewhat constrained by the interpretation of no damage at Buenaventura during the event. The Los Angeles Star of January 10, 1857, however, stated that the December 8 event severely damaged the church tower (Agnew and Sieh 1978). The same story attributed the collapse of the stone arch roof of the church at San Juan Capistrano to poor construction, a possibility made credible by the death of the master mason before completion of the church ( Figure 6.3; Duncan Agnew, oral commun., 1988).

Recently, Jacoby and others (1988) proposed that this event ruptured the San Andreas fault at Wrightwood ( Figure 6.3), on the basis of dendrochronologic dating of distress to trees growing on the fault trace. Sieh and others (1989) argued that this rupture extended at least 25 km northwestward into the peat bog at Pallet Creek. The fault rupture in this event preserved at Pallet Creek is comparable in size to the rupture formed in the 1857 earthquake.

The preferred location of the December 8, 1812, earthquake on the San Andreas fault as proposed by Jacoby and others appears in Table 6.1. A magnitude of about 7 is consistent with the inferred extent of damage. The lateral extent of rupture is unconstrained to the southeast and may well have extended into the San Bernardino Valley. However, the accounts of the earthquake from Indians living in the San Bernardino Valley that were thought to place some constraint on the rupture are now believed to be fictitious (Harley, 1988), leaving Mission San Gabriel, some 40 km from the rupture, as the nearest point of observation.