Triggering of 1812 Santa Barbara earthquake by a great San Andreas shock: Implications for future seismic hazards in southern California

We study the evolution of the stress field over the last 200 years in southern California using the stress buildup associated with major faults and stress drops associated with great earthquakes. In this report we calculate the change in the Coulomb Failure Function (Delta CFF) associated with the great Wrightwood earthquake of Dec. 8, 1812 on the San Andreas fault for the region near Santa Barbara that experienced a large shock 13 days later. The rupture length constrained by paleoseismic data is used for the Wrightwood earthquake of 1812; a uniform displacement of 3.5 m from the surface to seismogenic depths is assumed. The geologic setting and focal mechanisms of earthquakes in the Santa Barbara region are dominated by thrust, left-lateral strike-slip faulting and combinations of the two. For our preferred rupture model of the earthquake of Dec. 8 the Santa Barbara shock of Dec. 21 1812 occurred in a region of +0.01 to +0.1 MPa Delta CFF for reasonable fault orientations, indicating that it may have been advanced by years to decades and triggered by the great San Andreas earthquake 13 days earlier. We also calculate Delta CFF on typical kinds of faults in southern California for a future candidate great earthquake along the San Bernardino and Coachella Valley segments of San Andreas fault. This earthquake produces large positive values of Delta CFF for shallow-dipping thrust faults beneath the greater Los Angeles area and for NW-trending right-lateral strike-slip faults in three regions. This suggests that shocks of magnitude similar to the M 7.1 1812 Santa Barbara earthquake could be triggered by the next great event on the southern San Andreas fault.