Local magnetic field measurements, fault creep observations and local earthquakes on the San Andreas fault

Simultaneous records of local variations in magnetic field, fault creep and the occurrence times of local earthquakes have been obtained for the period early 1974 through mid-1977 along the San Andreas fault between the most southern extent of the 1906 earthquake fault break and the most northern extent of the 1857 break. The data utilized are primarily from stations located near the two ends of this section of the fault where strains accumulation is expected. The magnetic data show that local magnetic field variations up to 1.8 gammas with durations of a few minutes to several months have occurred. The creep data indicate both episodic events and changes in creep rate of up to 10 mm/yr lasting for six months or more. No clear magnetic transients or offsets are evident either simultaneous with or preceding the occurrence times of the episodic creep events by up to a day or so. Although some patterns of creep onset times at adjacent stations appear to correspond to some periods of longer term change in local magnetic field, these changes do not always occur and other groups of creep events have no corresponding changes in local magnetic field. Earthquakes with magnitudes less than 4.0 do not appear to correspond in time to local changes in magnetic field greater than 0.8 gamma or variations in the creep rate.

In order to explain the observations presented in this study, it appears necessary to allow for a substantial amount of deep aseismic slip without any obvious attendant changes in the time distribution or size of the local earthquakes. Changes in stress related to the surface expression of aseismic slip on the San Andreas fault can be estimated from dislocation models fit to these data and to observations of simultaneous strains and tilts at points near the fault. These stress values are on average less than one bar near the surface but are probably more than 10 bars in localized regions at depths of a kilometer or so.