Focal mechanisms have been determined from P wave first motion polarities for 138 small to moderate (2.6≤M≤4.3) earthquakes that occurred within 10 km of the surface trace of the San Andreas fault in southern California between 1978 and 1985. On the basis of these mechanisms the southern San Andreas fault has been divided into five segments with different stress regimes. Earthquakes in the Fort Tejon segment show oblique reverse slip on east-west and northwest striking faults. The Mojave segment has earthquakes with oblique reverse and right-lateral strike-slip motion on northwest strikes. The San Bernardino segment has normal faulting earthquakes on north-south striking planes, while the Banning segment has reverse, strike-slip, and normal faulting events all occurring in the same area. The earthquakes in the Indio segment show strike-slip and oblique normal faulting on northwest to north-south striking planes. These focal mechanism data have been inverted to determine how the stresses acting on the San Andreas fault in southern California vary with position along strike of the fault. One of the principal stresses is vertical in all of the regions.

The vertical stress is the minimum principal stress in Fort Tejon and Mojave, the intermediate principal stress in Banning and Indio, and the maximum principal stress in San Bernardino. The orientations of the horizontal principal stresses also vary between the regions. The trend of the maximum horizontal stress rotates over 35¿, from N15¿W at Fort Tejon to N20¿E at Indio. Except for the San Bernardino segment, the trend of the maximum horizontal stress is at a constant angle of about 65¿ to the local strike of the San Andreas fault, implying a weak fault. The largest change in the present stress state occurs at the end of the rupture zone of the 1857 Fort Tejon earthquake. It appears that the 1857 rupture ended when it propagated into an area of low stress amplitude, possibly caused by the 15¿ angle between the strikes of the San Jacinto and San Andrea faults. The strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation.