The San Andreas Fault: A state of stress analysis in central and northern California

The San Andreas Fault system is a network of faults extending from the Gulf of California to the Mendocino Triple Junction that accommodates the motion between the North American and Pacific tectonic plates. The faults' types, slip rates and distributions of seismicity varies from south to north; the question addressed by this dissertation is whether or not the mechanical behavior of this plate boundary varies as well.; We used suites of fault plane solutions of earthquakes occurring in central and northern California, and inverted them for the best stress tensors. We obtained a map of stress orientations close to and far away from the major strands of the San Andreas Fault system in these areas. In the creeping zone on the central San Andreas the maximum horizontal compression, S _H, is oriented almost perpendicular to the fault trend far away from it and as close as 2 km from it, whereas in the fault zone itself S_H lies at a smaller angle to the fault (∼50°). In northern California there is no clear difference between on-fault and off-fault orientations and S_H orientations are on average at 55° from the trend of major faults. The Bay Area shows an intermediate behavior between the two just mentioned.; This difference in the orientation of S_H from central to northern California suggest a change in the mechanical behavior of the plate boundary between these two regions. This situation could be related to the “young and multiple stranded” SAF system in northern California compared to the “old” SAF in central California where much more slip has accumulated on this one fault strand.; Using the same data we investigated possible temporal variations in the orientation of S_H before and after the occurrence of a major earthquake. Such variations could be related to the stress release produced by the mainshock. For the four events studied, 1986 Mt. Lewis, 1984 Morgan Hill, 1979 Coyote Lake, and 1989 Loma Prieta earthquakes, a rotation of S_H to an orientation more normal to the fault is apparent after the mainshock occurred. In some cases eventually S _H rotates back to its original orientation. However, due to relatively large confidence limits obtained on these orientations, the definition and interpretation of a temporal pattern of the orientation of the maximum horizontal compression remains elusive.