Absolute Strength of the San Andreas Fault Inferred From 3D Loading Simulation and Seismological Data Analysis

Heat flow observations and inferred tectonic stress orientations along the San Andreas fault system in California have raised a controversy about its absolute strength. In the last decade many mechanical models of the San Andreas fault have been proposed to explain the orientations of the axis of maximum horizontal compression. In this study we numerically computed the absolute stress fields around the San Andreas fault on the basis of the elastic dislocation theory and examined the relation between the pattern of the stress field and the absolute fault strength. We also investigated spatial changes in seismic activity and orientation of the axis of maximum horizontal compression around the fault to estimate the absolute fault strength. The strike of the San Andreas fault system is nearly parallel to the direction of relative plate motion (N40W) except for the big bend segment in Southern California. In Northern and Central California the San Andreas fault is simply modeled by an infinitely-long, pure strike-slip interface that divides the elastic surface layer overlying a viscoelastic substratum in two plates. In tectonic loading simulation we specify an absolute fault strength distribution on a model segment of the plate interface. The driving force of this system is steady slip at 40 mm/yr on the remaining parts of the plate interface. Through this simulation we can realize a steady stress field around the fault, caused by the steady slip with a frictional resistance at the plate interface. The background absolute stress fields in Northern and Central California are obtained by adding the litho-static pressure to these steady stress fields. From comparison of the computed absolute stress fields and spatial seismicity changes, we can estimate relative strength of these segments (e.g., the northern locked segment is about three times as strong as the central creeping segment), but not absolute strength. At the big bend segment in Southern California, on the other hand, the fault strike is oblique by 20 degrees to the direction of relative plate motion. In this case we model the fault system by an oblique transcurrent fault segment that connects the northern and southern semi-infinitely long, pure strike-slip plate interfaces. The driving force of this system is steady slip at 40 mm/yr on the northern and southern semi-infinite plate interfaces. At the oblique transcurrent segment the relative plate motion has a convergent component, which cannot be released by fault slip motion. Therefore, in tectonic loading simulation, the stress field caused by the convergent component increases linearly with time. In reality, the background absolute stress around the big bend segment is maintained at a constant level, since the tectonic stress caused by plate convergence is accommodated by inelastic crustal deformation and/or thrust faulting in the surrounding area. On the basis of such an idea, we computed the absolute stress field around the big bend segment, and compared the results with observations to determine the absolute strength of the San Andreas fault in Southern California.