The San Andreas Fault In Southern California Has a “Propeller” Shape—Implications for Tectonics and Seismic Hazard

The widely held perception that the San Andreas Fault (SAF) is in most places vertical or steeply dipping in southern California does not appear to be correct. Based on seismic-imaging, potential-field, earthquake-aftershock, and selected microseismicity studies, the dip of the SAF changes direction from southwest (55-75 deg.) in the Big Bend region to northeast (10-70 deg.) in the region of the eastern bend (San Bernardino Mountains and Coachella Valley), describing a crude "propeller" shape. In most places, background microseismicity along and across the SAF is consistent with our model SAF surface, but does not reveal the surface itself. Throughout its length, the model surface separates different microseismicity regimes on the Pacific and North American Plates (PAC and NAM, respectively).

Where traced through the crust in deep seismic studies or projected through the crust from upper-crustal studies, the SAF intersects the Moho on the immediate northeast side of the well documented upper-mantle high-velocity body of the Transverse Ranges, imaged from P-wave teleseismic tomography. It appears to continue to more than 150-km depth in the mantle along the northeast and north side of this body. In most places, the dip of the north side of this body is somewhat steeper than the dip of the SAF in the crust. We interpret this geometry to indicate a component of downwelling of PAC lithosphere along the plate boundary. Anisotropy may play a role in the visibility of the high-velocity body to teleseismic P-wave imaging owing to reorientation of the fast direction of olivine in the downwelling.

The dip of the SAF is important for estimating shaking potential for scenario major earthquakes and for calculating geodetic deformation.