Seismic evidence for conjugate slip and block rotation within the San Andreas Fault System, southern California
Authors: Nicholson Craig , Seeber Leonardo , Williams Patrick , Sykes Lynn R. 
Published: February 2008

The pattern of seismicity in southern California indicates that much of the activity is presently occurring on secondary structures, several of which are oriented nearly orthogonal to the strikes of the major through‐going faults. Slip along these secondary transverse features is predominantly left‐lateral and is consistent with the reactivation of conjugate faults by the current regional stress field. Near the intersection of the San Jacinto and San Andreas faults, however, these active left‐lateral faults appear to define a set of small crustal blocks, which in conjunction with both normal and reverse faulting earthquakes, suggests contemporary clockwise rotation as a result of regional right‐lateral shear. Other left‐lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data. Many of these structures predate the modern San Andreas system and may control the pattern of strain accumulation in southern California. Geodetic and paleomagnetic evidence confirm that block rotation by strike‐slip faulting is nearly ubiquitous, particularly in areas where shear is distributed, and that it accommodates both short‐term elastic and long‐term nonelastic strain. A rotating block model accounts for a number of structural styles characteristic of strike‐slip deformation in California, including: variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults; domains of evenly‐spaced antithetic faults that terminate against major fault boundaries; continued development of bends in faults with large lateral displacements; anomalous focal mechanisms; and differential uplift in areas otherwise expected to experience extension and subsidence. Since block rotation requires a detachment surface at depth to permit rotational movement, low‐angle structures like detachments, of either local or regional extent, may be involved in the contemporary strike‐slip deformation of southern California. A block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge‐bounding faults, but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction. This behavior, coupled with the presence of possible regional detachments, accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges. Although fault displacements along secondary structures associated with block rotations remain small, they may still influence the nucleation and the characteristic rupture length of large earthquakes. A more complete description of what these structures are, and how they interact, may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard.