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The in situ crustal stress field fundamentally governs, and is affected by, the active tectonic processes of plate boundary regions, yet questions remain about the characteristics of this field and the implications for active faults in the upper crust. We estimate the magnitude of differential stress at seismogenic depth in southern California by balancing in situ orientation indicated by earthquake focal mechanisms against the stress imposed by topography, which tends to resist the motion of strike-slip faults. Our results indicate that most regions require differential stress of at least 20 MPa at seismogenic depth. In the areas of most rugged topography along the San Andreas Fault System, differential stress at seismogenic depth must exceed 62 MPa consistent with differential stress estimates from complimentary methods. This furthermore suggests pore pressure must be less than lithostatic and that coefficient of friction cannot be very low (i.e. coefficient of static friction >0.3). [ABSTRACT FROM AUTHOR]

The central section of the San Andreas Fault (SAF) displays a range of seismic phenomena including normal earthquakes, low-frequency earthquakes (LFE), repeating microearthquakes (REQ) and aseismic creep. Although many lines of evidence suggest that LFEs are tied to the presence of fluids, their geological setting is still poorly understood. Here, we map the seismic velocity structures associated with LFEs beneath the central SAF using surface wave tomography from ambient seismic noise to provide constraints on the physical conditions that control LFE occurrence. Fault perpendicular sections show that the SAF, as revealed by lateral contrasts in relative velocities, is contiguous to depths of 50 km and appears to be relatively localized at depths between about 15 and 30 km. This is consistent with the hypothesis that LFEs are shear-slip events on a deep extension of the SAF. We find that along strike variations in seismic behaviour correspond to changes in the seismic structure, which support proposed connections between fluids and seismicity. LFEs and REQs occur within low-velocity structures, suggesting that the presence of fluids, weaker minerals, or hydrous phase minerals may play an important role in the generation of slow-slip phenomena. [ABSTRACT FROM AUTHOR]

Sets of E- to NE-trending sinistral and/or reverse faults occur within the San Andreas system, and are associated with palaeomagnetic evidence for clockwise vertical-axis rotations. These structures cut across the trend of active dextral faults, posing questions as to how displacement is transferred across them. Geodetic data show that they lie within an overall dextral shear field, but the data are commonly interpreted to indicate little or no slip, nor any significant rate of rotation. We model these structures as rotating by bookshelf slip in a dextral shear field, and show that a combination of sinistral slip and rotation can produce the observed velocity field. This allows prediction of rates of slip, rotation, fault-parallel extension and fault-normal shortening within the panel. We use this method to calculate the kinematics of the central segment of the Garlock Fault, which cuts across the eastern California shear zone at a high angle. We obtain a sinistral slip rate of 6.1 ± 1.1 mm yr–1, comparable to geological evidence, but higher than most previous geodetic estimates, and a rotation rate of 4.0 ± 0.7° Myr–1 clockwise. The western Transverse Ranges transect a similar shear zone in coastal and offshore California, but at an angle of only 40°. As a result, the faults, which were sinistral when they were at a higher angle to the shear zone, have been reactivated in a dextral sense at a low rate, and the rate of rotation of the panel has decreased from its long-term rate of ∼5° to 1.6° ± 0.2° Myr–1 clockwise. These results help to resolve some of the apparent discrepancies between geological and geodetic slip-rate estimates, and provide an enhanced understanding of the mechanics of intracontinental transform systems. [ABSTRACT FROM AUTHOR]

SUMMARY We performed forward modelling of seismic anisotropy beneath a migrating strike-slip plate boundary to: (1) test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the San Andreas Fault and (2) constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle. The modelling involves five steps: (1) thermomechanical modelling, using a finite-element code, of the deformation field, (2) viscoplastic self-consistent modelling of the resulting olivine and pyroxene crystal preferred orientations, (3) calculation of the elasticity tensors for different domains of the finite elements (FEs) model, (4) forward modelling of seismic wave propagation through the model using ray theory, finite-frequency theory and a full-wave approach and (5) performing splitting measurements on the synthetic seismograms. SKS splitting data in central California are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the Mendocino Triple Junction. The westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the San Andreas Fault in central California. Comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two-layer models accounting for the observations in the vicinity of the San Andreas Fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary. The ability of different synthetic approaches to localize horizontally and vertically the plate boundary-related deformation differs significantly. Splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model. In contrast, splitting analysis on full-wave synthetics, which should be more representative of actual long-period SKS waves, results in smooth apparent lateral variations of anisotropy; the exact location and width of the plate boundary may only be retrieved by comparing fast-polarization profiles obtained using a multichannel analysis on waves with different periods. [ABSTRACT FROM AUTHOR]

We investigate the seismic velocity contrast across the San Andreas fault (SAF) in the Parkfield area using fault zone head waves (FZHW) that propagate along the bimaterial fault interface and direct P waves. We systematically analyse large data sets of near-fault waveforms recorded by several seismic networks over the period 1984–2005. Clear FZHW are observed at many stations on the NE side of the fault in the creeping section of the SAF north of Middle Mountain (MM). This indicates the presence of a sharp bimaterial interface and that the NE side of the fault has lower seismic velocities in that region. The obtained P-wave velocity contrast is about 5–10 per cent north of MM, and it systematically decreases to 0–2 per cent near Gold Hill (GH). The along-strike variations of the velocity contrast are consistent with geological observations of a sliver of high-velocity rock immediately to the NE of the SAF near GH, associated with the GH fault, and existing 3-D seismic tomography results. The obtained imaging results offer an explanation for the mixed rupture directions of the M6-type Parkfield earthquakes. The strong velocity contrast around MM is expected to produce a preferred propagation direction to the SE for earthquakes that nucleate near MM (e.g. the 1934 and 1966 Parkfield earthquakes). In contrast, the near-zero velocity contrast and multiple fault branches near GH imply that earthquakes that nucleate near GH (e.g. the 2004 Parkfield earthquake) are not expected to have a preferred propagation direction to the SE, and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities. The observed systematic reduction of the velocity contrast along the SAF from NW of MM to SE of GH provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the Parkfield section and propagate to the SE, and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the NW. [ABSTRACT FROM AUTHOR]