Using GPS Derived Shear Strain Rates in Southern California to Constrain Fault Slip Rate, Locking Depth, and Residual Off-Fault Strain Rates
Authors: Campbell Lajhon , Holt William , Chen Yu 
Published: March 2020

&lt;p&gt;Strain rate fields within strike-slip regimes often possess complexity associated with along-strike&amp;#160; slip&amp;#160; rate&amp;#160; variations.&amp;#160; These along-strike&amp;#160; slip&amp;#160; rate&amp;#160; variations&amp;#160; produce dilatational&amp;#160; components&amp;#160; of&amp;#160; strain&amp;#160; rate&amp;#160; within&amp;#160; and&amp;#160; near&amp;#160; the&amp;#160; fault&amp;#160; zones&amp;#160; and&amp;#160; within&amp;#160; the adjacent block areas. These dilatation rates do not directly reflect the slip rate magnitude on the strike&amp;#160; slip&amp;#160; fault,&amp;#160; but&amp;#160; rather&amp;#160; the&amp;#160; relative&amp;#160; change in&amp;#160; along-strike&amp;#160; slip&amp;#160; rate.&amp;#160; Displacement rates measured&amp;#160; using&amp;#160; GPS&amp;#160; observations&amp;#160; reflect&amp;#160; the&amp;#160; full&amp;#160; deformation&amp;#160; gradient&amp;#160; field,&amp;#160; which may involve significant dilatational components and other off-fault deformation. Thus, using displacement rates&amp;#160; to&amp;#160; infer&amp;#160; slip&amp;#160; rate&amp;#160; and&amp;#160; locking&amp;#160; depth&amp;#160; of&amp;#160; major&amp;#160; strike-slip&amp;#160; faults&amp;#160; may introduce&amp;#160; errors&amp;#160; when&amp;#160; along-strike&amp;#160; slip&amp;#160; rate&amp;#160; variations&amp;#160; are&amp;#160; present.&amp;#160; On the&amp;#160; contrary,&amp;#160; true locking depth and slip rate can be obtained from the pure strike-slip component of shear strain rates. In this study we investigate the use of shear strain rates alone (obtained from the full displacement rate field of the SCEC 4.0 velocity field in southern California) to infer fault slip rate and locking depth parameters along the San Andreas (up to 37&amp;#176; N) as well as the San Jacinto fault zones. Such an&amp;#160; analysis is&amp;#160; critical&amp;#160; for&amp;#160; accurate&amp;#160; estimation&amp;#160; of&amp;#160; off-fault&amp;#160; strain&amp;#160; rates&amp;#160; outside&amp;#160; of&amp;#160; the major shear zones.&lt;/p&gt;&lt;p&gt;We conducted benchmarking tests to determine if accurate shear strain rates&amp;#160; can&amp;#160; be&amp;#160; obtained&amp;#160; from&amp;#160; a&amp;#160; synthetic&amp;#160; fault&amp;#160; slip&amp;#160; rate&amp;#160; field&amp;#160; possessing&amp;#160; the&amp;#160; same&amp;#160; station spacing as the SCEC 4.0 dataset. The synthetics were derived using Okada&amp;#8217;s [1992] elastic dislocation routine (Coulomb&amp;#160; 3.2).&amp;#160; These displacement&amp;#160; rates&amp;#160; were&amp;#160; interpolated&amp;#160; using&amp;#160; bi-cubic&amp;#160; Bessel&amp;#160; interpolation&amp;#160; to&amp;#160; infer the full&amp;#160; horizontal&amp;#160; velocity&amp;#160; gradient&amp;#160; tensor&amp;#160; field,&amp;#160; along with model uncertainties.&amp;#160; To test realistic conditions, along-strike slip&amp;#160; rates were put into the elastic dislocation model and model displacements were output at the true GPS station spacing in southern California from the SCEC 4.0 dataset. The modeled strain rate field shows negligible strain rate artifacts in most regions and both the shear strain and dilatation rates obtained&amp;#160; from&amp;#160; the&amp;#160; bi-cubic interpolation&amp;#160; were&amp;#160; well-resolved.&amp;#160; The inferred shear strain rate field was then&amp;#160; inverted, using a simple screw dislocation&amp;#160; forward model&amp;#160; for the best-fit&amp;#160; fault location,&amp;#160; fault locking depth, &amp;#160;and&amp;#160; fault&amp;#160; slip&amp;#160; rate.&amp;#160; Model parameter estimates were well resolved,&amp;#160; both&amp;#160; near&amp;#160; and away from fault slip rate transitions (&amp;#177; 1 km for fault locking depth; &amp;#177; 1-2 mm/yr for fault slip rate). Test results to date show the method can resolve slip rates and locking depth within the zones of along-strike transition. Results to date from this methodology applied to southern California using the SCEC 4.0 GPS velocity field show&amp;#160; remarkably&amp;#160; well-resolved&amp;#160; and&amp;#160; prominent&amp;#160; shear&amp;#160; strain&amp;#160; rate&amp;#160; bands&amp;#160; that follow&amp;#160; both&amp;#160; the San Andreas and San &amp;#160;Jacinto&amp;#160; fault&amp;#160; systems.&amp;#160;&amp;#160; The shear strain rates&amp;#160; reflect dramatic&amp;#160; along-strike&amp;#160; slip&amp;#160; rate&amp;#160; variations,&amp;#160; found&amp;#160; in &amp;#160;many&amp;#160; previous&amp;#160; studies.&amp;#160; However, fault locking depths are generally shallower than previously published results. Residual off-fault strain rates, not associated with the major strike-slip faults, appear to accommodate ~30% of the total Pacific-North American plate relative motion.&lt;/p&gt;