Brendan W. Crowell Yehuda Bock David T. Sandwell Interseismic Strain Accumulation in the Imperial Valley and Implications for Triggering of Large Earthquakes in Southern California Brendan W. Crowell Yehuda Bock David T. Sandwell Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California, San Diego Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Acknowledgements Funding by the Southern California Earthquake Center Special thanks to Yuri Fialko Thanks to everyone that participated in the field surveys: Andy Barbour, Xiaopeng Tong, Sylvain Barbot, Jim Broesch, Janine Buehler, Xiaowei Chen, Sam Haber, Leah Hogarth, Rowena Lohman, Jim Means, Jill Pearse, Pat Rentz, Matt Wei, Gidi Baer, Mindy Squibb, Ann Armsby, John Blum, Aaron Hieber, Robert Petersen and Virginia Tice Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Overview Performed 3 rapid static GPS surveys between March, 2008 and March, 2009 with respect to CRTN to capture the transfer of strain between the Imperial and the San Andreas faults Analyzed data from our surveys and earlier surveys since 2003 to compute station velocities Gridded velocities into a 0.01º grid Computed strain rate tensor on these velocities and SCEC CMM velocities to see changes between 2003-2009 Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Velocity Field since 2003 Imperial Valley is the transfer zone between the normal faulting East Pacific Rise and the strike-slip San Andreas fault system Strain transfer between the Imperial and San Andreas faults is poorly understood A series of transtensional faults exist in the southern Salton Sea into the Brawley Seismic Zone Seismic swarm activity is common, e.g. the 2009 Bombay Beach, 2005 Obsidian Buttes and 1981 Westmoreland swarms CGPS Campaign Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Maximum Shear Strain Rates 2003-2009 SCEC CMM – Prior to 2003 Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Change in Strain Rate Change in Dilatation Rate Change in Maximum Shear Strain Rate Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Is the strain anomaly real? CRRS GLRS -2.11 mm 1.68 mm -2.5 ± 0.16 mm/yr 1.57 ± 0.35 mm/yr -0.41 ± 0.14 mm/yr 0.52 ± 0.35 mm/yr Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Is the strain anomaly real? Change in Maximum Shear Strain Rate Change in Maximum Shear Strain Rate, with station density reduced to CMM Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Inversion Results Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Simulated Line-of-sight displacement Lohman and McGuire, JGR [2007] Results from fault inversion, this paper Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Slip deficit along the southern San Andreas fault Fault parallel slip deficit is between 5 and 7 mm/yr, equivalent to a shear stress increase of ~0.3 bars/yr Predicted motions – actual motions = slip deficit

Coulomb Stress Change – Slip on OBF N45oW oriented right-lateral strike-slip faults N15oE oriented right-lateral strike-slip faults Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

What causes strain transients in the Salton Trough? We propose that the simple act of the San Andreas fault and the Imperial fault slipping at their far-field rates causes an increase of stress along preferred fault planes When stress is relieved along these fault planes, a strain transient can occur, evidenced by swarm activity and potentially afterslip Brothers et al., Nature Geo. [2009] Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Coulomb Stress Change – Transtensional Basin Optimally oriented strike-slip faults Right-lateral: N15oW Left-lateral: N53oE (OBF is N66oE) Optimally oriented normal faults Orientation: N18oE Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Conclusions We measured a large strain anomaly feature along the southern edge of the Salton Sea, which we associate with the late August, 2005 Obsidian Buttes seismic swarm and continued motion along the Obsidian Buttes fault since then Slip along the Obsidian Buttes fault causes an increase in Coulomb stress along the southern San Andreas fault as well as faults in the Salton Sea Long term motion along the San Andreas and Imperial faults causes an increase in Coulomb stress along preferred fault planes that are seen in the field and from seismicity Events along the auxiliary faults in the Salton Trough can raise stress levels along the San Andreas, in what could be classified as preseismic deformation Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009

Future Directions Very dense GPS surveys around the southern rim of the Salton Sea over the next few years to better understand strain transfer and separate tectonic and non-tectonic subsidence Use ALOS InSAR in the area to perform joint inversions of the two data sets Study the Cerro Prieto – Imperial fault transfer zone as a parallel to the Brawley Seismic Zone Crowell, Bock, Sandwell - AGU Fall Meeting 2009 - T33E-02 - December 16, 2009