1. 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 T33E December 16, 2009

2. 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 T33E December 16, 2009

3. 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
Crowell, Bock, Sandwell - AGU Fall Meeting T33E December 16, 2009

4. 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 T33E December 16, 2009

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

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

7. 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 T33E December 16, 2009

8. 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 T33E December 16, 2009

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

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

11. 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

12. 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 T33E December 16, 2009

13. 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 T33E December 16, 2009

14. 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 T33E December 16, 2009

15. 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 T33E December 16, 2009

16. 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 T33E December 16, 2009