Strain accumulation on faults beneath Los Angeles: a geodesy-based picture accounting for the effects of sedimentary basins and anthropogenic surface deformation

Chris Rollins, Donald F. Argus, Walter Landry, Sylvain D. Barbot, & Jean-Philippe Avouac

Submitted August 12, 2016, SCEC Contribution #6654, 2016 SCEC Annual Meeting Poster #148

The Los Angeles region is contracting at ~8 mm/yr in the N 5° E direction due to the misalignment of the Mojave section of the San Andreas Fault with the direction of relative Pacific-North American plate motion. This contraction is accommodated by the accumulation of strain on thrust faults such as the Sierra Madre, Puente Hills and other systems and the release of that strain in damaging earthquakes such as the 1971 San Fernando, 1987 Whittier Narrows and 1994 Northridge shocks. A larger earthquake on one of these systems could constitute a worst-case-scenario event for Los Angeles, and so it is essential to use geodetic data to constrain where, and how quickly, tectonic strain is accumulating on these faults. This estimation problem is affected by 1) anthropogenic surface deformation that overprints tectonic contraction in geodetic data, 2) the complex 3D geometries of the relevant faults, and 3) the soft sedimentary basin underlying Los Angeles, which affects the elastostatic Green’s functions that map slip rates on faults to velocities at the surface. Using 1) a GPS velocity field corrected for anthropogenic motions [Argus et al, 2005, and in prep.], 2) a detailed quadrilateral mesh of fault geometry based on an updated version of that in Marshall et al [2009] and on the SCEC CFM5.0 [Shaw et al, 2015], and 3) elastostatic Green’s functions that incorporate the lateral and vertical heterogeneities in elastic properties represented by the SCEC CVM-H15.1, we obtain the most accurate geodesy-based picture of strain accumulation beneath Los Angeles to date. Among other results, we find that strain accumulation on strike-slip faults such as the Palos Verdes, Whittier-Elsinore and Raymond-Hollywood systems may cause an apparent N-S contractional gradient of ~2 mm/yr across Los Angeles that is unrelated to thrust faulting, and that inferred strain accumulation rates on thrust faults are more readily reconciled with geologic slip rates when this strike-slip motion is taken into account.

Key Words
GPS, geodesy, basin, thrust fault

Citation
Rollins, C., Argus, D. F., Landry, W., Barbot, S. D., & Avouac, J. (2016, 08). Strain accumulation on faults beneath Los Angeles: a geodesy-based picture accounting for the effects of sedimentary basins and anthropogenic surface deformation. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Tectonic Geodesy