Analysis of Stress and Strain in the Central Coast, California

Monday, 15 December 2014
Nora S Lewandowski, Lettis Consultants International, Inc., Valencia, CA, United States, Jeffrey R Unruh, Lettis Consultants International, Inc., Walnut Creek, CA, United States and Marcia K McLaren, Pacific Gas and Electric Company, San Francisco, CA, United States
We evaluate the orientations of the principal stresses and strains regionally in the Central Coast, California and locally in the Irish Hills using two independent datasets: the GPS velocity field and earthquake focal mechanisms. We use two analytical tools to investigate the geometry and rate of strain from the geodetic velocity data: GPSTRN and SSPX. The GPS inversions indicate the maximum shortening strain, d3, in the Central Coast is directed NE to NNE and the sense of vertical axis rotation is clockwise, consistent with distributed dextral shear. Within the Irish Hills, the direction of d3 ranges from ~N1°W to N20°E, with a mean value of ~N13°E, and negative dilatation which suggests deformation is characterized by net horizontal shortening and vertical thickening. Using focal mechanism datasets, we evaluate strain and stress in the Central Coast using the analytical tools FLTSLP and SATSI, respectively. Regionally, d3 derived from FLTSLP is directed NE to NNE; within the Irish Hills, the mean orientation of d3 is ~N21°E and ranges from ~N15°E to N26°E. Assuming that d3 is parallel to the maximum compressive stress, σ1, the SATSI inversions produced similar results as the FLTSLP inversions, with σ1 oriented in the NS to NNE direction, specifically ranging from ~N9°E to N17°E, with a mean value of ~N13°E, for the Irish Hills. The results of the GPS and focal mechanism data inversions generally are in good agreement, suggesting distributed brittle faulting represented by small earthquakes is releasing a small component of the elastic strain accumulating on the major active faults in the region. In the vicinity of the Irish Hills, all analyses indicate NE to NNE directed d3 and σ1 orientations, with average orientations of ~N10°E to N20°E, consistent with distributed transpressional dextral shear sub-parallel to local motion between the Pacific plate and Sierra Nevada-Central Valley microplate, and previous results of N15°E ±4° for the orientation of P-axes of earthquakes in the Central Coast region north of 35° N.