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Crustal Deformation near the San Andreas Fault: Estimating Elastic Parameters of the Upper Crust with Space Geodesy

 

Timothy H. Dixon

University of Miami, Florida

 

Major faults juxtapose geologically dissimilar terrains. These terrains, especially in the upper crust, may vary in mechanical properties, leading to asymmetric strain accumulation, and perhaps asymmetric strain release in earthquakes. Improved knowledge of the variability of upper crustal mechanical properties is important in a wide variety of fields, including construction of large building and dams, oil drilling, and interpretation of seismic data. Here I discuss a new way of estimating such properties in regions of active faulting, using GPS. New GPS data on the Carrizo segment of the San Andreas Fault, separating the Salinian block southwest of the fault from Franciscan terrane to the northeast, allow us to better quantify asymmetric strain accumulation here. The asymmetry reaches a maximum of about 6 mm/yr, ~25 km from the fault. Finite element models allow investigation of the possible role of variable elastic layer thickness and upper crustal material properties. The data are well fit with a simple model comprising a weak upper crustal (<10 km depth) zone, 10-25 km wide northeast of the fault. Seismic and magneto-telluric data support this model. We can also estimate a “long term” (averaged over several seismic cycles) slip rate for the San Andreas Fault that agrees with the known Holocene rate, and a viscosity for the combined lower crust/upper mantle of 2-5x1019 Pa.s.


 

 

AAPG Search and Discovery Article #90064©2006-2007 AAPG Distinguished Lecturers