Crustal Deformation near the San Andreas Fault : Estimating Elastic Parameters of the Upper Crust with
Space Geodesy
Timothy H.
Dixon
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
AAPG Search and Discovery Article #90064©2006-2007 AAPG Distinguished Lecturers