A stress model of the San Andreas fault and SAFOD pilot hole
Overview. This is a 3D mechanical model aiming to
explain stress orientation and magnitude in the San Andreas fault vicinity. This
study is closely linked to the SAFOD project (see
cross section).
3D Model parameters are defined in files iessai
and jessai . The initial thermal field (file
t8440) is created using a 2D thermal model
(files ie8440 and
je8440). Note that an interpolation is needed
to between the 2D thermal output (irregular grid) and the 3D thermal input (regular
grid). An elastoplastic model (Drucker-Prager plasticity) and a viscoelastic model
(Maxwell model) are used to model respectively the frictional crust and the
ductile crust. The uppermost mantle is simulated using linear viscoelasticity.
The low strength of the San Andreas fault is simulated by a narrow vertical
layer (figure) with a small
effective friction angle. Model boundary conditions are 1) lateral velocities imposed
on the y-z (vertical) sides of the model with 35 mm/yr tangential velocity and
3.5 mm/yr normal velocity; 2) hydrostatic pressure at the base of the crust. A local isostatic compensation is assumed
at the base of the model at 35 km depth..
Results. Steady state (constant stress and strain
rate) is obtained after 50 000 yrs. Principal stress values on three vertical
profiles display an regular increase of the differential stress up to 350°C
temperatures (figure). However,
stress components inside the fault zone appear to be anomalous and largely
overcome the lithostatic stress. Orientations of the larger horizontal stress
Shmax are normal to the fault trace in the far field and are at 45° to the fault
trace in the fault zone. Near field orientations display intermediate values
similar to the ones provided by in-situ stress measurements inside the SAFOD
pilot hole. The variation of Shmax orientations with depth and along the San Andreas fault
reveals that largest variations occur near the fault zone (figure).
Related publication. This study has been published by
Chery, Zoback and Hickman,2004 (pdf)
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