CAVEAT regarding the Shells and FlatMaxwell models in SCEC-CSM

by Peter Bird
Department of Earth, Planetary, and Space Sciences
UCLA
pbird@epss.ucla.edu
2015.09.23

CAVEAT:
There are good reasons to conclude that active faults in southern California have lower
effective friction (e.g., 0.15) than the crustal continuum between these faults (e.g., 0.85).
This causes concentration of long-term-average strain-rates in active faults, elevation of
dislocation-creep strain-rates and shear-stresses at all depths, and deeper frictional/
dislocation-creep (i.e., "brittle/ductile") transitions in active faults.
However, these special stress and strain-rate conditions occur only in narrow wedges
of the upper crust, which taper to nominal thicknesses of zero at the surface.
Therefore, the 3-D sampling grids on which stress models are reported to SCEC CSM
are not able to capture these special conditions in the cores of active faults.
Another issue is that the limited spatial resolution of the FlatMaxwell model would not
be able to represent these local fault conditions even if the CSM grid were refined.
Instead, the stress tensors reported in both the Shells and the FlatMaxwell stress models
at CSM should be regarded as representing the regional or background stresses in strong
crust (with "Byerlee's Law" friction of 0.85) adjacent to, but outside, the active faults.

SUGGESTION:
Those who desire dynamic-model estimates of shear stresses and brittle/ductile transition depths
in active faults of southern California should refer to the table labelled,
"Fault element properties (at mid-points):"
which occurs in the log-file CSM2013001.log.txt produced by program Shells,
which is currently available in the folder with URL of: 
http://peterbird.name/oldFTP/SCEC_Community_Stress_Model/Stress_models_by_Peter_Bird/1_Shells_model/3_output_files
.