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Local-Scale Models
CDM group researchers are also developing
highly focused numerical models of long term
deformation and fault system development,
taking into account plastic deformation and
damage generation. In his research project, which
straddles the FARM and CDM research areas,
Benchuan Duan (TAMU) models plasticity
evolution due to earthquake shaking. He neatly
illustrates how this results in a component of
coseismic strain in nearby fault damage zones
which is consistent with the background stress,
rather than the coseismic stress change (Figure 44;
Duan et al., 2011 ). Together with earlier SCECfunded
work (Hearn and Fialko, 2009), this work
highlights the possibility that absolute stress
levels in the upper crust might be inferred from
the coseismic deformation of fault damage zones.
SCEC Research Accomplishments | Report
Yuri Fialko and Yoshi Kaneko (Scripps) have also been looking into whether inelastic failure in the shallow crust due to
dynamic earthquake rupture can explain the apparent deficit in shallow slip which is often noted in coseismic ruptures. They
find that the amount of shallow slip deficit is indeed proportional to the amount of inelastic deformation near the Earth's
surface. However, the largest magnitude of slip deficit in models accounting for off-fault yielding is 2-4 times smaller than that
inferred from kinematic inversions of geodetic data (Kaneko and Fialko, 2011).
In addition to the aforementioned numerical studies, Michele Cooke (U M Amherst) is continuing her work with wet kaolin
(clay) analogue models of fault system development, with the goal of understanding the complex geometry of the SAF system
in the Big Bend region. Her models show how faults form and are abandoned at restraining bends under progressive (oblique)
shear strain, and she makes the argument that the chronology of fault formation and abandonment (i.e. development of
secondary faults at progressively increasing distances from the original SAF bend) is consistent with the geologic record.
Viscoelastic models of the earthquake cycle
Two CDM research groups (Brendan Meade at Harvard and Elizabeth Hearn at UBC) tackled the question of how fault slip
rates inferred from classical elastic half-space dislocation models might be affected by viscoelastic earthquake cycle effects,
using earthquake-cycle models. To evaluate the magnitude of the viscoelastic deformation effects, both groups used forward
models of two-dimensional faults with Maxwell and other rheologies to model velocity profiles at different times in the
interseismic interval. These
profiles were inverted for slip
rates on a buried dislocation in an
elastic halfspace, providing an
effective means of assessing the
predicted range of variability in
fault slip rate estimates due to an
idealized rheological
parameterization. Both groups
have found that if earthquake
cycle variability is assumed to
take place entirely within the
upper mantle as a result of stress
relaxation, then certain Burgers
rheologies can simultaneously
explain the general agreement
between geologic and geodetic
Figure 44. Fault-parallel component of the residual displacement field after
an earthquake on the fault (solid black line) within a low-velocity fault zone
FZ2. Motion across FZ1 is retrograde along AA', while it is sympathetic along
BB'. They correspond to elastic and inelastic response of FZ1 to the rupture,
respectively. From Duan et al., 2011.
Figure 45. Predicted and observed variation in geologic/geodetic slip rate estimates as a
function of upper mantle rheology. Left- and right-hand panels are Maxwell and Burgers
rheologies, respectively. Hotter colors indicate larger postseismic deformation signals. Of the
models considered here, only those to the far right hand side of the rightmost figure are
consistent with both the general agreement of between geologic and geodetic slip rate
estimates and the observed magnitude of postseismic deformation. From Brendan Meade.
2011 SCEC Annual Meeting | 73