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SCEC Research Accomplishments | Report
plan to continue this work with a revised version of their boundary element model which generates surface velocities that
may be compared with GPS velocities.
Inferring fault slip and stressing rates from
the Southern California GPS Velocity Field
Brendan Meade and Kaj Johnson continued
their research using block models based on
the SCEC CFM to infer fault slip and moment
accumulation rates on active faults in
southern California. Both of these PI’s have
also begun to tackle the problem of how
viscoelastic relaxation could affect their
results. Direct, analytical calculation (Okada,
1992) of stress rates along the SAF (in the
context of a block model) inherently
incorporates kinematically consistent fault slip
rates and the influence of all fault segments
considered in the analysis. Brendan Meade
and his student John Loveless (Harvard) have
applied this strategy to the SAF, determining
the shear and Coulomb stressing rates both
due to slip on SAF segments alone (“self
stress”) and due to slip on all segments in the
block model (“total stress”) (Figure 41).
Comparing the self and total shear stress rates,
τSAF and τTOT, respectively, provides
indication of how structures nearby and
intersecting the SAF influence patterns of
stress accumulation (Figure 41). The shear
stress rate difference, defined as Δτ = (τTOT
−τSAF)/τTOT ×100, reaches values of up to
+30% along the Mojave and San Bernardino
segments of the SAF and is nearly zero to the
north along the Carrizo segment and to the
south along the Indio and Imperial segments,
except near the isolated fault junctions. The
enhanced stressing rate along Big Bend
segments demonstrates that interseismic
activity on structures that intersect the SAF,
including the San Gabriel, Garlock, North
Frontal, and Eureka Peak faults, act to
increase the likelihood and/or frequency of
seismic failure as compared to estimates based
on τSAF alone. Interpretations of paleoseismic offset data suggest that Big Bend segments have ruptured in roughly twice the
number of earthquakes as the more isolated Carrizo and Indio segments (Weldon et al., 2004), illustrating a correlation
between long-term, macro-scale seismicity and interseismic stress enhancement due to off-SAF processes (Figure 41).
Long-term deformation with large strains and plastic deformation
Figure 41. Modeled shear stress accumulation rates along the SAF (left panel),
with and without contributions from activity on other faults (left and right color
traces, respectively). The difference reaches values of up to 30% along the Mojave
and San Bernardino segments of the SAF and is nearly zero to the north along the
Carrizo segment and to the south along the Indio and Imperial segments, except
near the isolated fault junctions. The enhanced stressing rate along the Big Bend
segments demonstrates that interseismic activity on structures that intersect the
SAF, including the San Gabriel, Garlock, North Frontal, and Eureka Peak faults,
may act to increase the likelihood and/or frequency of seismic failure as compared
to estimates based on the SAF shear alone. Interpretations of paleoseismic offset
data suggest that Big Bend segments have ruptured in roughly twice the number of
earthquakes as the more isolated Carrizo and Indio segments (Weldon et al.,
2004), illustrating a correlation between long-term, macro-scale seismicity and
interseismic stress enhancement due to off-SAF processes (right panel). From
Brendan Meade.
Regional Scale Models
Benjamin Hooks (UT Martin) and Bridget Smith-Konter (UT El Paso) are developing regional-scale numerical models of longterm
deformation associated with the SAF system and its surroundings.They use FLAC, a 3-D dynamic numerical model (e.g.
Cundall and Board, 1988) to reproduce the first-order vertical deformation associated with the Southern San Andreas Fault
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