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Report | SCEC Research Accomplishments
medium (Harris and Day, 1993), they find that plastic yielding
has little influence on compressional step-over ruptures, but
significantly reduces the along strike overlap required for
jumping and allows for a wider stepover region along
dilatation stepovers (Figure 29). Time-dependent porepressure
changes allow rupture jumping over wider
compressional stepovers, but significantly reduce the ability
Figure 28. Rupture contours every 0.5 s are mapped on the fault.
for rupture jumping at dilatational stepovers. When
considered together, time-dependent pore-pressure changes have the greatest effect on the resultant behavior (Figure 30).
Several research groups examined rupture propagation along branching faults. Rice and colleagues address the physical and
modeling issues associated with rupture propagation through branched triple junctions using 2D finite element methods
(DeDontney et al., 2011). When a non-cohesive
elastic-plastic material is employed, the elevated stresses around the rupture tip and at the branch junction promote material
failure and inhibit opening, especially in the damage zones of mature faults (Figure 31). Opening may occur in intact
crystalline rock and deformed zones that have experienced significant cementation (i.e., rocks having large cohesive strength),
but is accompanied by significant material softening, decreasing the material strength with shear. Off-fault-plasticity also
promotes rupture of extensional side branches and inhibits rupture on compressional side branches. Oglesby and Bowman
continue to use the finite-element method to model the dynamics of branched faults, extending the work of King et al. (2005).
Over the past year, SCEC intern Jennifer Tarnowski, investigated how the size and shape of the barrier on the vertical fault
affected rupture propagation to the dipping fault. Her results suggest that the barrier on the vertical fault must be larger than
some minimum area for the rupture to move to the dipping fault (Figure 32). To test the hypothesis that rupture propagation
preferentially occurs in the direction of the more compliant side of a bi-material interface, producing greater damage on the
stiffer side, and to understand the influence of material contrast on fault branching in 2D, DeDontney et al. (2011 in press;
2011; 2011 submitted) examined the role of stress state on the distribution of plastic strain and the direction of rupture
propagation. Their results suggest several factors contribute to determining rupture directivity and branch location. For plastic
deformations, the most critical parameter is the orientation of the greatest compressive stress. DeDontney et al. demonstrate
that plastic yielding is predicted in both the stiff and compliant sides, depending on the stress orientation, and rupture
propagation can occur in the direction of slip displacement in the stiffer material if the most compressive stress is at a low
angle to the master fault. Furthermore, fault branches are more
likely to rupture when the branch is in the more compliant
material, regardless of the stress state (Figure 33). Returning to
the theme of planar faults and bimaterials (e.g., Andrews and
Ben-Zion, 1997; Harris and Day, 1997), Rubin and Wang use
observations to test the hypothesis that directivity is related to
across-fault contrast in elastic properties. They (Wang and
Rubin, 2011) performed inversions of spectral ratios of
microearthquakes on the northern creeping section of the SAF
to determine if SE propagation is favored. The spectral ratio
data were fit with the moving point source model to give
estimates of rupture lengths. The data suggest that of the 40% of
earthquake events that can be classified as bilateral ruptures,
the rupture length of the SE end is longer in ~67% of the cases,
Figure 29. Effects of off-fault off- off fault plastic yielding on locations on fault 2 where
and for rupture segments that can be defined as more nearly
propagating ruptures on fault 1 (blue line) jump onto fault 2 through dilational or unilateral, more than ~75% propagate to the SE, and when
compressional stepovers with different widths. Internal friction of 0.85 and defined, have approximately 10% faster propagation velocities.
cohesion of 25 MPa are used to characterize off-fault rock strength in the models They conclude that the asymmetry in aftershock occurrence and
with off-fault plastic deformation.
SE directivity reflect the across-fault material contrast.
Although the inversion results show that the directivity is
somewhat broadly scattered, their analysis clearly demonstrates
that several SE-propagating, strongly unilateral events do not
have NW-propagating counterparts, regardless of the model
62 | Southern California Earthquake Center
Figure 30. Combined effects of off-fault plastic yielding and time-dependent
pore-pressure on the ability of rupture jumping across a stepover.