Annual Meeting - SCEC.org

Report | SCEC Research Accomplishments
subjected to transient creep, asperity failure occurs, initiating tremor sequences. Parametric studies are underway to define the
properties that control the propagation velocity and distance.
Pollard and Madden investigated the effects of non-planar and non-vertical fault geometry on aftershock focal mechanisms
and fault slip distributions for the 1992 Landers earthquake. They analyzed slip distributions compiled and digitized by the
California Geological Survey (CGS) (Bryant, 1992, 1994, 2004; CGS, 2002), used the HASH (Hardebeck and Shearer, 2002)
algorithm and aftershocks relocated by Zanzerkia (2003), and performed numerical modeling using the quasi-static, linear
elastic, boundary element program Poly3D (Thomas, 1993; Maerten et al., 2005). Their 3D characterization suggests that the
orientations of focal mechanisms may reflect the local
stress field produced by slip during the main shock
and are sensitive to fault geometry. They show that
failure is predicted on non-planar faults having a
wide range of orientations (Figure 22 and Figure 23),
not all consistent with the orientation of and sense-ofslip
on the mainshock faults if they were assumed to
be vertical and planar. Also exploring aspects of fault
complexity, Cooke performed claybox models to
study fault formation and demise in restraining
stepovers along strike-slip faults. Part of her efforts
focused on characterizing the rheologic properties of
wet kaolin (in collaboration with van der Elst and
Brodsky) demonstrating that below the yield stress
wet kaolin displays elastic-plastic behavior, and at
failure exhibits velocity weakening rate-and-state
behavior, thereby serving as a sufficient analog
material for upper crustal deformation.
Micromechanics and Constitutive Behavior
Using micromechanical damage mechanics, Sammis et al. have extended their investigation of the effect of strain rate on the
generation of off-fault damage in the process zone of faults. They demonstrate direct agreement with laboratory data on
fracture strength as a function of slip rate from quasistatic to coseismic rates (Figure 24), and are working to extend their
analysis to address the relation of off-fault damage to earthquake rupture mechanics specifically for the case of a large fault
subjected to multiple earthquakes. Oglesby and Beeler are investigating the dependence of fault strength on rapid changes in
normal stress and their implications for
dynamic rupture in the laboratory by
shearing bare granite surfaces at normal
stress between 5 and 7 MPa. They find that
the response of shear stress to changes in
normal stress evolves with time or
displacement, therefore more closely
resembling the results of Prakash (1998)
and that constitutive relationship of
Prakash (1998) provides the best model of
the results (Figure 25). Oglesby and Beeler
interpret these results to reflect the
enhanced resolution and digital recording
capabilities that were not available at the
time of the Linker and Dieterich (1992)
study. Continuing their work investigating
plastic deformation and strain localization
and their implications for dynamic rupture,
Carlson et al. are generalizing their STZ
58 | Southern California Earthquake Center
Figure 22. Preferred model geometry in (a) map view with icons showing
the locations of the analyzed aftershocks, and (b) oblique view, looking
northwest. Vertical bars indicate fault extents.
Figure 23. Normalized preferred model slip distributions along the (a) Johnson
Valley fault, JVF and (b) Homestead Valley fault, HVF. Horizontal bars below the slip
distributions show the extents of faults, as in Figs. 1 and 2 of SCEC Report #10035.