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SCEC Research Accomplishments | Report
Figure 36. Stress redistribution on faults with heterogeneous strength significantly affects the long-term seismic fault
behavior. Left: Model of a rate-and-state fault with non-uniform distribution of normal stress motivated by fault nonplanarity.
Right: Distribution of rupture speeds over the fault for selected events. When the initial shear prestress is arbitrarily
chosen to be uniform, the rupture speed in the 1st event varies significantly over the fault (top panel, the case of 10%
normal stress variation). However, the 5th event of the same simulation (second panel) has nearly constant rupture speed,
similar to the case with uniform normal stress (not shown). The other panels show the 5th events for cases with 20% and
40% variation.
Large-scale dynamic rupture simulations carried out by SCEC teams have the potential to provide novel and critical
information for the assessment of seismic hazard in Southern California. It is important to understand how to assign initial
conditions for these simulations that are consistent with other assumed ingredients for dynamic rupture simulations,
including fault geometry, materials, initial stresses, and friction (e.g., Harris, 2004). Several groups are focusing on these
critical issues. As an example, Lapusta and colleagues are studying how dynamic rupture interacts with fault heterogeneity
over many earthquake cycles, taking into account the effect of prior fault slip. Dynamic rupture simulations suggest that both
variations in fault strength and fault prestress can strongly influence the development of dynamic ruptures, e.g. induce or
suppress supershear rupture speeds (e.g., Day, 1982; Madariaga et al., 1998; Madariaga and Olsen, 2000; Fukuyama and Olsen,
2002; Dunham et al., 2003; Liu and Lapusta, 2008). Two distributions – of fault strength and shear stress – are often assumed
independently, but they are related due to prior fault slip. To assess how the distribution of fault strength affects typical fault
prestress before large events, Lapusta’s group simulates long-term slip on a fault segment that includes multiple earthquake
cycles and (i) resolves all the stages of every single earthquake in detail, including earthquake nucleation, dynamic rupture
propagation and arrest, and (ii) reproduces post-seismic slippage and interseismic creep (Lapusta et al., 2000; Lapusta and Liu,
2009). These simulations of faults with heterogeneous normal stress distributions motivated by non-planar faults (Jiang and
Lapusta, AGU, 2010) show that the RMS mismatch between the representative static fault strength and prestress before modelspanning
events evolves to near-constant values in the long-term history of the fault. In a set of models, comparison between
the first simulated earthquake and subsequent events shows that shear stress redistribution over time at least partially
compensates for the heterogeneity in fault strength, diminishing its effect on seismic events (Figure 36). Quantifying the longterm
mismatch between strength and prestress in terms of the model parameters, such as the degree of fault heterogeneity,
could provide physically based guidance for assigning initial conditions in simulations of single dynamic earthquake ruptures.
Observational Studies of Fault Slip and Fault Structure
Wechsler et al. (2011) characterize the composition, structure and particle size distribution (PSD) of pulverized and damaged
granitic rocks in a 42-m-deep core adjacent to the San Andreas Fault near Littlerock, CA. The cored section is composed of
pulverized granites and granodiorites, and is cut by numerous mesoscopic secondary shears. The most pronounced faultrelated
alteration occurs along the shears, and is a function of both composition and depth. Alteration to clay appears to be the
result of fluid–rock interaction and brittle deformation under low temperature conditions, rather than of surface-related
weathering. The zones of pulverization that lack significant weathering likely reflect repeating episodes of dynamic dilation
and contraction. The mean particle size for the majority of pulverized and cataclastic samples falls between 50 and 470 µm,
and PSDs can be fit by a power law, with D-values ranging between 2.5 and 3.1. To better understand the response of damage
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