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Group 2 – FARM | Poster Abstracts
2-075
SHEAR STRAIN LOCALIZATION IN DYNAMIC RUPTURE AND STICK-SLIP
MODELS Daub EG, Manning ME, and Carlson JM
We study the impact of shear strain localization in models of earthquake faults, investigating the
dynamics of interface-scale elastic sliders and fault-scale ruptures. We account for strain
localization using Shear Transformation Zone (STZ) Theory, a continuum approximation for plastic
deformation in amorphous materials. STZ Theory ties fault weakening to an effective disorder
temperature. While the effective temperature is distinct from thermal temperature, it will evolve in
a similar manner and we include shear heating, diffusion, and relaxation terms in its governing
partial differential equation. Strain localization is incorporated into faulting studies by resolving
the effective temperature dynamics on a spatial grid spanning the width of the fault zone. This
approach differs from the common practice of modeling fault dynamics with a slip-weakening or
rate and state friction law, as the STZ law dynamically chooses how to distribute shear strain in the
gouge. In a model of an interface-scale slider, localized slip increases the spring stiffness for which
a stick-slip instability occurs. At low driving rates, localization can also lead to period doubling,
irregular recurrence times, and varying stress drops. Localization of slip also alters the
spontaneous propagation of elastodynamic ruptures. Ruptures where strain localizes exhibit larger
peak slip rates and stress drops, decreased shear stress at which supershear rupture can occur, and
ruptures can propagate with lower initial shear stresses as self-healing pulses.
2-076
SUPERSHEAR RUPTURES IN 3D SIMULATIONS OF EARTHQUAKE SEQUENCES
AND ASEISMIC SLIP: THE EFFECT OF RHEOLOGICAL BOUNDARIES AND
WEAKER PATCHES Liu Y, and Lapusta N
We study supershear transition and propagation of dynamic rupture through simulations of
earthquake sequences and aseismic slip in a 3D fault model. The model of a planar strike-slip fault
governed by Dieterich-Ruina rate and state friction contains a potentially seismogenic velocityweakening
region surrounded by velocity-strengthening regions.
We find that the rheological boundary between the velocity-weakening and velocity-strengthening
regions promotes supershear transition. During interseismic periods, velocity-strengthening
regions move with slip velocity comparable to the plate loading rate, while the velocity-weakening
region is essentially locked. This disparity in slip concentrates shear stress next to the rheological
boundary. Once earthquake rupture nucleates, it propagates faster over these areas of higher
prestress than over the rest of the seismogenic region, transitioning to supershear speeds in some
cases. Since the presence of such rheological boundaries on natural faults can be inferred from
laboratory studies and fault observations, this factor may significantly contribute to supershear
transition on natural faults. The occurrence of supershear transition in our 3D model depends on
friction properties and fault stress that develops in the model before large earthquakes and can be
explained by the distribution of the effective seismic ratio (Andrews, 1976) on the fault before large
events.
The phenomenon of supershear transition due to rheological boundaries could not be established
in prior studies, as it can only be observed in simulations that include all of the following factors: (i)
inertial effects to enable supershear transition; (ii) a 3D model to include the rheological boundary
in the direction of rupture propagation; and (iii) long-term slip history to establish the
corresponding stress distribution on the fault before large events.
2008 SCEC Annual Meeting | 181