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Poster Abstracts | Group 2 – FARM
different from that expected due to the lower elastic stiffness in the damaged material alone. When
the tensile lobe of a rupture tip propagated through the damaged material the velocity of that
rupture was reduced or stopped. By contrast, when the compressive lobe of a rupture tip passed
through the damage, the velocity of that rupture was unaffected by the damage. A physical
interpretation is that passage of a tensile lobe through the damage expends energy by lowering the
normal stress on pre-existing cracks thus allowing frictional sliding along the crack surfaces. When
the compressive lobe of the rupture passes through the damage, compressive stresses prevent
sliding, only minor energy is dissipated, and the damage has almost no effect on the velocity. This
effect can produce asymmetric propagation for earthquake ruptures on slip surfaces near the edge
of the highly damaged fault zone.
2-091
THE EFFECT OF ASYMMETRIC DAMAGE ON DYNAMIC SHEAR RUPTURE
PROPAGATION II: WITH MISMATCH IN BULK ELASTICITY Bhat HS, Biegel RL,
Sammis CG, and Rosakis AJ
We investigate the asymmetry of ruputure velocity on an inteface that combines a bulk elastic
mismatch with a contrast in off-fault damage. dynamic ruptures on the interface. Dynamic
ruptures were propagated on the interface between thermally shocked (Damaged) Homalite and
polycarbonate plates. The anelastic asymmetry introduced by damage is defined by ‘T’ and ‘C’
directions depending on whether the tensile or compressive lobe of the rupture tip stress
concentration lies on the damaged side of the fault. The elastic asymmetry is commonly defined by
‘+’ and ‘-‘ directions where ‘+’ is the direction of slip of the softer material. Since damaged
Homalite is stiffer than polycarbonate our directions are ‘T+’ and ‘C-‘. Theoretical and numerical
studies predict that shear rupture on elastic bimaterial interfaces propagates in the ‘+’ direction at
the generalized Rayleigh wave speed or in some numerical cases at the P-wave speed of the harder
material, Pfast, while in the ‘-‘ direction the rupture propagates at sub-shear speed or at the P-wave
speed of the softer material, Pslow, depending on the loading conditions. We observe that the offfault
damage effect overcomes the elastic bimaterial effect in dynamic rupture propagation. In the
‘C-‘ direction the rupture propagates at sub-shear to supershear speeds, as in undamaged
bimaterial systems, reaching a maximum speed of Pslow. In the ‘T+’ direction however the rupture
propagates at sub-shear speeds or comes to a complete stop due to increased damaged activation
(slip and opening along micro-cracks) which results in the reduction of stored elastic potential
energy and energy dissipation along the micro-cracks. Biegel et al. [2008] found similar results for
propagation on the interface between Homalite and damaged Homalite where rupture speeds
were slowed or even stopped in the ‘T-‘ direction but were almost unaffected in the ‘C+’ direction.
2-092
EXPERIMENTAL INVESTIGATION OF A TRANSITION BETWEEN STICK-SLIP AND
CREEP AS A FUNCTION OF TEMPERATURE, SLIP RATE, AND NORMAL STRESS
Mitchell EK, Brown KM, and Fialko Y
We investigate frictional properties of crystalline rocks to map the transition between stick-slip
(velocity-weakening) and stable creep (velocity-strengthening) behavior as a function of: slip
velocity, temperature and normal stress. We performed a series of direct shear tests on diabase and
novaculite for velocities of 10-5 - 3x10-2 mm/s, temperatures of 25-500°C and normal stresses of 1-
15MPa. Analysis of data reveals four basic types of frictional behavior: stick-slip, episodic slow-slip
events, quasi-sinusoidal accelerated creep and stable sliding. Episodic acceleration and peak slip
velocities progressively decrease through these phases in the above order. As temperature
increases and forcing velocity decreases, the sample progresses from stable sliding, to sinusoidal
accelerated creep, to stick creep, and then to stick-slip. The transition seems to occur around 200°C
190 | Southern California Earthquake Center