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Poster Abstracts | Group 2 – FARM
excess. We found that transitions to chaos for a single block occur for ? ≈ 9. We also studied the
behavior of a system of three blocks under D-R friction, finding that transitions to chaos occurred
for smaller values of this parameter ?. Taking this study a step further, we derive the elastic wave
equation in 1-d under Dieterich-Ruina friction to obtain the position u(x, t), the slip relative to the
driver plate. Using uniformly distributed initial conditions and periodic boundary conditions, we
find that numerical solutions to this PDE bifurcate under critical values of the same parameter ?.
For ? ≈ 4.7, transitions to chaotic solutions occur. For this range of parameter values, the chaotic
behavior occurs only in time; the spatial structure is preserved. We compute the structure function,
S_1(x,t), to explore statistically stationary states of the solution. In these cases, the structure
function will have the property of being only a function of the lag variable t. We also compute the
average of the spectra computed for many runs with different initial conditions in order to view
the distribution of frequency.
2-095
FORCE CHAINS IN SEISMOGENIC FAULTS VISUALIZED WITH PHOTOELASTIC
GRANULAR SHEAR EXPERIMENTS Hayman NW, and Daniels KE
Natural faults have many granular characteristics, including granular fault rocks and
spatiotemporal patterns of slip behaviors akin to granular systems. We present experimental
results which provide insight into granular behavior in natural faults. The experiments allow us to
directly image force chains within a deforming granular media through the use of photoelastic
particles. The apparatus consists of a spring-pulled slider block which deforms a photoelastic
granular aggregate at a constant velocity. Particles that carry more of the load appear brighter
when viewed through crossed polarizers, making the internal stresses optically accessible. The
resulting pattern is a branched, anisotropic force chain network inclined to the shear zone
boundaries. Under both constant-volume and dilational boundary conditions, deformation occurs
predominantly through stick-slip displacements and corresponding force drops. The particle
motion and force chain changes associated with the deformation can either be localized to the
central slip zone or span the system. The sizes of the experimental slip events are observed to have
power-law (Gutenberg-Richter-like) distributions; a particle scale controls the lower limits of the
power-law distributions. For large drops in pulling force with slip, the power-law tail of the size
distributions is strongly affected by the choice of boundary condition. For small force drops the
probability distributions are approximately independent of boundary condition. These sizedependent
variations in stick-slip behavior correspond to changes in the force chain network: on
average, small events correspond to local force chain or particle rearrangements, whereas large
events correspond to system-spanning force chain changes and/or particle-slip. Such force chain
behavior may be responsible for similar size-dependent behaviors of natural faults.
2-096
PORE PRESSURE OSCILLATIONS ENHANCE PERMEABILITY IN THE
LABORATORY Elkhoury JE, Niemeijer AR, Brodsky EE, and Marone CJ
Shaking produced by earthquakes triggers other distant and not so distant earthquakes [Hill et a.,
l1993,Felzer and Brodsky 2006]. It also increases stream flows and spring discharges and even
enhances oil production [Rojstaczer and Wolf 1992, Manga et al., 2003, Beresnev and Johnson 1994].
These observations have been explained by an increase in permeability. If this effect is harnessed
and engineered, it would have major impacts on oil recovery and environmental engineering
[Beresnev and Johnson 1994]. Recent observations show that indeed seismic waves from
earthquakes can triple the effective permeability in natural systems [Elkhoury et al., 2006]. Here we
present the first experimental evidence of permeability increases in fractured rock samples subject
to dynamic stresses. We oscillate the pore pressure through a rock with a fracture and measure the
192 | Southern California Earthquake Center