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Poster Abstracts | Group 2 – FARM
bar (SHPB) is utilized to subject cylindrical rock specimens to well defined uniaxial compressive
stress-wave loading. In these experiments the amplitude as well as the duration of the compressive
loading pulse is systematically varied to study the initiation and progression of fragmentation in
both confined and unconfined granite samples. Well characterized lateral confinement can be
generated in the cylindrical specimens during SHPB testing by utilizing metal sleeves/jackets (of
known yield and flow strengths) around the specimens during the dynamic loading process. In the
second series of experiments, plate-impact experiments are conducted to obtain the stress threshold
for inelasticity in Westerly granite by estimating its Hugoniot Elastic Limit (HEL) under shockinduced
compression. These experiments are designed to also provide spall (tensile) strength
following shock-induced compression in the granite samples. The results of the SHPB experiments
indicate that the peak stress for Westerly granite under uniaxial compression is ~ 210 MPa (with a
strain to failure of about 0.7%) in the unconfined state; the peak stress increases to 1 GPa with a
confinement pressure of 60 MPa. The HEL for the granite is estimated to be between 4.2 to 5.0 GPa.
The spall strength following the shock-compression is measured to be small (~ 50 MPa), and nearly
independent of the applied compression level in the range 1.2 to 5.0 GPa. The post-impact samples
show a well defined spall plane with no apparent fragmentation at a shock compression level of 1.2
GPa. At higher impact velocities, fragmentation is observed up-to 1.8 GPa, and the rock samples
are reduced to a powder when the impact stress level is above 2.6 GPa.
2-081
PORE FLUID PRESSURIZATION WITHIN A LOW-PERMEABILITY FAULT CORE
Vredevoogd MA, Oglesby DD, and Park SK
We investigate the impact of pore fluid pressurization on earthquake faulting when the fault occurs
within a narrow zone of low permeability (the fault core). In particular, we explore the effect of
placing the slip plane at different positions within the fault core, and how the proximity to the
surrounding higher permeability material affects the maximum pressurization.
We also look at fault cores of various widths, to see how the distance from the slip surface to the
edge of the fault core changes the timing of the pressurization effect. We also discuss how the
highest pressurization can occur off the slip surface, due to the permeability contrast (Vredevoogd
et. al., 2007).
2-082
FRICTIONAL EXPERIMENTS AT INTERMEDIATE SLIP RATES WITH
CONTROLLING TEMPERATURE Noda H, Kanagawa K, and Senda T
One of the most renovative discoveries over the last decade in the fault-related science is the
extreme velocity-weakening frictional behavior observed in laboratory experiments for variety of
rocks [e.g. Tsutsumi and Shimamoto, 1997; Tullis and Goldsby, 2003; Prakash 2004]. These
observations are explained by several mechanisms such as melt-lubrication [e.g. Shirono et al.,
2006; Nielsen et al., 2008], silica-gel formation [Di Toro et al., 2004], build-up of pore pressure due
to decomposition or desorption of water [O’hara et al., 2006, Rice 2007], phase transformation and
generation of weak material [e.g. Han et al., 2007], and flash heating of microscopic asperities [Rice
1999, 2006; Tullis and Goldsby 2003; Beeler et al., 2007; Noda 2008].
Some of these mechanisms are sensitive to the temperature on the sliding surface while most of the
existing experiments are at room temperature and the fault surface is heated by the friction. In
order to investigate the physical processes operating on the fault surface, it is essentially important
to conduct frictional experiments with controlling the temperature and the slip rates
independently. For this purpose we set up a rotary shear apparatus at Chiba University which was
184 | Southern California Earthquake Center