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Group 2 – FARM | Poster Abstracts
resulting permeability change. We use Berea sandstone samples under triaxial stresses with
confining pressure of 9MPa and 20 MPa of normal stress. We start with DI water flowing through
an intact rock sample at constant pore pressures. Then, we shear the sample in-situ developing a
through going fracture. Once the flow rate stabilizes, we oscillate the pore pressure for 120 sec at a
20 sec period with amplitudes of 0.02 to 0.3 MPa. We observe a clear transient increase in
permeability induced by the oscillatory pore pressures akin to shaking from earthquakes.
Permeability increases with increasing amplitude of the oscillatory pore pressure by up to 50\%.
The maximum value of the permeability increases is $5$x$10^{-16}\,m^2$. After the oscillations
are over, the permeability recovers as the inverse square root of time to the pre-oscillations level
over 10's of minutes. The recovery indicates a reversible mechanism like unclogging and clogging
of fractures, as opposed to an irreversible one, like micro-fracturing, as responsible for the transient
permeability enhancement. Our result has clear consequences. It points at the feasibility of
dynamically controlling permeability of fractured systems. Its application ranges from hydrology
and oil reservoir engineering to geophysics and earthquakes triggering mediated by permeability
enhancement in fault zones due to shaking from other earthquakes.
2-097
A PHYSICAL MODEL FOR WIDESPREAD NEAR-SURFACE AND FAULT ZONE
DAMAGE INDUCED BY EARTHQUAKES Ma S
Seismic observations indicate that material velocities at shallow depths decrease over a large area
after large earthquakes. The reductions are widespread, and occur at distances of up to several
source dimensions. A persistent low-velocity fault zone has also been documented extensively
from seismic and geodetic observations, in which the velocity drops further after large
earthquakes. Dynamic stresses carried by seismic waves in the near surface or accompanying
rupture at depth in the fault zone, are thought to create these velocity reductions by causing
material damage. However, a rigorous physical interpretation as to why modest dynamic stresses
can cause widespread near-surface damage, and why fault damage zones form, is lacking. By using
a Drucker-Prager yielding criterion to simulate dynamic rupture propagation on a strike-slip fault,
I show that the widespread near-surface damage is caused by material yielding induced by seismic
waves under the low confining pressure. Because the confining pressure increases with depth,
materials yield more easily near the surface. The yielding zone at depth is narrowly confined near
the fault, but its thickness broadens dramatically near the surface, forming a ‘flower-like’ damage
zone, which is commonly observed in the geologic record. The fault zone damage at depth is
induced by the large dynamic stress associated with the rupture front, and can be induced by
strong seismic waves ahead of the rupture front near the Earth’s surface. These results have
important implications for the formation and evolution of fault zones, and possibly for the
dynamic triggering of earthquakes as well.
2-098
DAMAGE ASYMMETRY IN FAULT STRUCTURE OBSERVED GEOLOGICALLY
CAN BE GENERATED BY BILATERAL RUPTURES ALONG A BIMATERIAL
INTERFACE Duan B
Damage asymmetry in fault structure has been observed along some strike-slip faults recently (e.g.,
Dor et al., 2006a,b, 2008). These geological observations are very useful constraints for fault and
rupture mechanics. The previous theoretical study by Ben-Zion and Shi (2005) has shown that
unilateral ruptures along a bimaterial interface can produce asymmetric off-fault damage. In this
study, we find that asymmetric off-fault damage can also be generated by bilateral ruptures along a
bimaterial interface. We perform elastoplastic calculations on 2D plane strain models, with slipweakening
friction laws on faults to govern dynamic rupture propagation and Mohr-Coulomb
2008 SCEC Annual Meeting | 193