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Group 1 – GMP | Poster Abstracts
estimates strongly suggest that the PBRs have experienced strong ground motions from many
earthquakes in the region and provide very important seismic hazard constraints. The PBRs are
compared with simplified seismic hazard estimates based on ground motions from the largest
earthquakes contributing to hazard at each site; both older ground motion prediction equations
(GMPEs) and the recently developed Next Generation of Attenuation (NGA) GMPEs are utilized.
These data, combined with data from previous PBR field testing initiatives, will be utilized in the
future to constrain synthetic PetaSHA ground motion catalogues.
1-060
ROCK DAMAGE DURING STRONG SEISMIC GROUND MOTIONS Sleep NH, and
Hagin PN
The shallow subsurface (upper 10s of meters) behaves nonlinearly and anelastically during strong
ground motions. This nonlinear behavior causes attenuation of through-going seismic waves. We
estimate the total diminution of energy strong seismic waves using the difference in P-S delays
from small repeating earthquakes before and after strong shaking. We relate the change in lowamplitude
S-wave velocity to damage during strong shaking that increases porosity. Two effects
suggest that rate and state friction formalism is applicable. (1) The S-wave velocity then slowly
increases with the logarithm of post-seismic time. (2) Strong seismic motion triggers slick-slip small
seismic events in the shallow subsurface (Fischer et al, 2008 BSSA). The rock fails locally at prestressed
places on cracks. Failure on numerous cracks over numerous strong earthquakes
maintains heterogeneous pre-stress. We distinguish quantities from rate and state friction. (1) The
ratio of dynamic shear stress to normal traction that would pulverize the rock if sustained. This
quantity is measured in the laboratory as the starting coefficient of friction of intact rock. The
Linker and Dieterich (1992) relationship adequately represents its dependence on normal traction.
Well-bore failure provides a convenient method of calibration. (2) Given the presence of pre-stress,
there is a somewhat lower dynamic stress that if sustained through a vibration cycle would
attenuate the energy supplied by the seismic wave. It represents the practical limit above which
augmentation of the amplitude of the incoming wave does not significantly augment shaking at the
surface. The maximum stress in Coulomb-based Masing rules has similar implications. The
limiting sustained acceleration for extreme ground motions is less than 1.5 g and does not depend
strongly on rock type. We apply percolation-theory relationships between S-wave velocity and
porosity change provide an energy balance associated with changes in low-amplitude S-wave
velocity. We then use this relationship to estimate the work done to open pore space and damage
rock during strong ground motion. Work to open pore space against lithostatic pressure is a major
cause of nonlinear attenuation in intact rock at depth. This situation differs from that of mature
laboratory gouge where dilatant work against normal traction is only ~1/17 of the macroscopic
frictional work. Micromechanically after strong shaking has ceased, failure leaves residual stress
and strains in its wake. These stresses do work against confining pressure and surface free energy
when they extend crack tips. Failure during strong seismic shaking involves inelastic strains
comparable to the elastic strains. The work to open pore space is thus a significant fraction of the
anelastic work. In contrast, the strains on real contacts in gouge and on sliding faults greatly exceed
the elastic strains. Only a small fraction of the anelastic work scaling to the ratio of real grain
strength to grain shear modulus is available to do work. Furthermore, kinematic dilatancy (rolling
square blocks and broken things not fitting back together where inelastic shear and dilatant strains
are comparable) is important at small strains, but cannot have a net effect at large inelastic strains
in gouge. Energy balance indicates that failure under repeated dynamic strains that just exceed the
elastic limit is applicable to pulverized rocks near major fault zones.
2008 SCEC Annual Meeting | 101