Annual Meeting - SCEC.org

Group 1 – CSEP | Poster Abstracts
relative shear displacement of each fault node. The fault nodes are tested for contact with the
opposite fault member at each time step. This DEM model does not suffer from numerical
instability when discontinuities occur (e.g., instantaneous friction changes, fault separation, etc.).
Thus arbitrary friction laws and fault surface roughness' can be incorporated.
A smooth fault is used to emulate AEA08; the DEM model incorporates the AEA08 material
properties and surface topography. The initial stress state is obtained by displacing the nodal
positions and cycling the damped model to quiescence. Initial investigations used linear time
weakening friction following AEA08. The DEM model produces ground motions for the maximum
slip event at the repository location which are very similar to AEA08 motions. An instantaneous
friction law which drops from static to dynamic value in one time step very slightly increases the
rupture velocity and yields nearly identical ground motions. Fault separation occurs locally near
the free surface for very short time periods. AEA08 results for the “smooth” supershear and
subshear 2.7 m slip cases compare favorably with the DEM ground motions with and without time
weakening. These results indicate that beyond modulating the rupture velocity, frictional
weakening is not a necessary component of dynamic rupture simulations utilizing DEM. These
results strongly suggest that the physical significance of the so-called critical slip weakening
distance must be scrutinized.
1-071
OBSERVED RELATIONS AMONG FAULT STRENGTH LOSS, STRESS DROP, SLIP
VELOCITY, AND NEAR-FAULT PARTICLE VELOCITY DURING DYNAMIC
RUPTURE PROPAGATION, AND THEIR IMPLICATIONS FOR LIMITS ON GROUND
MOTION Beeler N, Kilgore BD, Boettcher M, McGarr A, Fletcher J, Evans J, and Baker S
Extreme ground motions implied by the PSHA at Yucca Mountain at low exceedance probability
require earthquake stress drop to approach the in situ shear stress. Lab rupture propagation
experiments and seismic studies are not generally consistent with this requirement; static stress
drops are