Annual Meeting - SCEC.org

Poster Abstracts | Group 1 – ExGM
One group of PBRs in southern California—known as the Grass Valley site—sits in close proximity
to major regional faults, making it an important location for studying ground motions. At least
eight major faults are within this structurally complex area of the Transverse Ranges, including
those of the North Frontal Thrust System of the San Bernardino Mountains, the San Andreas, and
the southwest segment of the Eastern California shear zone. Some of these faults have been
interpreted to have ruptured during the Holocene and have high (> 1mm/yr) reported slip rates.
The southern Cleghorn, a sinistral strike-slip fault that traverses the Grass Valley PBR site has a
reported Holocene slip rate of 3 mm/yr (Meisling, 1984) that has recently been questioned (Bryant,
QFFDB, 2003). Numerous PBRs are present within 1 km of the Cleghorn fault in the Grass Valley
drainage. Samples of two pedestals and a PBR from the Grass Valley site yielded preliminary Be-10
exposure ages of 23-28 ka for two pedestal rocks and 50 ka for one PBR (see Rood et al., 2008 SCEC
abstract). The late Pleistocene exposure ages suggest that the rocks have been precariously
balanced during the Holocene and have not been toppled by an earthquake on the southern
Cleghorn fault. Therefore, we infer that the southern Cleghorn fault has not been active during the
Holocene.
1-064
SENSITIVITY OF GROUND MOTION AT YUCCA MOUNTAIN TO UNCERTAINTIES
IN FAULT GEOMETRY AND FAULT ZONE STRUCTURE Duan B, and Day SM
To explore how uncertainties in fault geometry and fault zone structure may affect physical limits
on ground motion at Yucca Mountain, we conduct dynamic, deterministic 2D finite element
calculations. We start from the model with maximum slip (nearly complete stress drop) on Solitario
Canyon fault studied by Andrews et al. (2007), in which the 60° west-dipping fault is embedded in
a tilted and layered geologic structure offset by normal faults with fluctuating topography. The
velocity waveforms at the underground repository site that we obtained from this reference model
are very close to those obtained by Andrews et al. (2007), with difference in velocity peaks within
10-15% (probably due to small difference in setup of the initial stress). Based on this reference
model, we also find that different treatments of pore-pressure (i.e., time-dependent or non-timedependent)
can have significant effects on velocity peaks when off-fault material behaves elastoplastically.
In this case, we obtain a vertical velocity peak of ~ 3 m/s with time-dependent porepressure,
compared with ~ 4 m/s with non-time-dependent pore-pressure at the site.
Inclusion of a low-velocity fault zone with a width of 50 m on each side of the fault, within which
the seismic velocities are reduced by 20% compared with the corresponding layer's country rock,
has no obvious effects on velocity peaks at the site in either elastic and elastoplastic off-fault
response calculations, though the introduction of the low-velocity zone appears to introduce more
high frequency motion. A change of 10° of the fault dip from 60° at shallow depth (< 1 km) to 50° at
greater depth results in a larger fault slip and higher velocity peaks at the site, particularly in the
elastic off-fault response calculation, mainly because the shallower dip increases the down-dip
fault dimension. The numerical methodology accommodates the introduction of further
complexities in the fault-plane and rock-mass constitutive models and in the fault geometry, and
also enables future simulations in 3D.
1-065
NONLINEAR RESPONSES OF HIGH-RISE BUILDINGS IN SEATTLE FOR
SIMULATED GROUND MOTIONS FROM GIANT CASCADIA SUBDUCTION
EARTHQUAKES (MW 9.2) Yang J, and Heaton TH
With the exception of the 2003 Tokachi-oki earthquake, strong ground recordings from large
subduction earthquakes (Mw >8.0) are meager. Furthermore there are no strong motion recordings
104 | Southern California Earthquake Center