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Poster Abstracts | Group 1 – GMP
1-058
GROUND MOTION SIMULATIONS OF SCENARIO EARTHQUAKE RUPTURES OF
THE HAYWARD FAULT Aagaard B, Graves RW, Larsen S, Ma S, Rodgers A, Brocher T,
Graymer RW, Harris RA, Lienkaemper J, Ponce DA, Schwartz D, Simpson R, Spudich P, Dreger
D, Petersson A, and Boatwright J
We compute ground motions in the San Francisco Bay area for a suite of 35 magnitude 6.7-7.2
scenario earthquake ruptures involving the Hayward fault. The suite of scenarios encompasses
variability in rupture length, hypocenter, distribution of slip, rupture speed, and rise time. The five
rupture lengths include the Hayward fault and portions thereof, as well as combined rupture of the
Hayward and Rodgers Creek faults and the Hayward and Calaveras faults. For most rupture
lengths, we consider three hypocenters, yielding north-to-south rupture, bilateral rupture, and
south-to-north rupture. We also consider multiple random realizations of the slip distribution,
accounting for creeping patches (Funning et al., 2007) either through simple assumptions about
how creep reduces coseismic slip or a slip-predictable approach. The kinematic rupture models
include local variations in rupture speed and use a ray-tracing algorithm to propagate the rupture
front. Although we are not attempting to simulate the 1868 Hayward fault earthquake in detail, a
few of the scenarios are designed to have source parameters that might be similar to this event.
This collaborative effort involves four modeling groups, using different wave propagation codes
and domains of various sizes and resolutions, computing long-period (T > 1-2 s) or broadband (T >
0.1 s) synthetic ground motions for overlapping subsets of the suite of scenarios. The simulations
incorporate the 3-D geologic structure as described by the USGS 3-D Geologic Model (Jachens et
al., 2006; Watt et al., 2007) and USGS Bay Area Velocity Model (Brocher et al., 2007). The
simulations illustrate the dramatic increase in intensity of shaking for a magnitude 7.0 bilateral
rupture of the entire Hayward fault compared with a magnitude 6.8 bilateral rupture of the
southern two-thirds of the fault; the area subjected to shaking stronger than MMI VII increases
from about 10% to more than 40% of the San Francisco Bay urban area. For a given rupture length,
the synthetic ground motions exhibit the strongest sensitivity to the distribution of slip and
proximity to sedimentary basins. The hypocenter also exerts a strong influence on the amplitude of
the shaking due to rupture directivity. The synthetic waveforms exhibit a weaker sensitivity to the
rupture speed and are relatively insensitive to the rise time.
1-059
FIELD-TESTING PRECARIOUSLY BALANCED ROCKS IN THE VICINITY OF SAN
BERNARDINO, CALIFORNIA: SEISMIC HAZARD RAMIFICATIONS Anooshehpoor R,
Purvance MD, and Brune JN
A number of faults exist in the immediate vicinity of San Bernardino which contribute substantially
to the seismic hazard. These include the San Andreas, San Jacinto, Cleghorn, and Cucamonga faults
among others. Groups of precariously balanced rocks (PBRs) exist in close proximity to many of
these fault zones. In the absence of extensive near-source strong motion recordings over
sufficiently long periods of time, we utilize these PBRs to constrain the ground motion amplitudes
which have not been exceeded during the PBR residence times. Field tests have recently been
conducted on seven PBRs near Lake Arrowhead, two PBRs near Silverwood Lake, and two PBRs
south of Beaumont. These field tests consist of accurate determination of the PBR shapes using
photogrammetry along with quasi-static tilting tests of a subgroup of these PBRs to determine
accurate restoring force versus tilt curves. This information is vital for accurate PBR fragility
determination. As outlined in Rood et al. (2008), recent Be-10 cosmogenic age dates are consistent
with the PBRs having been exhumed no more recently than 16-23 ka which are broadly consistent
with VML minimum surface exposure ages on granite corestones in Southern California. These age
100 | Southern California Earthquake Center