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Report | SCEC Research Accomplishments
earthquakes, and in terms of directly interfacing with earthquake engineers in the analysis of built structures. Activities in
these areas are highlighted by the projects described below.
Model Validation
Precariously Balanced Rocks
Recent activity in studying precariously balanced rocks has shifted towards quantifying the dates of the rocks in order to
facilitate comparison with fault behavior. Grant-Ludwig et al. are using Be-10 dating techniques to constrain the ages of
precariously balanced rocks at several locations along the San Andreas and San Jacinto faults. Analysis of one of the Grass
Valley rocks in the western San Bernardino mountains, which lies near the intersection of the San Andreas and San Jacinto
faults, supports the assessment by Bryant (1987) that the Cleghorn fault has not ruptured in the Holocene. Furthermore,
Grant-Ludwig et al. hypothesize that the Grass Valley and other precariously balanced rocks near Silverwood Lake may
indicate that ground motions have been relatively lower near the stepover between the San Andreas and San Jacinto faults
near Cajon Pass as a result of earthquake ruptures jumping between these two faults. If corroborated with additional
observations, these results would directly impact the development of earthquake rupture scenarios and forecasts for these two
major faults as well as the corresponding ground motions.
Ambient Noise Analysis
Several SCEC researchers continue to exploit ambient noise analysis in validating SCEC Community Velocity Models as a
complement to ground-motion recordings from earthquakes. Chen and Olsen demonstrated improvements made to the SCEC
Community Velocity Model 4.0 via full waveform tomography analysis through comparison of synthetic Green's functions
derived from the seismic velocity model and those from ambient noise observations (Figure 61). These results suggest that the
revised seismic velocity model will yield more accurate travel times and shaking duration for sites throughout southern
California. Hirakawa and Ma evaluated whether observations derived from ambient noise support the waveguide along a
chain of sedimentary basins from San Bernardino to Los Angeles identified in the TeraShake simulations (Olsen et al., 2006).
From four years of ambient seismic noise correlations for 50 station pairs, they generated 570 Green's functions separated into
four frequency bands. Hirakawa and Ma found reverberations for Green's functions involving propagation paths across the
basins, consistent with the waveguide (Figure 62). However, they caution that these Green's functions also contain noise,
which must be reduced to reach a more definitive conclusion about the the presence or absence of the waveguide.
Nonlinear Site Response
Focus group activities have continued to include efforts to understand the physics involved in ground motion at high
frequencies. Wu and Peng are applying a technique developed using Japanese KIK-Net data to determine the peak ground
acceleration at which nonlinear site response becomes significant. In Japan they found that the threshold appears to be in the
range of 60-100 gal with a logarithmic recovery in time. They hypothesize that smaller nonlinear responses below this
threshold with near instantaneous recovery coincide with pre-existing levels of damage, where as the nonlinear responses
above this threshold reflect development of further damage during strong shaking. Wu and Peng are currently applying this
technique to the El Mayor-Cucapah earthquake sequence to determine if the threshold observed in the KIK-Net data applies to
California as well. These results will have a direct impact on further development of the SCEC Broadband Platform, which
relies on stochastic methods and nonlinear site corrections to produce broadband synthetic ground motions.
Ground-Motion Simulations
Broadband Simulations
As described in Section SPECIAL PROJECTS, Community Modeling Environment, the recently released Broadband Platform
provides scientists with a suite of tools to compute broadband synthetic ground motions, including the effects of
heterogeneous rupture propagation and nonlinear site effects. These capabilities continue to be refined as additional studies
provide improved methodologies. Research activities within the last year have focused on nonlinear site response (as
described earlier and in the next paragraph) and rupture model characterization.
Assimaki et al. developed Site 1D as a component of the Broadband Platform in order to provide a more realistic model of
damping in the nonlinear response. Site 1D uses a viscoelastic formulation for frequency-independent Q with a hysteretic
damping scheme. (Li and Assimaki, 2010). In addition to the implementation of the nonlinear site response, the code uses the
peak ground acceleration on rock and a frequency index (FI, a dimensionless quantity that describes the alignment of the site
82 | Southern California Earthquake Center