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Group 1 – GMP | Poster Abstracts
Wald & Allen (2007). Our measurements were taken at 509 sites in California and Nevada. Most of
the southern California measurements (188 sites) were taken along the San Gabriel River (Thelen et
al., 2006) and were used by Wald & Allen (2007) in their regressions. An additional 96 sites were
measured as a part of smaller studies in other parts of California. In Nevada, 109 sites were
measured in the Reno basin affected by fluvial, alluvial, lacustrine, and tectonically-driven
processes. On the basis of our Reno observations, the low correlation between Vs30 and slope we
interpret to be influenced by the complexity of landscape evolution and less by topographic slope
alone. Measurements in southern Nevada (116 sites) were influenced by the caliche development
that dominates local soil development. Here, higher velocities are observed where topographic
slope is lowest, contrary to the Wald & Allen (2007) model. Our analysis finds that 67% of our Vs30
measurements (338 of 509) lie outside ±20% of the predicted values based on topographic slope.
The RMS topographic prediction error is 161 m/s, or 45% of the measured Vs30. These results
suggest that the Wald & Allen (2007) first-order approach based on slope is not able to reliably
predict measured Vs30 values in regions of complex geology. Our results also highlight the
inherent variability of shallow shear-wave velocity in complex environments and the need for
more detailed Vs30 measurements to determine more refined predictive methods for
microzonation.
1-053
CALCULATING THE SOURCE SENSITIVITY OF BASIN GUIDED WAVES BY TIME-
REVERSED SIMULATIONS Day SM, Roten D, and Olsen KB
Simulations of earthquake rupture on the southern San Andreas fault (e.g., TeraShake; ShakeOut)
reveal large amplifications associated with channeling of seismic energy along contiguous
sedimentary basins. Geometrically similar excitation patterns can be recognized repeatedly in
different SAF simulations (e.g., Love wave-like energy with predominant period around 4 seconds,
channeled southwestwardly from the San Gabriel basin into Los Angeles basin), yet the amplitudes
with which these distinct wavefield patterns are excited differ, depending upon source details (slip
distribution, direction and velocity of rupture).
To improve understanding of the excitation of the high-amplitude patterns, we propose a
numerical method for determining the sensitivity of a given wavefield pattern (i.e., one identified
in a simulation, such as the above-cited sedimentary channeling effect identified in the ShakeOut
simulations) to perturbations of the source kinematics. We first define a functional (phi(u), where u
is the wavefield perturbation) that isolates the wavefield feature of interest and is proportional to
its level of excitation. We then calculate the pullback of that functional onto the source by means of
a single time-reversed (i.e., adjoint) simulation. The resulting functional (G*phi) now acts on the
space of sources (slip functions) rather than wavefields, so given any source perturbation, we can
calculate the resulting feature excitation without actually doing any forward wavefield
simulations. In practice, the kernel of the pulled-back functional G*phi itself gives much insight
into the feature-excitation sensitivity, and the time-reversal simulation itself helps ellucidate the
wave propagation process leading to the wavefield feature in question.
We applied this method to analyze the source sensitivity of the San Gabriel/Los Angeles channeled
wave seen in ShakeOut simulations, finding: (i) Excitation is relatively insensitive to slip on the
southernmost ~60 km long segment of the SAF (between Bombay Beach and Indio). (ii) Excitation
is highly sensitive to slip on the ~100 km long SAF segment between Indio and San Bernardino, but
only when the rupture is toward the NW, and especially when rupture velocity is near the
maximum S wavespeed. (iii) Excitation is very insensitive to slip on the SAF north of San
2008 SCEC Annual Meeting | 97