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Institute for Crustal Studies
UCSB, Santa Barbara, CA
93106-1100
It is well established that sedimentary basins can significantly amplify
earthquake ground motion. However, the amplification at any given site can
vary with earthquake location. To account for basin response in probabilistic
seismic hazard analysis, therefore, we need to know the average amplification
and intrinsic variability (standard deviation) at each site, given all
earthquakes of concern in the region. Due to a dearth of empirical
ground-motion observations, theoretical simulations constitute our best hope
of addressing this issue. Here, 0–0.5 Hz finite-difference, finite-fault
simulations are used to estimate the three-dimensional (3D) response of the
Los Angeles basin to nine different earthquake scenarios. Amplification is
quantified as the peak velocity obtained from the 3D simulation divided by
that predicted using a regional one-dimensional (1D) crustal model. Average
amplification factors are up to a factor of 4, with the values from individual
scenarios typically differing by as much as a factor of 2.5. The average
amplification correlates with basin depth, with values near unity at sites
above sediments with thickness less than 2 km, and up to factors near 6 above
the deepest (
9 km) and steepest-dipping parts of the basin. There is also
some indication that amplification factors are greater for events located
farther from the basin edge. If the 3D amplification factors are divided by
the 1D vertical SH-wave amplification below each site, they are
lowered by up to a factor of 1.7. The duration of shaking in the 3D model is
found to be longer, by up to more than 60 seconds, relative to the 1D basin
response. The simulation of the 1994 Northridge earthquake reproduces recorded
0–0.5 Hz particle velocities relatively well, in particular at
near-source stations. The synthetic and observed peak velocities agree within
a factor of two and the log standard deviation of the residuals is 0.36. This
is a reduction of 54% and 51% compared to the values obtained for the regional
1D model and a 1D model defined by the velocity and density profile below a
site in the middle of the basin (DOW), respectively. This result suggests that
long-period ground-motion estimation can be improved considerably by including
the 3D basin structure. However, there are uncertainties concerning accuracy
of the basin model, model resolution, the omission of material with shear
velocities lower than 1 km/s, and the fact that only nine scenarios have been
considered. Therefore, the amplification factors reported here should be used
with caution until they can be further tested against observations. However,
the results do serve as a guide to what should be expected, particularly with
respect to increased amplification factors at sites located above the deeper
parts of the basin.
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