Download 9.3 Mb pdf - IRIS

Figure 11: High-resolution seismic reflection profile from urban Los
Angeles showing shallow folding above the backlimb of the Compton
blind thrust fault. The profile shows a narrow “kink band” above
the location where a thrust ramp leaves a horizontal basal fault.
Kinematic model is shown at top. Yellow lines show the locations
of cores used to obtain ages and measure the thickness of the shallow
strata, from which slip rates can be estimated. (Modified from
Leon et al., submitted)
sively moving into more urbanized areas as the number of
available channels and quality of shallow seismic sources has
increased. In addition to imaging the faults themselves, these
studies have been imaging the shallow folds above deep
“blind” thrust faults that lie kilometers below the surface,
helping assess risks from faults which do not have a surface
rupture (Figure 11).
Figure 12. Ground shaking over the sediment-filled Seattle basin
resulting from teleseismic signals from the 1999 Chi-Chi, Taiwan,
earthquake, and during local earthquakes and blasts, as measured
using PASSCAL instruments. The tomographic image of the basin was
made using over 1000 seismometers that recorded large blasts. (Figure
from Pratt et al., 2003 and Snelson et al., 2007)
The availability of large numbers of instruments allows determination
of the spatial distribution of strong-motion amplification
and duration caused by the excitation of shallow sedimentary
basins and deeper structures during earthquakes.
In the Puget Sound region, for example, large numbers of
PASSCAL sensors have been used to monitor ground shaking
created by teleseismic and local earthquakes, large blasts, and
even during the demolition of the King Dome sports stadium
(Snelson et al., 2007; Figure 12). Similar studies have been
carried out in Anchorage, Alaska, and Hawaii to map seismic
amplification beneath urban areas.
PASSCAL high-resolution seismic equipment also is used for
geotechnical studies to characterize the shallow subsurface.
In particular, measurements of the shallow S-wave velocity
structure, either through small-scale refraction profiles or
measurement of surface wave speeds via ambient noise analysis,
are used to determine amplification factors to estimate
damage likelihoods from shear waves during earthquakes.
For example, one recent study mapped the shallow S-wave
velocities along profiles in the Reno and Las Vegas, Nevada,
urban areas, and in Los Angeles.
13