hazard curves for three ground motion parameters (peak
specific response spectra are presented. These examples
ground acceleration and response spectral values at periods
illustrate the characterization of analysis inputs, analysis of
of vibration of 0.3 and 3.0 seconds) at a site are illustrated
the results, and development of equal hazard response
in Figure E-6. The contributions of different earthquake
spectra. The first example is a relatively high-hazard site
magnitudes to the seismic hazard at the same site are
in the San Francisco Bay Area in California; the second
illustrated in Figure E-7. As shown in the example in
example is a moderate hazard site in southern Illinois.
Figure E-7, there is increasing contribution to the hazard
b.
Site in San Francisco Bay Area.
from large magnitude earthquakes as the response spectral
period of vibration increases. This result is typical and
reflects the larger influence of magnitude on ground
(1) Seismic source characterization.
motions at longer periods, as illustrated in the attenuation
curves in Figure 3-10 of Chapter 3. Also as shown, the
(a) The site is a rock site located
contribution of larger magnitude earthquakes increases as
approximately 21 km (13 miles) east of the San Andreas
the return period increases (probability level decreases).
fault and 7 km (4.3 miles) west of the Hayward fault, as
This result is also typical and reflects the lesser ability of
shown in Figure E-10. The seismic sources, including
smaller magnitude earthquakes to produce high levels of
discrete faults and area sources, are shown in Figure E-11.
ground motion. An analysis similar to that shown in
The corridors shown around the faults are for the purposes
Figure E-7can also be made to identify the dominant
of analyzing the seismicity that is likely associated with the
distance ranges contributing to the seismic hazard
faults.
(although the distance contributions may be adequately
described by the source contributions in many cases). In
(b) For each fault, cumulative earthquake
cases where site-specific acceleration time histories are
recurrence based on seismicity was plotted and compared
required, such analyses of the dominant contributors to the
with earthquake recurrence based on geologic slip rate data
site ground motion hazard are essential to the process of
for the fault. For the slip-rate-based recurrence
selecting or developing time histories that have appropriate
assessments, two magnitude distribution models were
characteristics, including an appropriate duration of strong
initially used exponential model; and characteristic model.
:
shaking (duration is strongly correlated with earthquake
Comparisons of recurrence estimated for each model with
magnitude).
seismicity were made. Examples of these comparisons for
the San Andreas fault and Hayward fault are shown in
c. Analysis of Contributions to Uncertainty in the
Figures E-12 and E-13. These comparisons and
Seismic Hazard. The results of a PSHA can also be
comparisons for other faults indicate that the characteristic
analyzed to identify those components of the seismic hazard
magnitude distribution used in conjunction with fault slip
model that primarily contribute to uncertainty in the hazard
rate data provided recurrence characterizations in good
results, as reflected in the hazard curve distributions such
agreement with seismicity data. On the other hand, the
as illustrated in Figure E-5. This uncertainty is due to the
exponential magnitude distribution used with the fault slip
alternative models and parameter values incorporated in the
rate data resulted in recurrence rates that exceeded the rates
logic tree. The analysis of two potential contributors to
from seismicity data. From these comparisons and
uncertainty in seismic hazard results is illustrated in
comparisons for the other faults, it was concluded that the
Figures E-8 and E-9. In Figure E-8, it can be seen that
fault-specific recurrence was appropriately modeled using
uncertainty in the choice of ground motion attenuation
the characteristic magnitude distribution model and this
relationships contributed substantially to the overall
model was used for all the fault-specific sources.
th
uncertainty in seismic hazard (as measured by the 5 to
Recurrence on the area sources was modeled using both:
95th percentile hazard curve results) for this particular site.
(1) the exponential magnitude distribution and seismicity
In Figure E-9, it can be similarly seen that uncertainty in
data; and (2) both the exponential and characteristic
maximum earthquake magnitude contributed only
magnitude distributions and tectonic data on plate
moderately to the overall uncertainty in seismic hazard for
convergence rates in the San Francisco Bay Area. For the
the same site.
entire central Bay Area, a comparison was made between
the recurrence predicted by the adopted recurrence models
and the observed seismicity. This comparison is shown in
E-5. Examples of PSHA Usage in Developing Site-
Figure E-14 and illustrates good agreement. The faults
Specific Response Spectra.
contribute much more to the regional recurrence than the
area sources. Because the fault recurrence is modeled using
a. Introduction. In the following two subsections,
geologic slip-rate data, the comparison in Figure E-14 is
examples of the application of PSHA in developing site-
indicative of good agreement between seismicity and
E-9
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