geologic data in defining the regional rate of earthquake
magnitude earthquakes on the San Andreas fault than on
activity.
the Hayward fault and the relatively greater influence of
magnitude on long-period motions than short-period
(c) Figure E-15 illustrates the generic logic
motions).
tree for seismic source characterization used for the PSHA.
As shown, the study incorporated uncertainty and
(c) Magnitude contributions to the mean
alternative hypotheses and parameter values for
hazard curves are illustrated in Figure E-19. The
segmentation, maximum rupture length (influencing
contributions of higher magnitudes increase both with
maximum earthquake magnitude), maximum magnitude
increasing period of vibration and with increasing return
estimate correlations, recurrence approach (alternatives of
period (RP).
using seismicity data and tectonic convergence rate data for
source zones), recurrence rates and b-values, and
(d) Analyses of two of the components of the
magnitude distribution model for recurrence assessments
seismic hazard model that contribute to the uncertainty in
(characteristic for faults and characteristic and exponential
the hazard curves are illustrated in Figures E-20 and E-21.
for area sources).
From Figure E-20 it can be seen that much of the
uncertainty in the hazard curves is associated with
uncertainties as to the appropriate attenuation relationship.
(2) Ground motion attenuation characterization.
The uncertainty in the hazard associated with different
Three different sets of rock ground motion attenuation
models of earthquake recurrence for the San Andreas fault
relationships for response spectral acceleration at different
(different segmentation models) (Figure E-21) is small,
periods of vibration (5 % damping) as well as for peak
particularly at lower frequencies of exceedance.
acceleration were utilized. Median values for these
relationships (for magnitudes 5, 6, and 7) are illustrated in
(e) Equal hazard response spectra (expressed
Figure E-16 for peak acceleration and spectral acceleration
in the form of tripartite plots) constructed from the mean
at two periods of vibration. Each set of these relationships
hazard results are shown in Figure E-22 for return periods
also has its associated model of uncertainty (dispersion)
varying from 100 to 2000 years.
around the median curves. The dispersion relationships for
c. Site in Illinois
the preferred model (designated Caltrans, 1991, in Figure
E-16 are summarized in Table E-1. (The attenuation
model designated Caltrans, 1991, is the relationship of
(1) Seismic Source Characterization. The site
Sadigh et al., 1993). Note that this model predicts
location is shown in Figure E-23 and is in southern Illinois
increasing dispersion for decreasing magnitude and
on the Ohio River. The dominant source zone for this site
increasing period of vibration, based on analysis of ground
is the Iapetan Continental Rifts source zone (ICR), which
motion data. The three sets of attenuation relationships
represents an interconnected system of partially developed
comprise three additional branches that are added to the
and failed continental rifts that lie within the mid-continent
logic tree in Figure E-15.
region of the United States and includes the New Madrid
source zone (NSZ) where the large 1811 and 1812
(3) PSHA Results
earthquakes occurred. The extent of ICR is shown by the
heavy line in Figure E-23 along with source zones outside
(a) Typical results of the PSHA are illustrated
ICR and the historical seismicity. Modeling of earthquake
in Figure E-17 in terms of the hazard curves obtained for
recurrence within the dominant ICR can be summarized as
peak acceleration and response spectral acceleration at two
follows:
periods of vibration. The distribution about the mean
hazard curve represents the uncertainty in seismic source
(a) The recurrence rate for large (1811-1812
characterization and ground motion attenuation
type) earthquakes in NSZ is modeled based on paleoseismic
characterization modeled in the logic tree.
evidence. As shown in Figure E-24, the paleoseismic-
determined rate of these earthquakes exceed the rate of
(b) Figure E-18 shows the contributions of
large earthquakes predicted from the historical seismicity.
different seismic sources to the hazard (sources are shown
in Figures E-10 and E-11). As shown, the Hayward fault,
(b) The recurrence rate for smaller
which is closest to the site, dominates the hazard for PGA
earthquakes in ICR is determined by the historical
and spectral values at low periods of vibration, but the San
seismicity. Two basic models are used within a logic tree
Andreas fault contribution increases with increasing
framework for defining subzones for characterizing
vibrational period (reflecting the potential for larger
E-19
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