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

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

Figures E-12 and E-13. These comparisons and

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

Figure E-14 and illustrates good agreement. The faults

contribute much more to the regional recurrence than the

area sources. Because the fault recurrence is modeled using

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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