zone environments. Attenuation relationships have

also been developed for different broad categories of

subsurface conditions, particularly for the categories

Described below are two alternative

of rock and firm soils. In some cases, attenuation

approaches for obtaining response spectra based on

relationships have distinguished the effects of

PSHA: Approach 1 - anchoring a response spectrum

different types of faulting (e.g., strike-slip vs. reverse

shape to the PGAs determined from PSHA;

faulting). It is important to select a set of attenuation

Approach 2 - developing equal-hazard spectra

relationships that are most applicable to the site

directly from the PSHA. The two approaches are

under consideration.

Several recently developed

schematically illustrated in Figures 3-3 and 3-4,

relationships are summarized in Seismological

respectively.

Research Letters (1997).

(1)

Approach 1 - Anchoring Response

g*.*

Spectrum Shape to PGA Determined from PSHA. In

The

seismic

source

this alternative, the hazard analysis is carried out

characterization and ground motion attenuation

only for PGA, and the PGAs for the design ground

characterization are combined in a probabilistic

motions are obtained from the hazard curve

model

to

develop

relationships

between

the

developed for the site. The response spectra are then

amplitude of a ground motion parameter and the

constructed by anchoring appropriate response

probability or frequency of its exceedance (diagram c

spectrum shapes to the PGA values.

Typically,

of Figures 3-3 and 3-4).

These relationships are

spectrum shapes for the appropriate category of

termed hazard curves. A hazard curve for PGA is

subsurface condition, such as the shapes contained in

illustrated in Figure 3-12. Appendix E describes the

the 1994 Uniform Building Code (UBC), are used.

mathematical formulation for the seismic hazard

It should be noted, however, that widely used

model, and provides examples of its usage in

spectrum shapes, such as those in the 1994 UBC,

obtaining hazard curves.

The appendix also

were developed on the basis of predominantly WUS

discusses the quantification of uncertainty in hazard

shallow crustal earthquake ground-motion data, and

curves as related to the uncertainty involved in the

they may not be appropriate for EUS earthquakes or

relationships and parameters of the model (i.e.,

subduction zone earthquakes.

Furthermore, such

uncertainty in seismic source parameters such as

spectrum shapes are considered to be most applicable

to moderate-magnitude earthquakes (magnitude . 6

maximum earthquake magnitude, frequency of

earthquake occurrence, etc., and uncertainty in the

1/2) and close to moderate distances (distance < 100

choice of appropriate attenuation relationships). It is

km).

For larger magnitudes and distances, the

important to incorporate these uncertainties in a

shapes may be unconservative in the long-period

PSHA in order to provide robust estimates of the

range; conversely, for smaller magnitudes, the

mean hazard, and evaluate the uncertainties in the

shapes may be overly conservative for long periods.

hazard.

To assess the appropriateness of the spectrum

3 - 36

Integrated Publishing, Inc. |