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1 Geodeco S.p.A
Via Aurelia
24
16031 Bogliasco
(GE)
Italy
paolo{at}stanfordalumni.org
(P.B.)
2 Civil and Environmental Engineering
Department
Stanford University
Stanford, California
94305
(C.A.C.)
* Present address: AIR Worldwide, San Francisco, California.
This study presents effective probabilistic procedures for evaluating
ground-motion hazard at the free-field surface of a nonlinear soil deposit
located at a specific site. Ground motion at the surface, or at any depth of
interest within the soil formation (e.g., at the structure foundation level), is
defined here in terms either of a suite of oscillator-frequency-dependent hazard
curves for spectral acceleration,
, or of one or more
spectral acceleration uniform-hazard spectra, each associated with a given mean
return period. It is presumed that similar information is available for the
rock-outcrop input. The effects of uncertainty in soil properties are directly
included.
This methodology incorporates the amplification of the local soil deposit
into the framework of probabilistic seismic hazard analysis (PSHA).
The soil amplification is characterized by a frequency-dependent amplification
function, AF(f), where f is a generic oscillator
frequency. AF(f) is defined as the ratio of
to the spectral
acceleration at the bedrock level,
. The estimates of
the statistics of the amplification function are obtained by a limited number of
nonlinear dynamic analyses of the soil column with uncertain properties, as
discussed in a companion article in this issue
(Bazzurro and Cornell,
2004). The hazard at the soil surface (or at any desired depth) is
computed by convolving the site-specific hazard curve at the bedrock level with
the probability distribution of the amplification function.
The approach presented here provides more precise surface ground-motion-hazard estimates than those found by means of standard attenuation laws for generic soil conditions. The use of generic ground-motion predictive equations may in fact lead to inaccurate results especially for soft-clay-soil sites, where considerable amplification is expected at long periods, and for saturated sandy sites, where high-intensity ground shaking may cause loss of shear strength owing to liquefaction or to cyclic mobility. Both such cases are considered in this article.
In addition to the proposed procedure, two alternative, easier-to-implement but approximate techniques for obtaining hazard estimates at the soil surface are also briefly discussed. One is based on running a conventional PSHA with a rock-attenuation relationship modified to include the soil response, whereas the other consists of using a simple, analytical, closed-form solution that appropriately modifies the hazard results at the rock level.
Related articles in Bulletin of the Seismological Society of America:
This article has been cited by other articles:
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  D. Arroyo and M. Ordaz Inelastic-Strength Spectra in Probabilistic Seismic-Hazard Analysis Bulletin of the Seismological Society of America, December 1, 2007; 97(6): 2171 - 2181. [Abstract] [Full Text] [PDF]
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J. P. Stewart and C. A. Goulet Comment on "Nonlinear Soil-Site Effects in Probabilistic Seismic-Hazard Analysis" by Paolo Bazzurro and C. Allin Cornell Bulletin of the Seismological Society of America, April 1, 2006; 96(2): 745 - 747. [Full Text] [PDF]
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P. Bazzurro and C. A. Cornell Reply to "Comment on 'Nonlinear Soil-Site Effects in Probabilistic Seismic-Hazard Analysis' by Paolo Bazzurro and C. Allin Cornell," by Jonathan P. Stewart and Christine A. Goulet Bulletin of the Seismological Society of America, April 1, 2006; 96(2): 748 - 749. [Full Text] [PDF]
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