Seismic Site Response for Near-Fault Forward Directivity Ground Motions
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 132, Issue 12
Abstract
Forward directivity effects in the near-fault region produce pulse-type motions that differ significantly from ordinary ground motions that occur at greater distances from the causative fault. Current code site factors are based on empirical observations and analyses involving less intense nonpulse ordinary ground motions. Nonlinear site response analyses with bidirectional shaking are performed using representative site profiles to quantify seismic site response effects for intense near-fault motions resulting from forward directivity. Input rock motions are represented with simplified velocity pulses that characterize the amplitude and period of forward directivity motions. Results indicate that site response affects both the amplitude and period of forward directivity pulses, and hence, local site conditions should be considered when evaluating seismic designs in the near-fault region. Stiff soil sites tend to amplify the peak ground velocity and increase the period of pulse-type motions, particularly, when the period of the rock motion coincides with the degraded period of the site. Amplification is limited at soft soil sites by the dynamic strength of the weak soil, so attenuation occurs for intense input motions. This nonlinearity is not reflected in the site factors in current building codes. Guidance is provided for estimating the amplitude and pulse period for velocity pulses at soil sites.
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Acknowledgments
Financial support was provided by the Pacific Earthquake Engineering Research Center through Award No. 2011999 and by the David and Lucile Packard Foundation. The writers wish to thank Dr. Walter Silva of Pacific Engineering and Analysis for sharing with us his soil profile information, and an early edition of the PEER strong motion database that he developed. Professor Pestana of the University of California at Berkeley provided useful comments through the review of Dr. Rodriguez-Marek’s Ph.D. thesis. An initial version of this paper was significantly improved by incorporating comments from four anonymous reviewers.
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Information
Published In
Journal of Geotechnical and Geoenvironmental Engineering
Volume 132 • Issue 12 • December 2006
Pages: 1611 - 1620
Copyright
© 2006 ASCE.
History
Received: Jun 30, 2005
Accepted: Jun 21, 2006
Published online: Dec 1, 2006
Published in print: Dec 2006
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