The earthquake hazard associated with ground motion amplification in the lower Portneuf River Valley, Idah

This thesis describes an investigation of the effect of the soil deposit in modifying earthquake-induced bedrock motion in the Pocatello South quadrangle, Idaho. The purpose of the investigation was to provide results regarding site period, peak horizontal acceleration, and amplification of peak horizontal bedrock acceleration at the surface of the soil deposit for use in local earthquake hazard planning and mitigation.;An essentially deterministic approach to earthquake hazard analysis was taken. First, bedrock motion in the study area was characterized by identifying seismic sources in terms of magnitude and epicentral distance, which provided parameters for specifying simulated accelerograms. Second, the soil deposit response in the study area was characterized by determining the material properties and stratigraphy of the soil from geotechnical and geological data. Third, the bedrock motion and soil deposit input data were subjected to an equivalent linear elastic method of analysis using the SHAKE91 computer program. Finally, the resulting site period and acceleration data were used to construct earthquake hazard maps depicting site period and amplification of bedrock acceleration in the study area.;The results of the analysis indicated (1) a mean site period of about 1.5 s, (2) the predominant period of short-period, high-amplitude bedrock acceleration was increased by a factor of about six, while the peak horizontal acceleration at the surface was decreased by a factor of about 0.90, and (3) the predominant period of long-period, low-amplitude bedrock acceleration was increased by a factor of about three, while the peak horizontal acceleration at the surface was increased by a factor of about two.;It may be concluded from the results of the analysis that (1) amplification of bedrock acceleration in the study area may be expected at periods of input motion exceeding about 0.5 second and at peak bedrock accelerations of less than about 0.4g, and (2) deamplification of bedrock acceleration in the study area may be expected at periods of input motion of less than about 0.3 second and at peak bedrock accelerations exceeding about 0.4g.