United States Department of Transportation
i
Evaluation of Site Effects Utilizing Cascadia Subduction Zone Ground Motions
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2023-06-01
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[PDF-8.67 MB]
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Edition:Final Report
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Abstract:The potential Cascadia Subduction Zone (CSZ) megathrust earthquake is recognized as one of the major natural hazards affecting the Pacific Northwest of the United States. The estimation of expected ground-motions is complicated by the long-duration motions from CSZ as well as site effects from deep sedimentary basins in northwest Oregon. This study evaluates the combined effects of long duration motions and basin effects on site amplification factors due to propagation of earthquake waves in surficial soils. Nonlinear and equivalent linear one-dimensional site response analyses were performed using broadband synthetic CSZ ground motions from the M9 Project. A web-based tool was developed to synthesize the vast data and geographically visualize the M9 ground motions as well as the subsequently computed response spectra. Five soil profiles representing a range of site classes from Site Class C to Site Class E were considered for the analyses. Ground motions were extracted at three locations inside major basins in Northwest Oregon (Portland basin, Tualatin basin, and North Willamette basin) and three comparable locations outside the basins. The effect of basin on soil amplification factor was characterized by comparing the soil amplifications inside and outside basins for the same soil profiles. The basin amplification factors calculated for CSZ broadband synthetic ground motions at bedrock (Site Class B/C) for selected sites within the three basins in Oregon were found to be noticeably larger than the ratios calculated from empirical correlations that are incorporated in New Generation Ground Motion Attenuation Models (NGA-West2). The soil amplification ratios calculated from the site response analyses were generally within the envelope of code-based site coefficients in ASCE 7, except for very short periods (<0.5 seconds). The effect of basins on soil amplification ratios ranged from 50% increase to 30% decrease at periods close to the natural period of the basin (generally between 1 sec and 2 sec). The implication of these findings on the use of code-based site coefficients and advantages of performing site-specific site response analysis are discussed.
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