Cascadia Subduction Zone Earthquake Vertical Ground Acceleration Investigation and Potential Impact on Bridges in the Pacific Northwest
Date of Award
Master of Science (M.S.) in Civil & Environmental Engineering
Civil and Environmental Engineering
Seismic, Engineering, Bridge, Subduction, Vertical, Earthquake
The effects of vertical ground accelerations during subduction zone earthquakes currently are not sufficiently understood. There are numerous case studies and evidence that effects of vertical ground accelerations can significantly impact the performance of bridges during a seismic event, but most previous research has been focused on shallow crustal earthquakes. Current bridge design codes provide little guidance for accounting for vertical ground accelerations in seismic design, in part because additional information is needed about the characteristics of vertical ground motions during Cascadia Subduction Zone (CSZ) earthquakes in the Pacific Northwest.
For this study, recorded seismic data from recent subduction zone earthquakes was reviewed and compared to predicted CSZ ground motions. Time history data obtained from subduction zone earthquakes in Japan and Chile was used to compute response spectral accelerations for the three axes of motion. These results were then binned based on peak ground acceleration, short period spectral acceleration and 1.0-second period spectral acceleration, and geometric mean results then compared to the geometric mean of the predicted CSZ response spectra from the M9 CSZ Tool. Stations located within deep sedimentary basins were reviewed and compared separately. This study found that recorded subduction zone vertical accelerations and the predicted CSZ earthquake vertical accelerations could impart significant forces and should likely be considered in design of earthquake resistant bridges.
Differences between recorded subduction zone accelerations and predicted CSZ accelerations included magnitude of vertical acceleration, magnitude of vertical to horizontal acceleration ratio (V/H ratio) and spectral period ranges of maximum V/H ratios and decay.
Bassil, Rachel C., "Cascadia Subduction Zone Earthquake Vertical Ground Acceleration Investigation and Potential Impact on Bridges in the Pacific Northwest" (2022). Civil and Environmental Engineering Master's Project Reports. 59.
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