Effects of Simulated Magnitude 9 Earthquake Motions on Structures Located in Deep Sedimentary Basins

Nasser Marafi

University of Washington

Date & Time
Building 3, Rambo Auditorium
alex grant

Presentation slides

The Cascadia Subduction Zone (CSZ) can produce long-duration, large-magnitude earthquakes, whose ground motions will be modified by the deep sedimentary basins, which underlie several cities in the Pacific Northwest (e.g., Portland, Seattle, and Vancouver, BC). The effects of these basins on the ground motion duration and frequency content are poorly understood because no recordings are available for large-magnitude earthquakes in this region.

To compensate for this paucity of recordings, researchers from the United States Geological Survey and the University of Washington generated ground motions for numerous scenarios of an M9 event. The simulations were generated using deterministic, finite-difference (T>1s) and stochastic (T<1s) approaches. This seminar will present the impacts of 30 motions simulated in Seattle for 32 archetypical, reinforced-concrete-core-wall structures (4- to 40-stories). The frequency dependent basin amplification increases the spectral accelerations at periods corresponding to tall structures and results in damaging spectral shapes. The duration of the M9 motions are long, but the basins have little effect on the significant duration of the motions. Tall buildings designed to enhanced building code requirements have collapse probabilities similar to that expected for a 2475-year motion, but the return period of an M9 earthquake is only about 500 years. Tall buildings designed to meet the minimum building code requirements were much more likely to collapse. A newly developed ground-motion intensity measure (effective spectral acceleration) explains these results and made it possible to evaluate the likely regional impacts of these motions.

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