Understanding risk to Seattle’s high-rise buildings from a giant Cascadian earthquake

Session: Deterministic Simulated Ground Motion Records Under ASCE/SEI (7-05: Guidance for the Geotechnical Industry
Location: DeAnza Ballroom 3, Thursday 9 April 2009, 8:30 a.m.

The Cascadia subduction zone is likely to produce the strongest shaking experienced in the lower 48 states. Although seismic activity in the Pacific Northwest has been relatively low in the past two centuries, there is a growing consensus that this fault zone ruptures in giant earthquakes (magnitude exceeding 9); the last rupture is inferred to have occurred in 1700. What is the risk to high-rise buildings from such a giant earthquake? Although four giant subduction earthquakes occurred elsewhere in the past century, there were no cities with high-rise buildings in the heavily shaken areas for any of these events. Furthermore, ground motions have never been recorded in areas strongly shaken by a giant earthquake.

T. Heaton and J. Yang of Caltech simulated ground shaking from an earthquake similar to the giant Sumatran earthquake (M 9.2) that occurred in 2004, and which is hypothesized to be similar to the giant earthquakes in the Pacific Northwest. The simulated shaking lasts for more than four minutes and it is dominated by low frequency motions. While smaller buildings (e.g., wooden houses) are not particularly susceptible to these low frequency motions, tall buildings resonate at the low frequencies contained in these simulations. The Seattle basin exacerbates the situation by significantly amplifying these long-period motions in the Seattle metropolitan area. Heaton and Yang simulated the response of modern steel 6- and 20-story buildings to the hypothetical ground motions. Although there are many unanswered questions about such a future earthquake, they report that severe damage and possible collapse is indicated in many of their simulations. Buildings that were constructed prior to important building code changes that were made as a result of the 1994 Northridge earthquake are especially vulnerable to this long period shaking. Heaton and Yang report that, given the current state of understanding, there is insufficient knowledge of ground shaking and of building response to ensure the integrity of tall buildings in such an earthquake.

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