Seattle buildings are less earthquake-resistant than expected

A new study suggests than many buildings in Seattle cannot withstand large earthquake incidents.

The research focuses on the performance of reinforced concrete walls under seismic load and shows that the buildings' response is inadequate, especially in the city of Seattle.

The team, including Nasser A. Marafi, a postdoctoral researcher, who studied the case of Seattle for his Ph.D. thesis, utilized ground motions that derive from a large data of simulated M 9 earthquakes in the Pacific Ocean. His research is part of a larger project carried out by scientists at the University of Washington and the U.S. Geological Survey (USGS). Massive earthquakes in the region have occurred before but no ground motions records exist. To address this issue, a set of ground motions that could be expected during a M 9 temblor was produced via computer simulations. Considering those ground motions, the buildings' performance was evaluated. "Then my work takes the ground motions that those simulations predict and asks what this means for building response. How do buildings respond to this kind of shaking that we're predicting from this simulation?," Dr. Marafi, stated.

Due to Seattle's sedimentary basin, ground motions are amplified and therefore structures must withstand much larger inertial forces. However, most building under 24 stories are not designed to take into consideration the amplification effects and consequently, they would suffer critical damage during a large earthquake. 

The results from the new analyses that include the basin effects, predict an 11% increase in the displacement between two consecutive floors of a building in comparison with the building codes predictions that do not take into account the amplification factor. According to Dr. Marafi, structural analysis cannot fully predict whether a building would totally collapse or not during a M 9 earthquake but statistically 1 out of 3 buildings designed with the minimum seismic standards will give way.

The inductions of the study are concerning. Fortunately, the forthcoming report of the National Seismic Hazard Maps that advises building code will include the amplification effects for Seattle. According to Dr. Marafi, city officials are determined to include the basin impact in the design codes for building under 24 stories by 2021.