The Seattle Basin is a 6 to 7 km deep basin, formed by geologic processes and earthquake faults, filled with soil and soft rock. These sediments within the basin amplify earthquake motions, as we saw from earthquake ground motion recordings and damage patterns during the Nisqually earthquake. These amplified motions primarily affect taller buildings.
The U.S. Geological Survey and the University of Washington have been working to quantify the amplification of motions within the Seattle basin. As part of the M9-Cascadia Megaquake research project, researchers found the amplification is larger than previously expected for subduction source earthquakes.
The United States Geological Survey (USGS) and SDCI co-hosted a workshop on March 22, 2018 for geotechnical engineers, structural engineers, and earthquake researchers to come to a consensus on how best to implement the M9 results. The results of this workshop are published by USGS. To provide engineers and developers adequate time to incorporate these findings into their designs for tall buildings, SDCI has published Directors Rule 20-2018 allowing building owners the option to delay implementation of the recent M9 findings into site-specific analysis until December 1, 2018. This date corresponds to a peer review kickoff meeting or permit intake date for prescriptive code buildings applying site-specific analysis.