Much of the " SODO District ", just South of Downtown Seattle, is actually built on the former mud flats of Elliott Bay. This includes King St. Station, the BNSF mainline South from the station to Boeing Field, Stacy St. yard and Seattle International Gateway intermodal yard, plus all of Union Pacific's Argo yard. Also included are all of Amtrak's yard & maintenance facilities where according to a friend who worked there at the time, the 2001 Nisqually Earthquake resulted in small mud geysers shooting up through the yard tracks !!!
Some 110 years ago much of the original Elliott Bay mud flats were filled in with earth dug & sluiced out of the Denny Regrade in downtown, plus dredgings from Elliott Bay itself. This filling process took many years, and when Northern Pacific built their new mainline from Argo Tower through King St. Station and the new rail tunnel under downtown, much of the mainline through what is now the SODO District was a continous wooden trestle above the mudflats. The East - West running Spokane St. line across SODO, from the original NP main along Airport Way to West Seattle, was also a trestle for many years. Note that the filled in area also include all of Seattle's Port container facilities. All in all, this area would suffer greatly in a major earthquake.
Much of the Tacoma Tideflats & Port area is built of mud and fill on the old Puyallup River and Hylebos Creek estuaries, and would also be a major problem area in a big earthquake.