For a Seattle resident, the main consequences are earthquake shaking and disruption, strongly modified by local ground conditions, plus location-dependent tsunami, landslide and volcanic hazards.
Three earthquake sources matter: the offshore Cascadia megathrust, deep events inside the Juan de Fuca Plate, and shallow upper-plate faults, including the Seattle Fault. Shaking can damage buildings, bridges, ports and utility networks, interrupting transport, electricity and water supply. Soft basin sediments can amplify or prolong shaking; saturated loose fill can undergo soil liquefaction and lateral spreading. Steep slopes can fail as landslides. The same regional event can therefore have very different consequences on bedrock and on artificial fill.[ref]
Large Cascadia tsunamis chiefly threaten the outer Pacific coast. Seattle's inland waterways also have local tsunami hazards from crustal faults and underwater landslides; one should not equate a Seattle waterfront site with an open-ocean coastal site. Cascade eruptions can affect the region through volcanic ash and transport disruption. Lahars are strongly channelled by river valleys, so their exposure is location dependent rather than uniform across Seattle. These geological differences influence infrastructure design, land-use decisions and emergency planning.[ref]
https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/earthquakes-and-faults. Washington Geological Survey, earthquake and fault mechanisms
[ref]https://cmgds.marine.usgs.gov/data/geotech/pugetposter/geologic.html. USGS, Puget Sound deltas, historic liquefaction damage and the Seattle Fault
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