Puget Sound faults about 1100 years ago (AD 900—930) an earthquake of magnitude 7+ uplifted Restoration Point and Alki Point, dropped West Point (the three white triangles in the Seattle Basin on the map), caused rockslides in the Olympics, landslides into Lake Washington, and a tsunami on Puget Sound. It extends as far east as (and probably terminates at) the Rattlesnake Mountain Fault Zone (RMFZ; the southern extension of the SWIF) near Fall City. This seems geologically reasonable, as both the SWIF and RMFZ appear to be the contact between Tertiary Crescent Formation basement of Puget Sound on the west and the older
Puget Sound faults geologic province is the approximately 50 million year (Ma) old marine basalts of the Crescent Formation, part of the Siletzia terrane that underlies western Washington and Oregon. East of Puget Sound the basement of the Cascades province is various pre-Tertiary (older than 65 Ma) metamorphic rock. Somewhere between Puget Sound and Cascades foothills these two geological provinces come into contact. As the juxtaposition of various disparate tectonic structures in northwest Washington requires significant strike-slip movement, it is further expected that this contact will be a major fault. The northern end of the Crescent Formation (aka Metchosin Formation) has been identified