[williamstein]

What is “the BIG ONE” in PNW that could happen without warning any second?

[dnemmers]

Earthquake or volcano would be my guess, with landslides being a secondary concern. Plate tectonics in that area creates some hidden (or in the case of Tahoma and others in the area, not so hidden) dangers that have struck in the recent history. The Nisqually earthquake comes to mind as well.

https://en.m.wikipedia.org/wiki/2001_Nisqually_earthquake

https://pnsn.org/earthquakes/recent/

[tjohns]

Earthquake. The entire North American west coast sits on the boundary of the Pacific tectonic plate, so it's a subduction zone.

Same reason California and Japan are known for large earthquakes.

See also: https://en.wikipedia.org/wiki/Ring_of_Fire

[LiquidPolymer]

Subduction zone earthquake.