Washington sits within reach of the Cascadia Subduction Zone, and King County additionally over the Seattle Fault. This is a genuine engineering consideration rather than a theoretical one, and it is why seismic retrofit is a real service here in a way it is not in most of the country.
The specific vulnerability
In a great many homes built before modern codes, the wooden frame sits on the concrete foundation under its own weight, with nothing meaningful connecting the two.
In an earthquake, the ground and foundation move while the house above has no structural reason to move with them. The house can slide off its foundation, which causes catastrophic damage to a building that might otherwise have survived intact.
Cripple walls
The second common weakness. Many older Washington homes have a short stud wall between the foundation and the floor framing, creating the crawl space. That is a cripple wall.
Unbraced, these walls have little resistance to sideways movement. They can collapse sideways in an earthquake, dropping the house.
Bracing them with structural sheathing is a large part of what retrofit work involves, and it is done from inside the crawl space.
What the work looks like
Most of it happens in the crawl space, which is why it is unglamorous and largely invisible when finished.
Anchor bolts or approved connectors are installed through the sill plate into the concrete foundation. Cripple walls are sheathed with structural plywood and connected properly to both the sill plate above the foundation and the floor framing above them. Additional connectors tie the framing together at critical points.
The living space above is generally untouched. This is one of the few substantial structural improvements that does not require you to move out.
Which homes are candidates
Broadly, houses built before modern seismic provisions entered the code, particularly wood-framed homes on raised foundations with crawl spaces. That describes a large share of older Seattle, Tacoma and Spokane housing.
Homes on concrete slabs are a different situation. So are newer homes built under current codes, which should already have these connections.
The honest first step is an assessment rather than an assumption. What is actually there varies, and some older homes have had partial work done at some point.
What retrofit does not do
Worth being clear, because expectations matter. It does not make a house earthquake-proof. It addresses specific, well-understood failure modes — sliding off the foundation and cripple wall collapse — that account for a large share of severe damage in wood-framed housing.
It also does not address chimneys, which are a separate vulnerability, or the contents of the house. Securing water heaters, tall furniture and heavy items on high shelves is separate, cheap, and worth doing regardless.
Permits and who does it
This is structural work and generally requires a permit and inspection. Some jurisdictions have standard plan sets for common retrofit configurations, which can simplify the process where the house fits the standard case.
Use a contractor who does this work regularly. It is a specific discipline, and the connections either transfer load properly or they do not — which is not something a homeowner can evaluate from looking at the finished result. That makes verified registration and genuine specialism more important here than in most trades.
Insurance
Earthquake cover in Washington is typically a separate policy or endorsement rather than part of standard homeowner cover, and it is worth understanding your position whether or not you retrofit. Ask your insurer how retrofit work affects your specific situation, since practice varies.