Washington's Seismic Design Category, explained

Under the seismic model IBC and ASCE 7 use for new construction, Washington is Category B. Low seismic hazard. Code requires basic seismic detailing (bracing, connections) but no special structural system restrictions. Nationally, Washington ranks above 41% of the 4,875 cities in this dataset for seismic hazard - the 41th percentile.

Washington by the numbers

The raw figures: peak ground acceleration 0.065g, short-period spectral response 0.135g, 1-second spectral response 0.077g. Among 330 Illinois cities in this dataset, Washington ranks 140 for seismic hazard, the 58th percentile within the state. About 15,857 people live in Washington, Illinois.

What a retrofit costs in Washington

Outside California, no state-level retrofit grant equivalent to Brace + Bolt could be found for Illinois. A typical retrofit still runs $5,000-$10,000 nationally, about $7,000 for a common single-family job.

What this means for insurance

Outside California, earthquake coverage is typically sold as an endorsement added to a standard homeowners policy, not a separate product - worth asking about directly when getting a home-insurance quote for a Washington address.

More on how this number was built

The methodology guide covers exactly how USGS's public design-maps API produced Washington's numbers. The guide to what a Seismic Design Category actually means covers why a category is never a substitute for an engineer's assessment of one specific house.