Johnston's Seismic Design Category, explained

Under the seismic model IBC and ASCE 7 use for new construction, Johnston is Category B. Low seismic hazard. Code requires basic seismic detailing (bracing, connections) but no special structural system restrictions. That puts Johnston in the 16th percentile nationally for seismic hazard - higher than 16% of the 4,875 US cities USGS was checked against here.

Johnston by the numbers

The raw figures: peak ground acceleration 0.029g, short-period spectral response 0.06g, 1-second spectral response 0.051g. Among 85 Iowa cities in this dataset, Johnston ranks 51 for seismic hazard, the 40th percentile within the state. About 25,022 people live in Johnston, Iowa.

Retrofit economics for Johnston

Outside California, no state-level retrofit grant equivalent to Brace + Bolt could be found for Iowa. 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 Johnston address.

More on how this number was built

The methodology guide covers exactly how USGS's public design-maps API produced Johnston'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.