Chelsea's Seismic Design Category, explained

Chelsea carries Seismic Design Category B under USGS's ASCE 7-16 model. Low seismic hazard. Code requires basic seismic detailing (bracing, connections) but no special structural system restrictions. That puts Chelsea in the 63th percentile nationally for seismic hazard - higher than 63% of the 4,875 US cities USGS was checked against here.

Where Chelsea sits in Massachusetts

The raw figures: peak ground acceleration 0.167g, short-period spectral response 0.279g, 1-second spectral response 0.066g. Among 58 Massachusetts cities in this dataset, Chelsea ranks 21 for seismic hazard, the 65th percentile within the state. About 40,245 people live in Chelsea, Massachusetts.

Bracing and bolting: the actual cost

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

Insuring against this, not just retrofitting

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 Chelsea address.

More on how this number was built

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