Finding out a mapped active fault runs near a house you already own is unsettling. It is also actionable. Here is what the distance means, what it does not, and the order to do things in.
Running a hazard check on a house you already own and seeing a mapped active fault nearby is a bad five minutes. Before it turns into a bad month, it helps to know exactly what that line means and what it does not.
A hazard report gives you the straight-line distance from your address to the nearest mapped active fault trace. Three things follow from that phrasing, and each one matters:
The active faults brief covers this in more detail, and how to check if a property is in a fault zone covers the check itself.
Ground rupture is the ground itself tearing along the fault. It affects a narrow band, and it is the reason setback distances from a mapped trace are taken seriously by planners.
Ground shaking affects a much wider area, is the cause of most earthquake damage to buildings, and does not require you to be near the fault at all. Soft ground can amplify it, which is why liquefaction is part of the same conversation.
Most owners who find a fault nearby are, practically, dealing with the second one. That is a structural question about the house, not a reason to abandon the address.
Do not assume an older house is unsafe because it is old. Age is not the variable; how it was built and maintained is. An engineer can tell the difference and a search engine cannot.
Do not assume a newer house is fine. Compliance with a building code is a floor, not a guarantee, and workmanship varies.
Do not sell in a panic. You would be selling to someone who runs the same screening, at a price shaped by the same information, without ever having found out what the actual condition of the structure is.
People imagine retrofitting as demolition. Often it is not, and knowing the range makes the engineer's visit less frightening to book.
Depending on the structure, the work can be as modest as strengthening connections and bracing, or as involved as adding structural elements and improving the foundation. The only way to know which end of that range applies to your house is to have someone qualified look at it. A photograph, a forum post, and a neighbour's opinion are all worthless here, in the specific sense that acting on them can cost you a great deal.
What you can do before that visit:
A large share of earthquake injuries come from contents, not collapse. This part is cheap, immediate, and entirely within your control:
None of that requires an engineer or a permit, and none of it depends on how far the nearest fault happens to be.
A hazard screening reads public national datasets and reports a distance. It does not assess your structure, define a rupture zone, or predict an earthquake. It is a prompt to get the one assessment that genuinely reduces your risk: a licensed structural engineer looking at the house you actually own.
CheckHazard does not replace a professional geotechnical or engineering survey.