When you look at the USGS website after an important earthquake has occurred, you will see a little tab at the top that says: DYFI? That means “Did You Feel It?”
The USGS uses this information to determine the “intensity” of the quake. How bad was the shaking? What did you feel? Was there any damage?
This data helps assess the quake on a real-time basis by those affected by the shaking. By responding to the questions, you in effect, are helping in the study of earthquakes. Some of that information can be used to make future “shake maps” and determine seismic hazards.
Even if you didn’t feel the quake in your area, or felt only very little, they want to know!
Here is why and how they use the info.
And here is the survey.