The USGS does not require a specific format when citing our websites. You should follow the citation guidelines provided by instructors or publication editors as appropriate.
Unless you are citing a specific USGS publication, the author of a USGS website is almost always "U.S. Geological Survey." Also acceptable: "United States Geological Survey" or "USGS."
Most USGS informational websites are updated/published daily. That does not include online datasets and some older USGS websites. Look for "page last modified" dates in the footer or using your browser's page information options. For datasets, look in the metadata.
U.S. Geological Survey, 2020, Earthquake Lists, Maps, and Statistics, accessed March 18, 2020 at URL https://www.usgs.gov/natural-hazards/earthquake-hazards/lists-maps-and-statistics
U.S. Geological Survey, 2018, Water Science School: Acid Rain and Water at https://www.usgs.gov/special-topic/water-science-school/science/acid-rain-and-water. (Accessed May 14, 2018)
U.S. Geological Survey, 2020, National Water Information System data available on the World Wide Web (USGS Water Data for the Nation), accessed January 10, 2020, at URL https://waterdata.usgs.gov/nwis/
Learn more: USGS Data Citation