On the USGS-website there is the following note concerning copyright:
USGS-authored or produced data and information are considered to be in the U.S. Public Domain. While the content of most USGS websites are in the U.S. Public Domain, not all information, illustrations, or photographs on our site are. Some non USGS photographs, images, and/or graphics that appear on USGS websites are used by the USGS with permission from the copyright holder (as required by USGS policy on use of copyrighted material). These materials are generally marked as being copyrighted. To use these copyrighted materials, you must obtain permission from the copyright holder under the copyright law.
When using information from USGS information products, publications, or websites, we ask that proper credit be given. Credit can be provided by including a citation such as the following:
Credit: U.S. Geological Survey; Department of the Interior/USGS; U.S. Geological Survey/photo by Jane Doe (if the photographer/artist is known)
Warranty
No guarantee can be given as to the correctness of facts implied or explicitly stated.