Volcano prediction
Not to mention monitoring and prediction of volcanic eruptions. By calling the Pinatubo eruption correctly, they saved the US Government probably half a billion dollars or more in equipment (i.e., airplanes) moved in time at Clark AFB, not to mention the airmen and families. That alone justified the whole USGS budget for 1991.
FOr more on the development of USGS volcano response teams, read the book Volcano Cowboys.