USA, What precautionary measures USA has been taken for Yellowstone Volcano a threat to entire USA?

There is nothing on earth that can prevent yellowstone volcano from doing whatever it wants, and there is nothing to reduce the impact of an eruption. The USGS does have constant monitoring of Yellowstone, even some online info that is constantly updated. Their job is to try and alert the public in case there is a pending event, if it ever happens; the alert would be to try and evacuate the immediate area but not a dang thing you can do 500 miles away to stop the ash from falling.

There aren't many precautions you CAN take against large-scale volcanic eruptions or major earthquakes. There are building codes to make homes and offices moderately safer, but that's about it.

none.