Carolmca, your translation of the word "seared" is correct. (When you "sear" meat, you burn the outside so that the juices are sealed inside the meat and not lost to the skillet or frying pan.) When you "sear" something, you leave a permanent mark, a scar. Obama was saying that the images of 9/11 are burned into our national memory, leaving a permanent mark, a scar. In other words, they are images that will never be forgotten.