Me, I personally feel that none of this will happen (sorry if take this thread in a religious direction) until after the Rapture of the Church and the last seven years of this present age begin.
I see the United States, indeed, much of the North American continent, destroyed by a combination of geological events. The San Andreas fault, the New Madrid fault, the Yellowstone Caldera and the East Coast fault all go at once destroying this part of the world. Very little life will be saved.
Rev 6:12-17, "And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood; And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind. And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places. And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains; And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb: For the great day of his wrath is come; and who shall be able to stand?
This sets the stage for the last of the great empires to emerge across the pond and for end time world events to occur.
Gamma ray burst from Sagittarius A. Only safe place will be under ground or beneath several feet of concrete.