There are 3 places in the US where Geologists are expecting a huge earthquake. All of California, New Madrid, Missouri, and Charleston, SC. All three are overdue and the vibration of any one of them could set off the other two. If all three go, then many of the other small earthquake locations could participate and as Elvis once said, "Whole Lotta Shakin' Goin' On." If all three go at the same time, that could also encourage the Yellowstone Super Volcano to join the party. Geology is one of my several hobbies so I keep up with that sort of thing.