The Red Twilight (The Great World Wide Earthquake of 1977)

All history seems to be the same as in our timeline up until that day in 1977, except the 1906 California Earthquake never happened, meaning that over the course of 71 years, pressure was building and building; so much so that when it ruptured in 1977 the whole world paid dearly for it!

The quake happened on April 19, 1977. The entire Soviet Union was turned upside down. The Kremlin collapsed, Leningrad was leveled, chaos regined over the Russian motherland.