April 18: The Great San Francisco Earthquake

A second event for April 18:

In 1906, the Great San Francisco Earthquake killed upwards of 3000 people and led to the city's gradual destruction by fire in the days after. The event was seen as apocalyptic by some. Of course there were doubters of this, as demonstrated in a bit of doggerel circulating at the time about the city's largest distillery:

If, as they say, God spanked this town For being much too frisky,
Why did He burn His churches down And save Hotaling's Whiskey?

April 18: William Miller's Second Disappointment, of Three

The New England Baptist preacher William Miller convinced a substantial following that the Parousia (Second Coming of Christ) would occur during the year 1844. When the world did not end by his first deadline on March 21, Miller recalculated and determined it would end on the current date, April 18. (When that date failed, he would go on to project a third Second Coming for October 22.) Miller's system drew loosely from the book of Daniel in claiming the world will end 2300 years after the Babylonian Captivity and reconstruction of the Temple in Jerusalem. Despite the failure of his predictions, Millerites went on to form the Seventh-Day Adventists.