Roger,

Of all the accounts of large or greats quakes that I have studied, I have not seen any mention of "cloud formations". So my comment is not statistical, but anthropologic.

North America has been home to many major, and "great" earthquakes. Some of these, such as New Madrid occurred at a time in history when weather meant life or death to those who lived here. How about Cholame in 1857? Think those settlers in the San Juaquin Valley would have missed a strange cloud formation?
How about Owens Valley, San Fransisco, or Landers? By the way, there was a large lenticular cloud up over the desert after Landers, but not before. Not sure if that means anything at all.

Also after hearing and studying local Native American oral traditions, it has never been mentioned that "cloud formations" have any relationship what so ever to earthquakes. Just another way to think about it.

I'm skeptical. At least here on North America. If cloud formations precede large earthquakes here, I would have read or heard about it.

Glen