The infamous earthquakes of 1811-12 called New Madrid were around the 8.0 intensity for the 3 biggest ones. Remember, each higher whole number is a multiple of 10 times the one below it. Within the same time period Caracas, Venezuela had a monster quake that killed at least 20,000 people.
I think that is the quake that made the Mississippi in the quake area run up river.
The Cascadia fault line has more potential to do damage than the San Andreas.