I read a great description of the “earthquake ecology” beneath California.
“Imagine you had 50 panes of glass. You lay one on the floor, then hit it somewhere randomly with a hammer, breaking it. Then you lay another pane on top of it, and hit it in a different relative place than the first, breaking it.
“When you have done all 50 panes of glass, the pile of broken panes is a good approximation of the earthquake faults. Several of the “faults” are similar in some of the panes, so it appears there are larger and smaller “faults”.”
Sounds about right.
And of course there there is the plate boundary.