I just recently started in the hobby again, and last time I had tanks I was on the East coast, where earthquakes weren't really a concern.
Most of the people I've talked to seem to be unconcerned.
I think some of it likely depends on the shape/orientation of the tank. Like, a 40B is probably too short and wide to tip over, whereas a 20XH wouldn't take much more then a nudge to tip it.
For my tanks, I've been using gate hooks to secure the stand to a wall stud.
I currently have a 10g on one stand that isn't secured (the stand is, not the tank). I haven't quite decided if it's a temporary setup, or how I'm going to secure it. Previously, I had a 20XH, and I just got some nylon webbing and buckles, and made a loop to go around the trip and mount to screws set in the wall.