I have eight Vision cages stacked four high. To prevent them from sliding during an earthquake I placed industrial strength stick-on Velcro between the cages. They are now locked securely together. While I don't think they would tip over as a group unless we had a large earthquake, I will be fastening them to the wall just in case.
Worthwhile thread. It should encourage all of us in earthquake prone areas to take precautions to protect our animals.