[cmsmith]

Current building codes are intended to design to a 10% probability of collapse in the "Maximum Considered Earthquake", which is roughly a 2475-year event. The 2475-year event has a 2% chance of occurring during the (assumed 50 year) lifetime of a building.

[gruez]

>the (assumed 50 year) lifetime of a building

what happens afterwards? is the building supposed to be torn down?

[cmsmith]

Nah, the building is (mostly) just as good at the end of that time. It's just a convenience to make the probabilities easier to conceptualize. If we designed buildings such that the design earthquake had a 5e-6 daily chance of occurring, we'd just be confused.