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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.

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>the (assumed 50 year) lifetime of a building

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

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.

This reminds me of my favorite unit of measurement: https://en.wikipedia.org/wiki/Micromort

If there is a 10% chance of collapse every 2475 years, that is ~1.1/10million per day chance, or 0.11 micromorts. That assumes 100% death during a collapse, and that the person spends all day in one of these buildings.

0.11 micromorts/day is very low, which makes the design criteria reasonable. Of course, I suspect our ability to predict infrequent events with any degree of accuracy is unlikely.

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