This is how we express rare events which threaten buildings and bridges. For example, the design level earthquake is chosen such that it has a 10% chance of bei

This is how we express rare events which threaten buildings and bridges. For example, the design level earthquake is chosen such that it has a 10% chance of being exceeded over a 50 year time span (475 year return period). The maximum considered earthquake has a 2% chance of occurring over 50 years (2475 year return period). Fifty years is around the typical lifespan of a structure.