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Some of those zones aren't static. Some of those zones are discovered after people have made a life there and getting people to leave an area where they've made a life is hard.

If you live somewhere with strong earthquakes on a regular basis and enforcable building codes, chances are most of the buildings will withstand a pretty good earthquake, but a really big one will cause trouble.

Areas without regular strong earthquakes are likely to be caught by surprise, although newer buildings are likely to do ok, if they enforce international standard building codes; IBC is good for reasonable earthquakes, but not for extreme earthquakes (earthquake prone areas have additional requirements). Areas without enforcable building codes unfortunately aren't going to be well prepared, it costs more to build buildings that won't fall over in a nearby 4.0, especially if they're taller than one story.