For those wondering about a reported closure of the airport after a 4.1, which admittedly isn't large.
The reported location of the earthquake was very close the Christchurch airport, and 16 k isn't very deep. The geology also influences the destructive power of a quake.
It was well within the expected pattern of aftershocks after the two big quakes, the second of which flattened a fair bit of the city, and killed a lot of people, so it won't have taken anyone by surprise.
So, they've had a lot of practice with earthquakes over the last few years, and are unlikely to have over-reacted.