Hi guys!! This is a repost from r/AskAnEngineer as that sub appears inactive.

So I'm very curious about one thing. In May of 2021, the Mexico City metro line 12 collapsed, specifically one of its overpasses that seems to have nor resisted the weight of the train and fell apart o to the traffic underneath. According to several reports, the most thorough being the New York Times investigation and the leaked DNV audit commissioned by the CDMX government, the line had issues since its inception and the shoddy construction created the condition for it to collapse in the way it did.

What makes me very curious, is that the line mostly survived the 2017 Mexico City earthquake that damaged most of the city. Some of it was closed but only for a couple of days, it was operational very quickly after the earthquake. How can a structure survive an earthquake, only to fall apart just 4 years later under the weight of the train it carried a zillion times everyday?

I'm guessing the force of an earthquake is not the same as wear and tear damage. But I'm so puzzled by this. If someone could help me understand better, that'd be great!

Reference on Wikipedia for some info: https://en.m.wikipedia.org/wiki/Mexico_City_Metro_overpass_collapse

Edit: sorry I forgot to include this - I'm from France, living in the UK, and posting this from CDMX where I try to take the metro as little as possible as there's been 3 accidents this week 😅