IIRC, the earthquake was in 1985 and another problem (besides lousy construction) was the ground - not stable, but filled in old lakebed.
Sediments that are water-wet (down at the water table) are subject to "ground liquefaction" during pronounced shakings, such as was documented during the 2011 Japan earthquake and tsunami. Buildings can (and did, in the 1985 Mexico City quake) suddenly sink two and three stories deep into liquefied ground, killing occupants of the lower floors in seconds.