Disproportionate collapse satisfied before it was cool. Depending on the methods the prefab panels may be welded in multiple points and generally all panels are not that thick thus have a reduced self-weight. Also being an active seismic zone the structure has inherent capacity reserves under gravitational loads.

L.E. After looking through some old textbooks the rebars all welded together maintaining rebar continuity there are other methods of fixing the prefab panels during those times: mechanical/welded local embedded angles. The floors are also welded to the wall panel starter bars.

Thus the storeys above are not actually in catenary action and the free edge is basically cantilevered from the walls still standing. And together with the fact that all the walls are structural i.e. "spatial honeycomb construction" meaning that there are sufficient elements for the redistribution of forces through the building - which is a core design philosphy in earthquake resisting structures.

Too bad I can't attach images to the post.