I don't know what the original source for the image was on this. The caption at studyof 9/11 reads as follows:
An actual "pancake" collapse, caused by an earthquake: concrete slabs not solidly supported (no steel trusses, etc. to support them) fall one upon another, creating a pancake-like stack of slabs, with support columns left standing but laterally unsupported. This is only known to occur in certain types of concrete structures.