We should probably get this moved to the OB forum. But basically, yes, there was a city on the Shattered Plains, Stormseat, aka Narak. It was shattered at some point between Aharietiam and the Recreance (we think).
Some of the buildings remained, and were eventually covered over in crem.
The pattern Shallan figures out is related to how they were shattered--they were shattered in a (6? or 8? fold) symmetrical pattern. So she was able to extrapolate the shape of the plateaus once she'd seen one iteration of the pattern.
The city that was built on the Plains was also shattered, but enough of it remains to figure out some of how it was laid out. And the plateaus those remnants are on would still follow the pattern of the shattered plateaus, anyway.