That would make sense if there were any sign (such as a contemporary document, the Turks were literate, or rather had a literate class) showing awareness of the water. It doesn’t sound like the water is much of a problem for the basilica if it antedates the Moslem takeover. There’s always the possibility that the water was deliberately introduced in order to alleviate the strain of earthquakes — trouble is, well, think of the Leaning Tower of Pisa. :’)
Yeah, but you don’t need a copy of an architects drawing or building permit to suspect when you’ve found a septic tank.
Let’s use Occam’s Razor here.
The design alone—ditches leading away filled with sand— makes a statement that these were probably drains.
Besides, whoever heard of ditches with sand being a cure for earthquakes? I doubt there’s any documentation for that idea either.