I also believe that there is something extremely significant about the striations in the walls of Urithiru. And I think their significance is tied to the deeper secrets of Urithiru, not merely some trigger effect they have on Shallan and her repressed memories.
The following lines from Chapter 13 of Oathbringer jumped out at me in particular, and I took special note of them:
A few things stood out to me:
- the repeated references to darkness, whether it be the strata or tunnels twisting towards darkness, or the ceiling of the market looming over the people;
- the twisting strata are repeatedly described as "lines of madness" and "mesmerising", suggesting an effect on the human mind;
- the fact that Shallan didn't realise that she'd made over twenty drawings, suggesting, that she has herself either been mesmerised, or lost some memories of seeing the walls and drawing them.
I know Shallan is an expert at repressing memories, but I don't believe this memory loss is related to her past. Rather, I agree much more with the idea that the patterns are some form of Urithiru defence mechanism. My personal guess is that they aren't a code, disguising information directly, but act as a sort of optical lock, hiding certain parts of Urithiru from discovery by non-Radiants.
We have been led to believe that little valuable information has been discovered in Urithiru because it is extremely large. But perhaps the real reason is that the important parts are hidden in plain sight. Shallan has stumbled across several tunnels that lead into darkness -- a foreboding image, but one that might simply suggest that no one has ventured inside and "switched the lights on" so to speak. The only evidence of these darkened tunnels are her sketches -- she has no memory of them, and presumably no one else does either. But her sketches, and perhaps Shallan's developing powers, are no doubt the key to unlocking them.
In fact, when we read thise lines back again, that series of sketches might form a map, starting with "a corridor she'd passed through the other day", perhaps near the "large market cavern", and then progressing through a sequence of darkened tunnels leading to a room where the strata "coiled about itself in a mesmerising manner". Could Shallan have already discovered the way to Urithiru's library, for example, only to have the memory erased by the mesmerising strata?