Free Standing Intermediate Deck Stair Landing Footing Question

[MGH PA]

So I'm building a free standing 5' x 10' intermediate landing 5' high to accommodate stairs coming off our new deck. The deck is 10' off the patio below, the plan is to go out to the landing, and then switch directions back to under the deck landing on the patio below. I have everything laid out, but my question is regarding the footings of the landing. I'm putting in 12" sonotubes, but I've read mixed reviews on whether we need to have footings that extend below the frost depth or not in this situation? The only reason I ask is that I'm digging in some SERIOUS shale here, and it's hard to get below 24+" without really running into some issues. However, I'm not one to cut corners if it's going to cost me down the line.

We are in Zone 6, btw.

Thanks!