Oklahoma? Perhaps NOT "OK"

Here I was thinking that Oklahoma, Kansas and Nebraska were pretty much the same thing (Iowa, too - am I forgetting any? Perhaps the Dakotas...). Okay, granted - Oklahoma has a namesake musical and therefore a title song, and Kansas has a namesake band. But other than that they consist essentially of vast FLAT expanses interrupted only by fields (corn, wheat - what have you). Then I received this FOTD:

Bore-hole seismometry indicates that the land in Oklahoma moves up and down 25 cm throughout the day, corresponding with the tides.

"Oklahoma - where the land undulates just enough each day to make one motion sick due to the tides..." No wonder this is NOT common knowledge. It completely RUINS the song. It also begs the question: WHAT TIDES? Oklahoma sits virtually centered horizontally in the Country (LANDLOCKED, need I add). So, which ocean one could not say. Perchance Lake Michigan? I positively know that I can't ever think of the plains in the same way; I will wonder how many centimeters they might be rising and falling right beneath one's feet.