The use of yea here in 3 Nephi 8:18 is wholly appropriate since the yea-clause amplifies the preceding clause (both clauses refer to the breaking up of the rocks). The original manuscript undoubtedly had the yea since both the 1830 edition and the printer’s manuscript have it. In the 1837 edition, the typesetter, it would appear, accidentally dropped out the yea; there seems to be no motivation for deleting it, nor was the deletion marked by Joseph Smith in 𝓟 when he edited the text for the 1837 edition.
Summary: In accord with the earliest textual sources, restore the yea in 3 Nephi 8:18: “yea they were broken up upon the face of the whole earth”.