I've come across the following clause in ノルウェイの森.
揺らせ here clearly seems to be functioning as a 連用形, but I'm not quite sure how this can be derived from 揺らす as it's not 揺らして. Looking just at morphology, I suppose it could be the 連用形 of the potential/imperative form, but neither of those make sense here. The causative form of 揺る could also fit, but it's unclear what the indirect object would be, in this case.
A similar sentence appears just a few lines later
For reference, the equivalent sentence of the first example in the book's English translation is, "The October breeze set white fronds of head-tall grasses swaying".
Morphologically, what form is 揺らせ here?