"offshore" as preposition?

[Mark Mandel]

>From Wikipedia's page about the novel *Storm* (
https://en.wikipedia.org/wiki/Storm_%28novel%29):

A cyclone develops offshore Japan, and becomes a significant storm that
moves into California as a blizzard of significance for the Sierra Nevada
range, with snowfall amounts of 20 feet (6.1 m).

I don't recall ever seeing this usage before, only Adv + "of": "offshore of
PLACE". It could easily be a typing error, but development of a
prepositional use makes sense too. I can't think of a way to search the Web
for it. Any observations, in either sense?

Mark Mandel

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