《朗文》词典释义：1 [intransitive usually in progressive] if business, trade, or a particular area is booming, it is increasing and being very successful.
这个释义告诉你，boom的主语可以是一个特定地区。虽然词典例句中的主语多为business或某个行业，但不能排除一个地区、城市或国家作boom的主语。以下为COCA语料库例句：
First of all, economically, had they not been there in the late 1990s when New York City suddenly boomed practically like no place on the planet, the city needed all 10, 15 million square feet of office space that those buildings provided. If they were not there, so many other companies would have had to go elsewhere.
A backwater farm town until the late 1980s, as China boomed Dongguan was transformed into one of the most important hi-tech manufacturing centres in the world.
As entertainment centers for GIs -- sources of war material, and ancillary services -- the region boomed.
所以你的句子中 South Korea 作主语是没有问题的。
