It is Spanish but I am not sure why it’s says US by it when I look in the dictionary. No one uses it in English unless you speak Spanish already.

I don't use the word, but I immediately understood it without registering anything strange about it. It didn't seem out of place until I saw this comment thread where I realized it is not actually a word I've ever used. I guess it must be fairly commonly used by (American?) journalists.

It's a perfectly cromulent word.

In american irregular usage, it's a known word, but the primary reason someone would use it (other than incidentally or acidentally) is to effect an affect; that is to say, the author might have used this word on purpose to draw your attention.

This type of usage is much more common in, for example, poetical writings; it is also used semifrequently in opinion or persuasive writings. The author Stephen King employs this mechanism very frequently. He loves using weird americanisms to evoke a feeling of alienation from his audiences.

most US earthquakes are in California where Spanish is widely spoken and lots of things get Spanish names.

however I don’t think this is common usage. it might be “headline-ese” as the word has fewer characters than earthquake.

I am pretty sure it is related to the Italian "tremore", which means something that vibrates, and is used when referring to earthquakes.