Question

Is this a sentence, if not why?
They cause approximately 10,000 earthquakes annually.

Answer 1

No because when your answering a question you have to explain in this case you have to write the this causes

Answer 2

This is a complete sentence. It may not give much detail, but it is technically correct. There is a subject (they) and predicate (cause approximately 10,000 earthquakes annually). Therefore, it is a sentence.