Student vocabulary exercises seem to give people the impression that every word has a natural "opposite" (and every sentence has a tag question, every sentence can be made passive, etc.) What idea would "the opposite of earthquake-resistant" express? An earthquake resistant building is intentionally constructed to withstand an earthquake. Do you want a word for a building building that is purposely built so that it will fall down in an earthquake? Or do you just mean a normal building (a non-earthquake resistant building, a building whose earthquake status is unknown because it wasn't specially built)?