"Mereka mempelajari jenis-jenis gempa yang terjadi di Indonesia."
Translation:They study the types of earthquakes that occur in Indonesia.
so this issue has happened kind of a lot on duolingo and it is rather frustrating. In english, the use category nouns (for example: types, modes, species, etc.) followed by the substantive they correlate with (in this case earthquake), the substantive does need to be congruent with the category noun. In fact, in formal english and academic language it is more common to to use the singular. this would mean that you are in many contexts more likely to find constructions like" types of earthquake" than "types of earthquakes"