The entire passage is about predicting earthquakes.
Now when you predict you are looking at TIME. This is given through our passage - including the final paragraphs about how they moved from short-term to long-term.
Let us look at the specific line that the question is referring to:
Researchers initially reported success in identifying
these possible precursors, but subsequent analyses
of their data proved disheartening.
So what can we say about researchers' ability? That they cannot seem to predict WHEN such earthquakes would occur.
Kindly re-read the passage with a focus on understanding the main point i.e. they were not able to guess WHEN it would happen.
Hope this helps,