Question

Why are geologists able to make long term predictions about earthquakes fairly well?

Answer 1

You can look at the past history for an area with earthquakes at the times spans between them 

Answer 2

Geologists are able to make long-term predictions about earthquakes fairly well since geological events occur in bigger time spans. This can be seen by looking at the history of earthquake occurrence in a particular area where one would observe an almost uniform time span in-between occurrences.