Are not considered a reliable tool for short-term prediction of earthquakes because they precede only about half of earthquakes and many times, are not followed by a large quake.

Foreshocks.

Explanation:

A foreshock is a kind of earthquake that happens before a greater seismic event.  Foreshocks have been used as a means of predicting earthquakes, but this approach has been proven unreliable because only some earthquakes show foreshock events, and several events resemble foreshocks but are not followed by a bigger earthquake, which can lead to false alarms.

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