earthquakes
When an earthquake will occur is much more difficult to predict. Stress on a fault builds up at the same rate over time. So earthquakes should occur at regular intervals. But, so far, scientists cannot predict when quakes will occur even to within a few years. Around Parkfield, California, an earthquake of magnitude 6.0 or higher occurs about every 22 years. So seismologists predicted that one would strike in 1993. But that quake came in 200411 years late Figure below .