The problem with earthquakes is that they are notoriously unpredictable. It is almost impossible to say exactly when an earthquake is going to happen. Occasionally, using scientific methods people are able to make educated guess as to whether an earthquake will occur, but often earthquakes happen without any warning at all.
Finding a method to predict earthquakes is an important goal for many agencies and programs. Predicting earthquakes would help save thousands of lives by allowing evacuation before the event. This is especially true of highly populate areas, such as California. Currently US Congress has created the National Earthquake Hazards Reduction Program to study how to mitigate earthquake damage and learn how to predict them.
Earthquake predictions are generally based on physical precursors to earthquakes such as seismicity (how prone the region is to earthquakes or amount of earthquakes that occur in a region), history of earthquakes, and changes in land elevation. As time goes on prediction gets a bit better, but it is still a long ways off. The best that can be done now is predict the possible magnitude of an earthquake and the range of years it may occur in.
There are plenty of earthquake prediction myths floating around though. Some people claim to have a sixth sense for predicting earthquakes. Many people believe that animals will begin to act strangely right before an earthquake. Earthquake weather is another common myth, along with the position of the moon and other planets having an effect on earthquakes. So far this is no scientific proof that any of these things will actually predict an earthquake. It is very possible that earthquake prediction will never be possible at all.
