After the devastating 1906 San Francisco, California earthquake, a fault trace was discovered that could be followed along the ground in a more or less straight line for 270 miles.

The earth on one side of the fault had slipped compared to the earth on the other side of the fault by up to 21 feet.

Harry Fielding Reid, after studying the fault trace of the 1906 earthquake, concluded that the forces causing earthquakes were not close to the earthquake source but very distant.

Reid's idea was that these distant forces cause a gradual build up of stress in the earth over tens or hundreds or thousands of years, slowly distorting the earth underneath our feet.

Eventually, a pre-existing weakness in the earth--called a fault or a fault zone--can not resist the strain any longer and fails catastrophically.

This is something like pulling a rubber band gradually until the band snaps.

This theory is known as the "elastic rebound theory."