WHAT CAUSES EARTHQUAKES? An earthquake is the result of a sudden release of stored energy in the Earth's crust triggered by shifting tectonic plates. The Earth's lithosphere is an elaborate network of interconnected plates that move constantly -- far too slow for us to be aware of them, but moving, nonetheless. Occasionally they lock up at the boundaries, and this creates frictional stress. When that gets to be too large a strain, the rocks give way and break and slide along fault lines. This can give rise to a violent displacement of the Earth's crust, which we feel as vibrations or tremors as the pent-up energy is released. However, only 10% or so of the total energy is released in the seismic waves. However, the rest is converted into heat, used to crush and deform rock, or released as friction.
The American Geophysical Union contributed to the information contained in the TV portion of this report.