Scientists discovered tremors deep inside the Earth can also be produced be shifting and sliding of tectonic plates. Usually this is caused by magma flowing beneath volcanoes.
Scientists have recorded vibrations from underground tremors at a geologic observatory along the San Andreas Fault, an 800-mile (1,280-kilometer) scar in the earth that runs through California. The fault marks the boundary between the Pacific Tectonic Plate and the North American Plate.
Tectonic plates are large pieces of Earth's crust that bump and grind like chunks of sea ice floating atop the ocean. Earth's surface is made up of about 10 major tectonic plates and many more minor ones.