Tremors deep inside the Earth are usually produced by magma flowing beneath volcanoes, but a new study suggests they can also be produced by the shifting and sliding of tectonic plates. Scientists have recorded vibrations from underground tremors at a geologic observatory along the San Andreas Fault, an 800 mile 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 the Earth's crust that bump and grind like chunks of sea ice floating atop the ocean. The Earth's surface is made...