We could go deeper into a mine or we could sink a deep shaft and gather rock samples. But the world's deepest caves extend down about a mile and the deepest shafts extend only a few miles. Since it is over 4000 miles from the surface of the earth to the center of the planet, we have literally only scratched the surface of our world. The earth is constantly shaking and jiggling, and we can record these vibrations on devices called seismographs . In particular, we pick up the large vibrations that occur because of earthquakes .
Earthquakes occur as a result of stresses on rocks.
Forces within the earth push and pull rocks, but rocks don't always move smoothly. Often, slabs of rocks will stick against each other, as frictional forces cause the slabs to lock up. But the forces pushing on the rocks continue to push, and eventually, exceed the strength of the rock. The rock will fracture and snap back, much like a rubber band. This elastic rebound releases the strain on the rock.