The Earth is formed of several layers that have very different physical and chemical properties. The outer layer, which averages about 70 kilometers in thickness, consists of about a dozen large, irregularly shaped plates that slide over, under and past each other on top of the partly molten inner layer.
Most earthquakes occur at the boundaries where the plates meet.
In fact, the locations of earthquakes and the kinds of ruptures they produce help scientists define the plate boundaries. At spreading zones , molten rock rises, pushing two plates apart and adding new material at their edges. Transform faults are found where plates slide past one another. An example of a transform-fault plate boundary is the San Andreas fault, along the coast of California and northwestern Mexico. Earthquakes at transform faults tend to occur at shallow depths and form fairly straight linear patterns.