A _fault_ is a fracture in the Earth’s crust along which two blocks of the crust have slipped with respect to each other. Faults are divided into three main groups, depending on how they move. _Normal faults_ occur in response to pulling or tension; the overlying block moves down the dip of the fault plane. _Thrust (reverse) faults_ occur in response to squeezing or compression; the overlying block moves up the dip of the fault plane. _Strike-slip (lateral) faults_ occur in response to either type of stress; the blocks move horizontally past one another. Most faulting along spreading zones is normal, along subduction zones is thrust, and along transform faults is strike-slip. [Illustration: _Normal Fault. Blocks are pulled apart_] [Illustration: _Thrust Fault. Blocks are pushed together_] [Illustration: _Strike-Slip Fault. Blocks slide past each other_] Geologists have found that earthquakes tend to reoccur along faults, which reflect zones of weakness in the Earth’s crust. Even if a fault zone has recently experienced an earthquake, however, there is no guarantee that all the stress has been relieved. Another earthquake could still occur. In New Madrid, a great earthquake was followed by a large aftershock within 6 hours on December 16, 1811. Furthermore, relieving stress along one part of the fault may increase stress in another part; the New Madrid earthquakes in January and February 1812 may have resulted from this phenomenon. [Illustration: Diagram of Earth’s layers and seismic wave propagation.] The _focal depth_ of an earthquake is the depth from the Earth’s surface to the region where an earthquake’s energy originates (the _focus_). Earthquakes with focal depths from the surface to about 70 kilometers (43.5 miles) are classified as shallow. Earthquakes with focal depths from 70 to 300 kilometers (43.5 to 186 miles) are classified as intermediate. The focus of deep earthquakes may reach depths of more than 700 kilometers (435 miles). The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth’s core is about 6,370 kilometers (3,960 miles), so even the deepest earthquakes originate in relatively shallow parts of the Earth’s interior. The _epicenter_ of an earthquake is the point on the Earth’s surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth.