Fault
A fault is a fracture in Earth's crust along which blocks of rock have moved relative to each other, classified as normal, reverse, or strike-slip.
Definition
A fault is a crack in Earth's crust where rock on one side has moved relative to the rock on the other, and it only counts as a fault once that displacement has occurred. Normal faults form where the crust stretches and the hanging wall drops, reverse or thrust faults form where the crust is squeezed and the hanging wall rises, and strike-slip faults form where blocks slide past each other horizontally, ranging from centimeters to over a thousand kilometers long. The type of fault reflects the orientation of the principal stresses under Anderson's theory: normal where the greatest stress is vertical, thrust where it is horizontal, strike-slip where both greatest and least stress are horizontal. Faults are zones rather than clean planes, filled with crushed gouge near the surface and ductile mylonite at depth, and they slip through sudden stick-slip earthquakes or, on some segments, steady aseismic creep.
Example
Sliding the top book of a stack sideways shows the basic idea, with the gap between the books standing in for the fault. The San Andreas Fault in California is a real strike-slip fault, the Brevard Fault Zone runs through the North Carolina mountains near Brevard and Boone as an ancient fault from the rise of the Appalachians, and normal faults bound the Triassic rift basins near Durham. The Wasatch Fault in Utah is a normal fault that has raised the Wasatch Range more than $3\ \text{km}$ above the valley floor over millions of years, while the Blue Ridge Thrust carried billion-year-old basement rock westward over younger Paleozoic strata for tens of kilometers.
Key Insight
Not every crack in a rock is a fault; it only earns the name once the two sides have actually moved past each other, and the type of movement tells you the kind of force involved, stretching for normal faults, squeezing for reverse faults, shearing for strike-slip faults. Faults also heal and weaken between earthquakes, as mineral precipitation and pressure solution slowly rebuild the frictional strength that a rupture destroyed, and how fast that healing happens sets how often a given fault ruptures again.