The plane or fracture of a fault, known as the fault-plane, is seldom vertical.
late 13c., faute, "deficiency," from Old French faute (12c.) "opening, gap; failure, flaw, blemish; lack, deficiency," from Vulgar Latin *fallita "a shortcoming, falling," noun use of fem. past participle, from Latin falsus "deceptive, feigned, spurious," past participle of fallere "deceive, disappoint" (see fail).
The -l- was restored 16c., probably in imitation of Latin, but was not pronounced till 18c. Sense of "physical defect" is from early 14c.; that of "moral culpability" is first recorded late 14c. Geological sense is from 1796. The use in tennis (c.1600) is closer to the etymological sense.
late 14c., Scottish, "be deficient;" see fault (n.). Meaning "find fault with" is from mid-15c. Related: Faulted; faulter; faulting.
A fracture in a rock formation along which there has been movement of the blocks of rock on either side of the plane of fracture. Faults are caused by plate-tectonic forces. See more at normal fault, reverse fault, strike-slip fault, thrust fault, transform fault. See Note at earthquake.
Our Living Language : Bedrock, the solid rock just below the soil, is often cracked along surfaces known as planes. Cracks can extend up to hundreds of kilometers in length. When tensional and compressional stresses cause rocks separated by a crack to move past each other, the crack is known as a fault. Faults can be horizontal, vertical, or oblique. The movement can occur in the sudden jerks known as earthquakes. Normal faults, or tensional faults, occur when the rocks above the fault plane move down relative to the rocks below it, pulling the rocks apart. Where there is compression and folding, such as in mountainous regions, the rocks above the plane move upward relative to the rocks below the plane; these are called reverse faults. Strike-slip faults occur when shearing stress causes rocks on either side of the crack to slide parallel to the fault plane between them. Transform faults are strike-slip faults in which the crack is part of a boundary between two tectonic plates. A well-known example is the San Andreas Fault in California. Geologists use sightings of displaced outcroppings to infer the presence of faults, and they study faults to learn the history of the forces that have acted on rocks.