At fault

a defect or imperfection; flaw; failing:
a fault in the brakes; a fault in one’s character.
responsibility for failure or a wrongful act:
It is my fault that we have not finished.
an error or mistake:
a fault in addition.
a misdeed or transgression:
to confess one’s faults.
Sports.

a ball that when served does not land in the proper section of an opponent’s court.
a failure to serve the ball according to the rules, as from within a certain area.

Geology, Mining. a break in the continuity of a body of rock or of a vein, with dislocation along the plane of the fracture (fault plane)
Manège. (of a horse jumping in a show) any of a number of improper executions in negotiating a jump, as a tick, knockdown, refusal, or run-out.
Electricity. a partial or total local failure in the insulation or continuity of a conductor or in the functioning of an electric system.
Hunting. a break in the line of scent; a losing of the scent; check.
Obsolete. lack; want.
to commit a fault; blunder; err.
Geology. to undergo faulting.
Geology. to cause a fault in.
to find fault with, blame, or censure.
at fault,

open to censure; blameworthy:
to be at fault for a mistake.
in a dilemma; puzzled:
to be at fault as to where to go.
(of hounds) unable to find the scent.

find fault, to seek and make known defects or flaws; complain; criticize:
He constantly found fault with my behavior.
to a fault, to an extreme degree; excessively:
She was generous to a fault.
noun
an imperfection; failing or defect; flaw
a mistake or error
an offence; misdeed
responsibility for a mistake or misdeed; culpability
(electronics) a defect in a circuit, component, or line, such as a short circuit
(geology) a fracture in the earth’s crust resulting in the relative displacement and loss of continuity of the rocks on either side of it
(tennis, squash, badminton) an invalid serve, such as one that lands outside a prescribed area
(in showjumping) a penalty mark given for failing to clear or refusing a fence, exceeding a time limit, etc
(hunting) an instance of the hounds losing the scent
deficiency; lack; want
at fault

guilty of error; culpable
perplexed
(of hounds) having temporarily lost the scent

find fault, to seek out minor imperfections or errors (in); carp (at)
to a fault, excessively
verb
(geology) to undergo or cause to undergo a fault
(transitive) to find a fault in, criticize, or blame
(intransitive) to commit a fault
n.

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.
v.

late 14c., Scottish, “be deficient;” see fault (n.). Meaning “find fault with” is from mid-15c. Related: Faulted; faulter; faulting.
fault
(fôlt)

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.

In geology, a place where sections of the crust of the Earth move relative to each other. (See earthquake and San Andreas fault.)

Note: Faults tend to occur near the edges of tectonic plates.

Responsible for a mistake, trouble, or failure; deserving blame. For example, At least three cars were involved in the accident, so it was hard to determine which driver was at fault , or He kept missing the target and wondered if the sight on his new rifle was at fault . In Britain this usage was formerly considered incorrect but is now acceptable; in America it has been widespread since the mid-1800s. Also see in the wrong
see:

at fault
find fault
to a fault