In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.
A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault.
Wikipedia, Fault (geology), 2019
Earth's crust is full of cracks and fault lines. This was only known recently. However the Quran said that Earth had cracks long before it was discovered:
[Quran 86.12] And the Earth that has faults.
"Tasaddaa in Arabic تصدَّعَ" means got cracked. In this verse "Sadaa صَّدْعِ" means the crack itself. Here the Earth has internal cracks.
How could an illiterate man who lived 1400 years ago have known that Earth has faults?