An extensional fault is a fault that vertically thins and horizontally extends portions of the Earth's crust and/or lithosphere

Lithosphere

The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

. In most cases such a fault is also a normal fault, but may be rotated to have a shallower geometry normally associated with a thrust fault

Thrust fault

A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...

. Extensional faults are generally planar and, so long as the stress field

Stress field

A stress field is a region in a body for which the stress is defined at every point. Stress fields are widely used in fluid dynamics and materials science....

 is orthogonal to the Earth's surface initiate with a dip of 60° typically continuing down to the base of the seismogenic layer

Seismogenic layer

The seismogenic layer is the range of depths within the crust or lithosphere over which most earthquakes are initiated. Typically in continental crust this is in the uppermost 15 km...

.