What is the action that defines a strike-slip fault?

A strike-slip fault is characterized by horizontal sliding of the blocks of crust on either side of the fault line. This means that the primary movement occurs parallel to the fault, rather than vertically or in other directions. In this type of fault, one side of the fault moves to the left or right relative to the other side, which is a defining feature of strike-slip faults.

The nature of this horizontal movement is caused by shear stress in the Earth's crust, typically in areas where tectonic plates are sliding past each other, such as at transform plate boundaries. This type of fault is significant in understanding tectonic processes and is commonly associated with earthquakes.

In contrast, vertical movement is associated with normal or reverse faults, curvilinear flow typically describes fluid dynamics and not fault movement, and rotational movement pertains to different geological processes not classified under strike-slip behavior. Therefore, the correct choice emphasizes the unique aspect of horizontal displacement that defines strike-slip faults.