Types of fault
Based on the direction of movement, faults are broadly categorised as one of the following types:
Strike-slip faults
For strike-slip faults, the failure surface is usually near vertical and the direction of movement only occurs laterally with no or very little vertical motion. Strike-slip faults with left-lateral motion are known as sinistral faults, and those with right-lateral motion are known as dextral faults. Each is defined by the direction of movement of the ground on the opposite side of the fault from an observer.
Dip-slip faults
For dip-slip faults, the direction of movement only occurs vertically with no or very little lateral motion. Dip-slip faults have either a reverse or normal direction of movement.
A normal dip-slip fault occurs when the crust is extended. This type of fault is also often called an extensional fault, and in this situation, the hanging wall moves downward relative to the footwall. A reverse dip-slip fault is the opposite of a normal dip-slip fault. In this situation, the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of the crust.
In some dip-slip fault situations, a block of ground is present between the footwall and hanging wall components of the fault. A downthrown block between two normal faults dipping towards each other is called a graben. An upthrown block between two normal faults dipping away from each other is called a horst.
Movement of a normal and reverse dip-slip fault (top and centre) and a strike-slip fault (bottom).
Oblique-slip faults
A fault that has a component of dip-slip and a component of strike-slip movement is termed an oblique-slip fault. Nearly all faults will have some component of both dip-slip and strike-slip, so a fault that is classified as oblique requires both dip and strike components to be significant and measurable.
In 1855, the Wairarapa Fault ruptured spectacularly. The event involved both reverse and dextral strike-slip fault movement, which lifted the Rimutaka Ranges by more than 6 m and displaced the ground horizontally by more than 18 m. It remains the largest ground movement ever observed in historical New Zealand.
It is very rare for a fault to rupture in a single, straight line. They often have several gaps along their length, and large faults that cover a considerable distance can abruptly change direction several times.
Aftershocks
For most earthquake events, not all of the movement occurs during the initial fault rupture. Subsequent smaller earthquakes, called aftershocks, often occur near the original earthquake as the earth surrounding the fault readjusts to the previous ground movement. Aftershocks can last for weeks, months or even years after a large earthquake and can be very damaging. The February 2011 earthquake in Christchurch was an aftershock of the main September 2010 earthquake in Darfield.
The exception is a subduction zone earthquake, where aftershocks may be rare or not occur at all. In this situation, the toe of a subducting tectonic plate breaks off into the Earth’s mantel and generally only results in a single large earthquake event (the 2001 Nisqually earthquake in Washington State, USA, is one example).