fault scarp

fault scarp A fault scarp is a type of escarpment that arises from repeated earthquake displacement on a fault in the Earth's crust. Strictly speaking, the term ‘fault scarp’ refers to the scarp formed along the intersection of a fault plane with the Earth's surface and should only be used in the case of a fault that reaches ground level. However, the term is also used loosely of an escarpment that follows the approximate surface line corresponding to a fault that is active at depth. The latter should properly be called a fault-line scarp.

For a true fault scarp to form, displacement at the hypocentre of the earthquake (the focus of the earthquake, typically 10–20 km beneath the surface) must be sufficiently large to propagate to the surface. Because displacement across the fault plane decreases away from the focus, this condition is generally met only for faults associated with earthquakes of magnitude 6 or greater, in extensional or strike-slip settings. Each fault movement in an earthquake results in a sudden increase in the size of the scarp, by an amount ranging from a few centimetres for a magnitude 5.5–6 event to several metres for a magnitude 7.5–8 earthquake. The scarp may also grow more gradually as creep occurs on the fault plane in the hours and days following the earthquake. The freshly exposed portion of the scarp is formed of shattered fragments (fault breccia) from the rocks on either side of the fault and is usually highly polished. This breccia is easily eroded, and the polished surface usually persists for only a few metres above the base of the scarp.

Thrust faulting occurs on shallow dipping faults and does not commonly produce a true fault scarp, because the tip of the upper, overthrust block tends to collapse, leaving a sinuous scarp that do not coincide with the fault plane. Strike-slip faulting may result in a scarp if there is a significant extensional or thrust component of motion, or if rock-types with different weathering characteristics are juxtaposed by faulting. Failing this, both active and inactive faults are often marked by a fault-line valley, a line of preferential erosion along the fault.

Fault scarps are generally visible only on faults that have been active within the past few tens to hundreds of thousand years. The competing effects of fault motion, erosion, and sediment deposition all affect the form of the scarp and the latter two will eventually eliminate it after movement on the fault ceases. If the rates of erosion of the rocks either side of the fault are known or can be assumed to be negligible, and the sediments deposited contain material that can be dated, then the magnitude of the scarp can be used to determine the age at which the scarp began to form and the long-term rate of movement on the fault. Additionally it may be possible to calculate the displacement in individual earthquakes and the recurrence interval of such events. More information about the relative significance and rates of movement of neighbouring faults can be gleaned by observing their effects on the drainage pattern in the area. Both major and minor faults influence the directions of river channels, and even minor faults can have wind gaps, low points on the scarp where a stream once crossed the fault but has now been diverted because of the changed topography.

Peter Clarke