A fault scarp is a distinct, step-like landform created when movement along a fault plane breaks and displaces the ground surface. This visible cliff or steep slope results from a sudden, typically earthquake-related, vertical shift in the Earth’s crust. It represents the exposed portion of the fault that has been uplifted relative to the adjacent block of land. The formation of a scarp provides geologists with immediate evidence of active tectonic forces.
Defining the Structure and Appearance
A newly formed fault scarp presents as a remarkably linear, steep, cliff-like feature cutting across the landscape. The steepness of this initial face, known as the free face, can often be near-vertical, especially when formed in cohesive bedrock. The height of the scarp corresponds to the vertical displacement, or throw, that occurred during the seismic event, ranging from a few centimeters to several meters in a single rupture event.
The scarp acts as a topographic boundary, dividing the landscape into an up-thrown block and a down-dropped block. In unconsolidated sediments, the initial free face may quickly collapse to a slightly gentler slope, around 60 degrees, due to gravity immediately following the rupture. The overall shape is a clear, abrupt offset in the ground, making the scarp one of the most recognizable surface expressions of a major fault.
The Mechanism of Formation
Fault scarps are formed by the sudden, brittle rupture of the Earth’s crust during a moderate to large earthquake, not by slow, gradual tectonic creep. The vertical displacement necessary to create a scarp occurs along dip-slip faults, which are characterized by primarily vertical movement along the fault plane.
Normal faults occur in areas where the crust is being pulled apart by tensional forces, causing the hanging wall block to move down relative to the footwall block. This extensional movement results in a scarp where the exposed face is the footwall. Conversely, reverse faults, which include low-angle thrust faults, form in compressional environments where one block is pushed up and over the other.
In a reverse fault setting, the hanging wall is pushed upward relative to the footwall, creating a scarp where the overlying rock mass forms the exposed face. While pure strike-slip faults involve almost entirely horizontal motion, even these faults can produce a scarp if there is a minor component of vertical movement. The type of faulting dictates the geometry of the scarp and the overall tectonic environment.
Degradation and Modification Over Time
Immediately after its formation, a fault scarp is subject to weathering and erosion, meaning the feature is not permanent. Processes like mass wasting, wind abrasion, and water runoff begin to break down the exposed, fractured rock face. This causes the scarp to lose its initial steep, sharp profile, gradually becoming smoother and more rounded over time.
In areas with unconsolidated sediments, this degradation occurs relatively quickly as material erodes from the up-thrown side and accumulates at the base of the scarp. This process changes the scarp’s profile from a steep cliff to a gentler, concave-up slope. Geologists study these degradation profiles, which can be modeled mathematically using concepts like hillslope diffusion, to estimate the time elapsed since the scarp first formed. The degree of rounding is a direct indicator of the scarp’s age, with older scarps exhibiting a much gentler angle than recent ones.
Importance in Hazard Assessment
Fault scarps are invaluable tools in paleoseismology, the study of ancient earthquakes, because they provide a direct record of past seismic events. By analyzing a scarp, geologists can determine the location of the active fault and the magnitude of the vertical displacement that occurred. This allows researchers to estimate the size of the earthquake that caused the rupture.
A common technique involves trenching, where a deep ditch is excavated perpendicular to the scarp. This exposes the layers of soil and sediment that have been offset by the fault. By dating organic material or sediment layers within the trench, scientists can establish a timeline of past ruptures.
This data is used to calculate the recurrence interval, or the average time between large earthquakes on that specific fault. Understanding the frequency and magnitude of past ruptures is a foundational step in assessing future seismic risk and creating hazard maps for local communities.