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The topography of the escarpments allows us to estimate the mode of slip of the faults

Fault slip, particularly during earthquakes, can generate topographic jumps on the surface, commonly known as fault escarpments. Since the 1970s, numerous studies have demonstrated that the topography of a fault escarpment can be used to date its formation and constrain the rate of fault slip.

The topography of the escarpments allows us to estimate the mode of slip of the faults

© Curioso Photography on Unsplash

Publication date: 22/03/2023

Press, Research

Related themes : Earth System Science

Once an escarpment has formed, it is progressively eroded according to a law of diffusion, resulting in a decrease in its slope and curvature. This decrease provides information about the length of time the escarpment has been eroded, and therefore about its age. The height of the escarpment divided by this age indicates the rate of sliding.

Escarpment associated with the extensive Kongur Shan fault system (north-west China). A large cumulative scarp has developed over several million years, allowing the rocks forming the chain front to be exhumed. Smaller escarpments are observed locally where the fault cuts through more recent Quaternary deposits. The extent and shape of these recent escarpments depend on the age of the deposits affected. While such observations allow us to say that the fault is active and to calculate the rate of slip, it is not currently possible to tell from these morphological data how (seismically/asismically) the fault is slipping. View ©️Google Earth and field photos (©️ Martine Simoes) from the road between Kashgar and Tashgorgan.

A study carried out by a team of scientists from the CNRS-INSU (see box) goes further and shows that the topography of escarpments also records the history of seismic or asismic landslides that formed the escarpment. A new analytical model of the topographic evolution of an escarpment has been developed, taking into account erosion by diffusion, the slip speed of the fault and, above all, the mode of slip of the fault, whether it be a single seismic rupture, several seismic ruptures or an asismic slip (i.e. a continuous slip). Using this model, the study shows that the final topographic shape of the escarpment generated by a single seismic rupture or by asismic slip, of the same amplitude, can deviate by 10 to 20%.

This result opens up the theoretical possibility of estimating not only cumulative slip or slip velocity, but also the number and size of earthquakes based on escarpment morphologies. In addition, this study is the first to show that the topography of landforms provides information not only on the speed of faults, but also on their mode of slip.

Slip mode and degradation of a fault scarp in an unconsolidated environment. The results show a significant difference in the final morphology of the escarpment depending on the slip mode: one earthquake (red), two earthquakes (green), or asismic slip (blue). Cumulative surrection and model duration are identical in all models. ©️ Rodolphe Cattin
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