Statistical modelling of co-seismic knickpoint formation and river response to fault slip

Abstract : Most landscape evolution models adopt the paradigm of constant and uniform uplift. It results that the role of fault 10 activity and earthquakes on landscape building is understood under simplistic boundary conditions. Here, we develop a numerical model to investigate river profile development subjected to fault displacement by earthquakes and erosion. The model generates earthquakes, including mainshocks and aftershocks, that respect the classical scaling laws observed for earthquakes. The distribution of seismic and aseismic slip can be partitioned following a spatial distribution of mainshocks along the fault plane. Slope patches, such as knickpoints, induced by fault slip are then migrated at a constant rate upstream a 15 river crossing the fault. A major result is that this new model produces co-seismic knickpoints with a uniform height distribution for a fully coupled fault, i.e. with only co-seismic slip. Increasing aseismic slip at shallow depths, and decreasing shallow seismicity, censors the range magnitude of earthquakes cutting the river towards large magnitudes and leads to less frequent but higher amplitude knickpoints, on average. Inter-knickpoint distance or time between successive knickpoints follows an exponential decay law. Using classical rates for fault slip, 15 mm.yr-1 and knickpoint retreat, 0.1 m.yr-1 , leads to 20 high spatial densities of knickpoints requiring sub-metric spatial resolution to distinguish them. The correlation between the topographic profiles of successive parallel rivers cutting the fault remains positive for distance along the fault of less than half the maximum earthquake rupture length. This suggests that river topography can be used for paleo-seismological analysis and to assess fault slip partitioning between aseismic and seismic slip. Yet, considering simple scenarios of fault burial by intermittent sediment cover, driven by climatic changes or linked to earthquake occurrence, leads to knickpoint distributions 25 and river profiles markedly different from the case with no sediment cover. This highlights the potential role of sediments in modulating and potentially altering the expression of tectonic activity in river profiles and surface topography.
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Philippe Steer, Thomas Croissant, Edwin Baynes, Dimitri Lague. Statistical modelling of co-seismic knickpoint formation and river response to fault slip. Earth Surface Dynamics, European Geosciences Union, 2019, pp.1-34. ⟨10.5194/esurf-2019-5⟩. ⟨insu-02023056⟩



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