EGU General Assembly 2021
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Scaling seismic fault thickness from the laboratory to the field

Thomas P. Ferrand1, Stefan Nielsen2, Loïc Labrousse3, and Alexandre Schubnel4
Thomas P. Ferrand et al.
  • 1Institut des Sciences de la Terre d'Orléans, Université d'Orléans / CNRS UMR-7327, Orléans, France.
  • 2Department of Earth Sciences, Durham University, Durham DH1 5ED, United Kingdom.
  • 3Institut des Sciences de la Terre de Paris, Sorbonne Université, Campus Pierre et Marie Curie / CNRS UMR-7193, Paris, France.
  • 4Laboratoire de Géologie de l'Ecole Normale Supérieure, PSL Research University / CNRS UMR-8538, Paris France.

Pseudotachylytes originate from the solidification of frictional melt, which transiently forms and lubricates the fault plane during an earthquake. Here we observe how the pseudotachylyte thickness a scales with the relative displacement D both at the laboratory and field scales, for measured slip varying from microns to meters, over six orders of magnitude. Considering all the data jointly, a bend appears in the scaling relationship when slip and thickness reach ∼1 mm and 100 µm, respectively, i.e. MW > 1. This bend can be attributed to the melt thickness reaching a steady‐state value due to melting dynamics under shear heating, as is suggested by the solution of a Stefan problem with a migrating boundary. Each increment of fault is heating up due to fast shearing near the rupture tip and starting cooling by thermal diffusion upon rupture. The building and sustainability of a connected melt layer depends on this energy balance. For plurimillimetric thicknesses (a > 1 mm), melt thickness growth reflects in first approximation the rate of shear heating which appears to decay in D−1/2 to D−1, likely due to melt lubrication controlled by melt + solid suspension viscosity and mobility. The pseudotachylyte thickness scales with moment M0 and magnitude MW; therefore, thickness alone may be used to estimate magnitude on fossil faults in the field in the absence of displacement markers within a reasonable error margin.

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