Abstract
Viscous flow in the deep crust and uppermost mantle can contribute to the accumulation of strain along seismogenic faults in the shallower crust1. It is difficult to evaluate this contribution to fault loading because it is unclear whether the viscous deformation occurs in localized shear zones or is more broadly distributed2. Furthermore, the rate of strain accumulation by viscous flow has a power law dependence on the stress applied, yet there are few direct estimates of what the power law exponent is, over the long term, for active faults. Here we measure topography and the offset along fault surfaces created during successive episodes of slip on seismically active extensional faults in the Italian Apennines during the Holocene epoch. We show that these data can be used to derive a relationship between the stress driving deformation and the fault strain rate, averaged over about 15 thousand years (kyr). We find that this relationship follows a well-defined power law with an exponent in the range of 3.0–3.3 (1σ). This exponent is consistent with nonlinear viscous deformation in the deep crust and, crucially, strain localization promoted by seismogenic faulting at shallower depths. Although we cannot rule out some distributed deformation, we suggest that fault strain and thus earthquake recurrence in the Apennines is largely controlled by viscous flow in deep, localized shear zones, over many earthquake cycles.
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Acknowledgements
N. D’Agostino supplied the long-wavelength topography data used in Figs 2 and 4. This work was supported by NERC grants: NER/S/A/2006/14042, NE/E01545X/1 and NE/I024127/1. Financial support was also provided by the Statoil Earth System Modelling project (P.S.) and the Statoil-University of Bergen Akademia agreement (P.A.C.). We thank R. Huismans for discussions and P. Molnar and M. Handy for their comments.
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P.A.C. led the interpretation of the scaling exponent in terms of mid-crustal shear zones. C.H.S. contributed to understanding the behaviour of coupled frictional–viscous fault systems. G.P.R. provided the structural data and analysed the strain rate versus elevation relation. J.P.F.W. carried out the strain rate calculations and quantified data uncertainties. P.S. contributed to understanding stress and strain rate variations in a layered lithosphere.
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Cowie, P., Scholz, C., Roberts, G. et al. Viscous roots of active seismogenic faults revealed by geologic slip rate variations. Nature Geosci 6, 1036–1040 (2013). https://doi.org/10.1038/ngeo1991
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DOI: https://doi.org/10.1038/ngeo1991
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