Coseismic throw variation across along-strike bends on active normal faults: implications for displacement versus length scaling of earthquake ruptures

Iezzi, Francesco and Mildon, Z. and Faure Walker, J. and Roberts, Gerald P. and Goodall, H. and Wilkinson, M. and Robertson, Jennifer (2018) Coseismic throw variation across along-strike bends on active normal faults: implications for displacement versus length scaling of earthquake ruptures. Journal of Geophysical Research: Solid Earth 123 (11), pp. 9817-9841. ISSN 0148-0227.

Abstract

Fault bends, and associated changes in fault dip, play a key role in explaining the scatter in maximum offset versus surface rupture length fault scaling relationships. Detailed field measurements of the fault geometry and magnitude of slip in the 2016-2017 central Italy earthquake sequence, alongside three examples from large historical normal-faulting earthquakes in different tectonic settings, provide multiple examples in which coseismic throw increases across bends in fault strike where dip also increases beyond what is necessary to accommodate a uniform slip vector. Coseismic surface ruptures produced by two mainshocks of the 2016-2017 central Italy earthquake sequence (24th August 2016 Mw 6.0, 30th October 2016 Mw 6.5) cross a ~0.83 km amplitude along-strike bend, and the coseismic throws for both earthquakes increase by a factor of 2-3 where the strike of the fault changes by ~30o and the dip increases by 20-25o. We present similar examples from historical normal faulting earthquakes (1887, Sonora earthquake, Mw 7.5; 1981, Corinth earthquakes, Mw 6.7-6.4;1983, Borah Peak earthquake, Mw 7.3). We demonstrate that it is possible to estimate the expected change in throw across a bend by applying equations that relate strike, dip and slip vector to horizontal strain conservation along a non-planar fault for a single earthquake rupture. The calculated slip enhancement in bends can explain the scatter in maximum displacement (Dmax) versus surface rupture length scaling relationships. If fault bends are un-recognized, they can introduce variation in Dmax that may lead to erroneous inferences of stress drop variability for earthquakes, and maximum earthquake magnitudes derived from vertical offsets in paleoseismic datasets.

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