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

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