Coseismic Throw Variation Across Along-Strike Bends on Active Normal Faults: Implications for Displacement/Length Scaling of Earthquake Ruptures.

We suggest that along-strike fault bends are a plausible explanation of the scatter of maximum displacement (Dmax) for normal faulting earthquakes in Dmax versus length relationships shown in existing fault scaling relationships (e.g. Wells and Coppersmith, 1994).
We present field measurements of coseismic offsets and fault geometry from five different large normal faulting earthquakes (2016-2017 Mw6.0-Mw6.5 Central Italy earthquakes; 1887, Sonora earthquake, Mw7.5; 1981, Corinth earthquake, Mw6.7-6.4; 1983, Borah Peak earthquake, Mw7.3). In all these examples Dmax occurs where coseismic throw increases across bends in the strike of the fault where the dip also increases beyond what is needed to maintain the slip vector. We show that it is possible to replicate change in throw across a bend by applying calculations based on the conservation of the strain-rate across the variable fault geometry (Faure Walker et al., 2009). In detail, field measurements show that although values for the slip vector azimuth and the coseismic heave are in general maintained across the bend, the coseismic Dmax increases by a factor of 2-3 where the strike of the fault changes by 15-30º and the fault dip increases by 20-30º. Our calculations show that anomalous throws at bends are required by the geometry and kinematics of the faulting to maintain the horizontal extensional strain along strike and across the fault bend.
We show that the Dmax measured in the field for each of the studied earthquakes, located within the bends, is always larger than the Dmax predicted by Wells & Coppersmith (1994), Dmax vs fault length scaling relationship. We use these findings to suggest that the variation of fault strike and dip across along-strike fault bends is one possible cause for the scatter of values in Dmax vs fault length scaling relationships. We discuss the implications of our finding in terms of inferring stress drops and palaeoearthquake magnitudes from Dmax values. We emphasize how to use fault bends to improve stress drop and palaeoearthquake magnitude calculations and what parameters should be measured across fault bends in future studies.