Coseismic throw variation across along-strike bends on active faults: implications for displacement/length scaling of seismic rupture
We suggest that fault bends play a key role in explaining the scatter seen in maximum offset vs fault length relationships shown in existing fault scaling relationships. Primary detailed field measurements of the fault geometry and coseismic throw in the 2016-2017 central Italy earthquake sequence, together with observations of other large historical normal-faulting earthquakes within the literature, provide multiple examples where coseismic throw increases across bends in the strike of faults. We can quantify the expected change in throw across a bend by applying the “geometry-dependent throw-rate theory” (Faure Walker et al., 2015) to a single rupture, based on conservation of the strain-rate across the variable fault geometry.
We measured the geometry and kinematics of earthquake surface ruptures for the 24th August and 30th October 2016 earthquakes (Mw 6.0, Mw 6.5) in central Italy, collecting measurements of strike, slip vector azimuth, plunge of the slip vector, displacement, heave and throw. Following the 24th August event, we covered the total extension of the coseismic surface ruptures, with measurements taken every 2-10 m along strike. Following the 30th October event, we focused on constraining the large coseismic throws evident around a prominent along-strike fault bend, present in the southern part of the Mt. Vettore fault, collecting measurements every 10-50 m along strike, and characterizing the surface ruptures across the along strike fault bend and on the fault segments either side along strike.
Both datasets show that although the slip-vector azimuth and the coseismic heave vary by <10-20% across the bend, yet the coseismic dmax increases by a factor of x2-3 where the strike of the host fault changes by ~30º and the dip increases by 20-25º. We explain the large increase of throw using calculations that relate it to strain-rate conservation across the varying geometry and kinematics of the host fault. This calculation predicts large variations in throw given changes in the strike and dip of the fault in the fault bend. This is consistent with the large increase of throws on fault bends we have observed on other historical large-offset normal faulting earthquakes, which ruptured across along-strike fault bends (e.g. 1887, Sonora earthquake, Mw 7.5 (Suter, 2015); 1981, Corinth earthquake, Mw 6.7-6.4 (Jackson et al., 1982; Morewood & Roberts, 2001); 1983, Borah Peak earthquake, Mw 7.3 (Crone et al., 1987)). The application of the same calculations can explain the large increase of throws across fault bends also for these other examples. The largest offsets, observed across fault bends, including those associated with the 30th October Mw 6.5 earthquake and with the other historical earthquakes, are always larger than the maximum displacement predicted by Wells & Coppersmith, 1994, Dmax /fault length scaling relationship. We use these findings to suggest that the varying geometry of faults, represent one of the possible causes of the scatter of values in Dmax /fault length scaling relationships. Hence, along-strike fault bends should be considered when scaling relationships are used to infer stress drop variability for earthquakes or maximum magnitudes from vertical offsets in palaeoseismic datasets.