Earthquake Gates of the Altyn Tagh Fault: Linking Rupture Length to Geologically Constrained Dynamics of Fault Complexity
Unusually large, rare, and unexpected earthquakes overwhelm mitigation measures and the societal capacity to respond, leading to a cascade of disastrous effects. In order to assess the potential for such rare events, an approach is required to calibrate how effectively geometrical complexities, such as stepovers and restraining bends along strike-slip faults, impede earthquake propagation. I define the term 'earthquake gate' to describe areas of fault complexity that halt earthquake ruptures conditionally as a result of proximal fault geometry, rupture direction, and prior earthquake history. To test and develop the earthquake gate concept I summarize collaborative efforts to document rupture history and slip rate gradients through major geometric barriers of the Altyn Tagh fault in western China. We compare numerical model results with these geologic observations in order to assess how prior earthquake history and fault geometry control whether an earthquake gate is open or closed to rupture propagation. Along the Aksay restraining double bend of the Altyn Tagh fault, opposing slip rate gradients on two parallel fault strands result from distributed deformation that occurs as earthquake ruptures terminate within the bend. From this pattern of fault activity we calibrate a two-dimensional, multi-cycle numerical rupture simulation with visco-elastic interseismic deformation, from which we predict that less than 10% of events are able to breach this earthquake gate. Paleoseismic results validate model results that (1) earthquake histories of the two strands are largely dissimilar, (2) frequency of rupture events decreases within the bend, and (3) events within the bend are age-correlative to sites outside of the bend on the same fault strand. We conclude that the earthquake gate concept provides a useful framework for synthesizing earthquake geology and numerical rupture modeling to improve estimates of large earthquake rupture probabilities.