Global distribution of earthquake-fault dip angles

Abstract

The dip angle is one of the fault parameters that most affect fault-related hazard analyses (ground shaking, tsunami) because it not only influences the inference of other fault parameters (e.g. down-dip width, earthquake maximum magnitude based on fault scaling relations) but also and most importantly, the dip angle controls: (a) the fault-to-site distance values of ground motion estimates based on predictive models (Ground Motion Models); (b) the ground shaking predicted by physics-based simulations; and (c) the vertical component of static surface displacement, which determines the initial conditions for tsunami simulations when the seafloor is displaced. We present the results of a global survey of earthquake-fault dip angles (G-DIP, short for Global Dip) and analyse their empirical distribution for various faulting categories (normal, reverse, transcurrent crustal faulting and subduction-interface reverse faulting). These new empirical statistics are derived from an extensive and homogeneous data set of 597 uniquely determined fault plane dip angles corresponding to 269 individual earthquakes. As such, our statistics of fault dip occurrences separated by fault types at a global scale improve previous fault dip-angle distributions. We found significant differences between the average empirical fault dip-angle distributions and the values usually assumed based on Anderson's theory. Dip-slip crustal faults show the same mode at 40°–50° for both normal and reverse mechanisms, whereas transcurrent faults have a large spread of values below the mode at 80°–90°. Regarding reverse crustal faults, our result became evident after separating them from subduction interface faults, which show significantly lower dip values, with a mode at 10°–20°. We remark on the importance of documented uniquely determined fault planes to develop dip-angle statistics. We also suggest that our results can effectively be used as distribution priors for characterizing the geometry of poorly known seismogenic faults in earthquake hazard analyses and earthquake-fault modelling experiments.