Surface slip distributions and geometric complexity of intraplate reverse-faulting earthquakes
Earthquake ground surface ruptures provide insights into faulting mechanics and inform seismic hazard analyses. Surface ruptures for eleven historical (1968 to 2018) moment magnitude (Mw) 4.7 to 6.6 reverse earthquakes in Australia are analyzed using statistical techniques and compared to magnetic, gravity, and stress trajectory datasets. Of the total combined (summative) length of all surface ruptures (~148 km), 133 km (90%) to 145 km (98%) align with geophysical structure in the host basement rocks. Surface rupture length (SRL), maximum displacement (MD), and probability of surface rupture at a specified Mw are high compared with equivalent Mw earthquakes globally. This is attributed to (i) steep cratonic crustal strength gradient at shallow depths promoting shallow hypocenters (~1 to 6 km) and limiting down-dip rupture widths (~1 to 8.5 km), and (ii) favorably-aligned crustal anisotropies (e.g., bedrock foliations , faults, fault intersections) that enhance lateral rupture propagation and/or surface displacements. Combined (modeled and observed) MDs are in the middle third of the SRL with 68% probability, and either the ≤ 33rd and ≥ 66th percentiles of SRL with 16% probability. MD occurs proximate to or directly within zones of enhanced fault geometric complexity (as evidenced from surface ruptures) in 8 of 11 earthquakes (73%). MD can be approximated by 3.3 ± 1.6 (1σ) × AD (average displacement). S-transform analysis indicates high-frequency slip maxima also coincide with fault geometric complexities, consistent with stress amplifications and enhanced slip variability due to geometric and kinematic interactions with neighboring faults. Rupture slip taper angles exhibit large variations (-90 % to + 380 % with respect to the mean value) towards rupture termini and are steepest where ruptures terminate at obliquely-oriented magnetic lineaments and/or lithology changes. Incremental slip approximates AD between the 10th and 90th percentiles of the SRL. The average static stress drop of the studied earthquakes is 4.8 ± 2.8 MPa. A surface rupture classification scheme in cratonic stable regions is provided to describe the prevailing characteristics of intraplate earthquakes across diverse crustal structural-geophysical settings. New scaling relationships and suggestions for logic tree weights are provided to enhance probabilistic fault displacement hazard analyses for bedrock-dominated intraplate continental regions.