Tamarah King, Mark Quigley, Dan Clark

The 30th March 1986 Mw 5.7 Marryat Creek earthquake produced a highly arcuate 13 km long surface rupture with maximum vertical displacement of 0.9 m. Sinistral displacement on the NE-SW limb, dextral displacement on the NNE-SSW limb, and maximum vertical displacement in the central apex of rupture supports SW over NE movement of a hanging-wall block. Epicentre locations are poorly constrained and inaccurate, locally exceeding distances of 30 km from the surface rupture. The most geologically and seismologically reasonable fault rupture model involves 3 bedrock-controlled faults. Assuming simple planar geometry, these would intersect 5.5 km SW of the rupture at 3 km depth, which is consistent with centroid depths of 3 – 4.5 km. Two trenches across the 1986 rupture trace show no preceding discrete offset since deposition of overlying sediments (100 – 130 ka). Strong evidence exists to suggest historic surface rupture was controlled by basement structures including a large pre-existing fault, but only circumstantial evidence supports any prior neotectonic rupture. This earthquake is one of the most structurally complex (as proxied by the number of discrete faults) for its magnitude, as evidenced by comparison with a global compilation.