1. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964; and Department of Earth and Environmental Sciences, Columbia University; and
2. Columbia Earth Institute, Columbia University, New York 10027, USA
3. School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, Hawaii 96822, USA

Correspondence to: J. S. Floyd¹ Correspondence and requests for materials should be addressed to J.F. (e-mail: Email: jsfloyd@ldeo.columbia.edu).

Determining the composition and physical properties of shallow-dipping, active normal faults (dips < 35° with respect to the horizontal) is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere. Seismic reflection images¹ and earthquake source parameters² show that a magnitude 6.2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25–30° located ahead of a propagating spreading centre in the Woodlark basin. Here we present results from a genetic algorithm inversion of seismic reflection data, which shows that the fault at 4–5 km depth contains a 33-m-thick layer with seismic velocities of about 4.3 km s^-1, which we interpret to be composed of serpentinite fault gouge. Isolated zones exhibit velocities as low as 1.7 km s^-1 with high porosities, which we suggest are maintained by high fluid pressures. We propose that hydrothermal fluid flow, possibly driven by a deep magmatic heat source, and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low-angle normal fault.