Geophysical imaging of the Quaternary Wairoa North Fault, New Zealand: a case study

An integrated geophysical investigation, including gravity, vertical electric sounding (VES), 2D resistivity and seismic reflection/refraction techniques, was carried out along two profiles across the extensional Wairoa North Fault, New Zealand, prior to a trenching and paleoseismic study. The geophysical data clearly imaged the fault plane lying directly beneath a suspected fault scarp on one profile. The location of the intersection of the fault plane with the surface was deduced from 2.5D gravity and 2D resistivity modeling and subsequently corroborated by trenching to within 5 m. A 60° dip (±10–20°) for the fault plane along both profiles, similar to that revealed in the trench, was modeled to about 70 m depth at both locations, indicating a minimum vertical displacement. Seismic methods, together with a trial ground penetrating radar (GPR) profile, were less successful in imaging deformation above the fault plane, partly because of a lack of coherent reflectors within the upper Quaternary sequence, but mainly because of the deep weathering profiles on both sides of the fault. Despite this deeply weathered environment, geophysical methods (especially 2D resistivity) have proved very effective in confirming the significance of a suspected fault scarp, facilitating the accurate siting of a trench and providing valuable deeper information about fault morphology.