Imaging the Wairarapa Fault at Tauwharenikau using magnetotelluric sounding

Abstract

To test the ability of closely spaced magneto‐telluric data to provide high resolution images of fault structures, measurements have been made across the Wairarapa Fault at Tauwharenikau, southern North Island, New Zealand. 2D inversion and modelling of the TM‐mode data from 11 sites on a 1.5 km long transect show the transition from conductive Tertiary sediments southeast of the fault zone to resistive greywacke to the northwest, and also illustrate some of the difficulties in interpretation that can arise. The existence of dual fault traces separated by some 100 m allows for two possible interpretations of the electrical structure: (1) that the southeastern fault trace represents the main trace of the Wairarapa Fault and that low resistivity to the northwest is the result of fluid percolation into fractured rock between the fault traces; or (2) that the northwestern fault trace marks the actual contact between Tertiary sediments and greywacke. In this case, the near‐surface dip of the fault is constrained to be c. 80° to the northwest.