Location of Active Faults using Geomorphic Indices in Eroded Landscapes, South Taranaki, New Zealand

Abstract

South Taranaki region has a number of active faults that show surface expression in the younger and harder materials near the coast and central volcanoes of the North Island, but these traces finish abruptly inland when they cross into older, heavily eroded, mudstone and sandstone. Current methods to locate surface evidence of active faults (i.e. geomorphic interpretation of stereographic aerial photography) are not fully successful in this region. Erosion occurs here at a greater rate than surface rupture of faults which causes the removal of surface expression, and/or dense tree cover obscures surface expression. International studies of tectonic activity in eroded landscapes have identified geomorphic indices as useful reconnaissance tools to locate active faults. This research applies geomorphic indices to the Taranaki region for the first time. Four indices are tested; stream length-gradient index, stream channel sinuosity, hypsometry and drainage basin asymmetry. Results are obtained by applying the indices to four freely available national Digital Elevation Models (DEMs) of differing resolutions. This allowed comparison between DEMs, providing the ability to test DEM quality and at what scale geomorphic indices could be applied to these freely available data. Results show that the geomorphic indices used can identify changes to the equilibrium state at a catchment and stream channel scales. However, the myriad of physical processes occurring at a range of temporal scales within this area make identification of a tectonic signature a challenging task. Conclusive examples of tectonic processes are evident and analysis suggests that these examples are likely to be due to recent or large fault ruptures. There are many areas where a tectonic influence to the equilibrium state of a stream or catchment can be inferred. These areas could then be targeted for detailed geophysical or ground based geological studies. Furthermore, the comparison of results from the four DEMs highlights a range of issues with DEM collection methods and resolution. This study concludes that geomorphic indices can provide an effective method to locate evidence of recent and large faulting events. However, this type of analysis is hindered by the resolution of available digital elevation data.