Abstract:
Ground penetrating radar profiles were acquired in the Rangitaiki plains, which is a rapidly resurfacing environment that is dominated by unconsolidated sediments. The 200MHz antenna was primarily used to image the subsurface to depths of 5 m with an aim of resolving the geometry and paleoseismic history of faults at selected areas. Sites were selected based on the height of the scarp ; sites with poor scarp visibility were identified from LiDAR DEM studies. Four sites were chosen across the Matata Fault Zone, Awaiti Fault Zone and the Edgecumbe Fault Zone. Profiles were converted to depth and migrated to enhance data; which showed discontinuous zones of horizons inferred to be fault zones. One strand of the Matata Fault Zone was selected for a paleoseismic trench which reached depths of 5 m. Analysis of trenching results show variable displacement rates during the late Holocene, including one rupture that produced a vertical offset of >1m, consistent with an earthquake of M> 6.5. The results from the trenching analysis have contributed to a greater understanding of the Matata Fault Zone, and indeed, of the whole Whakatane Graben. Ground penetrating radar data was acquired from within the trench and was successfully used to image 4 m below its base despite the effect of the water table. This application of GPR extended the paleoseismic history by one rupture event. Direct correlation between GPR profiles and stratigraphy, which was identified from augering, vibracoring and trenching at profile sites, was successful in identifying geometries of stratigraphy, although differences with the dielectric constant mean that correlation of the stratigraphy was not exact.