Coffey, G.L.; Litchfield, N.J. 2023 Ground truthing and preliminary slip rates and recurrence intervals for four newly identified faults in the Wairarapa region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2023/39. 37 p.; doi: 10.21420/VYTA-3574
Abstract
Four previously unrecognised active faults have recently been identified near towns in the Wairarapa region (Litchfield et al. 2022). These are: the Ruamahanga Fault, north of Masterton, the Carters Line and Papawai faults, southeast of Carterton, and the Woodside Fault, westof Greytown. Little was known about these faults and their recurrence interval class was assigned from comparison of their geomorphic expression in Light Detection and Ranging (LiDAR) data to other better understood faults across the valley. This report summarises a study that builds upon this initial work by ground truthing active fault mapping and providing slip rate and recurrence interval data on these newly identified faults. Ground-Penetrating Radar (GPR) data was collected across all four faults to explore whether any evidence of faulting could be observed in the upper few metres of the subsurface and validate the mapping of these features as active faults, rather than other geomorphic features, such as terrace risers. Evidence of disrupted stratigraphy was identified in profiles crossing the Ruamahanga, Papawai and Woodside faults, while data quality was insufficient to interpret stratigraphy across the Carters Line Fault. The disrupted stratigraphy identified across the Ruamahanga, Papawai and Woodside faults is indicative of past offset and increases our confidence that these features are active faults as originally mapped. However, we could not definitively confirm them as active faults from GPR data alone. The first preliminary estimates of slip rate across each of the four newly identified faults are presented in this report and were developed using topographic profiling and terrace mapping. Vertical slip rates on these faults range from 0.1 to 0.3 mm/yr. Horizontal slip rates were calculated for the Papawai and Woodside faults, and these range from 0.1 to 0.7 mm/yr. Vertical and horizontal slip rates were used to place bounds on the net slip rate for each fault and these ranged from >0.1 to 1.1 mm/yr. Preliminary recurrence intervals were calculated using net slip rates and fault length-displacement scaling laws and these were compared with recurrence interval class estimates from Litchfield et al. (2022). Preliminary recurrence intervals are <4600 years for the Ruamahanga Fault, <16,000 years for the Carters Line Fault and 2200–2900 years for the Papawai Fault. The recurrence intervals calculated for these faults are consistent with the Recurrence Interval Class (RI Class) II assigned to each of them by Litchfield et al. (2022). Recurrence interval constraints for the Woodside Fault were also developed in this report, but these constraints underestimate the true recurrence interval for the Woodside Fault. Instead, we recommend that the RI Class of the Woodside Fault is changed from RI Class III (Litchfield et al. 2022) to RI Class I to reflect that the Woodside Fault likely ruptures at the same time as the Wairarapa Fault (RI Class I). We also recommend that further field-based studies of these faults are undertaken to further increase our confidence that they are active faults and improve estimates of slip rates and recurrence intervals. Further field study may include the excavation of paleoseismic trenches across selected faults to identify on-fault evidence of faulting and/or develop paleoearthquake records that can be used to better constrain recurrence interval. A prospective paleoseismic trenching site on the Woodside Fault was identified during this work that has the potential to provide earthquake timing and recurrence interval information on this fault. Additionally, pits could be excavated into offset surfaces to collect samples for dating, which will improve slip rate constraints.
