LATE-HOLOCENE CO-SEISMIC DISPLACEMENTS ON THE KEKERENGU FAULT, NEW ZEALAND: WAS SLIP DURING THE 2016 MW 7.8 KAIKOURA EARTHQUAKE ANOMALOUSLY LARGE?

The Kekerengu fault is one of the fastest-slipping onshore faults of inland New Zealand and its 2016 rupture generated much of the seismic moment in the Mw=7.8 Kaikōura earthquake. We study the Kekerengu fault at the Bluff Station and Shag Bend sites, along a linear, ~12-km stretch of the fault, where the fault exhibited ~10 m and ~9 m respectively of right-lateral coseismic displacement during this earthquake. We document additional cumulative displacements recorded by fluvial terrace risers at these two sites using geomorphic field mapping and analysis of post-2016 lidar data. We use a radiocarbon age and more than 15 InfraRed-stimulated luminescence dates to constrain the ages of these offsets. By combining the ages and offsets of faulted fluvial terrace risers, together with published paleo-earthquake ages from a paleoseismic trench excavated near the Bluff Station site (Morris et al. 2022; NZ J Geol Geophys v 66, n 2), we determine the potential range of displacements for the past five earthquakes which ruptured this section of the Kekerengu fault, including the Kaikōura earthquake. We show that the past slip-per-event behavior of the Kekerengu fault is consistent with, and likely exceeds, the slip observed during the Kaikōura earthquake. The average slip-per-event prior to the 2016 event likely ranged between 12-15 m for the previous four earthquakes. This amount is similar to the large right-lateral displacements (≥ 15 m) that were observed in the 1855 surface rupture on the Wairarapa fault, which is the northern continuation of the Kekerengu fault. Such large displacements suggest a strong connection between the Kekerengu fault and the Hikurangi megathrust, as has been suggested for the Kaikōura earthquake and the 1855 Wairarapa earthquake. These results have key implications for earthquake hazard analyses in New Zealand, and more generally for seismic hazard assessment in other similar oblique-subduction tectonic settings around the world, where information about the behavior of upper-plate faults may provide insight into the past behavior of the subduction megathrust itself.