LONG-TERM SLIP RATE OF THE KEKERENGU FAULT AND POSSIBLE RANGE OF PRE-2016 KAIKOURA EARTHQUAKE DISPLACEMENTS AT THE BLUFF STATION SITE, NEW ZEALAND

The largest displacements observed during the M_w 7.8 2016 Kaikōura earthquake were 8 to 12 m along the Kekerengu fault, rupture of which generated most of the seismic moment during that earthquake. This fault is probably one of the fastest-slipping faults in New Zealand. We study the Bluff Station site, located in the northwestern part of the Kekerengu fault, where offsets of the Kaikōura earthquake produced ~10 m of right-lateral slip. At this site, we used geomorphic field mapping and analysis of digital post-2016 lidar data to observe fluvial terrace risers offset by ~625 m, ~230 m, 29 m and 20 m. Dating of several fluvial terraces with the pIR-IRSL technique helps us constrain the ages of these displacements. The minimum age of the ~625 m terrace riser offset is constrained by the ~23 ka abandonment age of the younger terrace, resulting in a maximum long-term slip-rate at the Bluff Station site of ~27 mm/yr. The maximum age of the offset is partially constrained by the presence of ~25 ky Kawakawa tephra in loess overlying the older terrace. We also explore smaller offsets in pre-2016 Kekerengu fault ruptures to determine whether earlier displacements were similar to those in 2016. Combining our preliminary results with published Kekerengu fault paleo-earthquake ages allows us to constrain the possible range of displacements that might have occurred during the past several pre-2016 events. The available age data suggest that the 19 m of pre-2016 displacement likely occurred during three earthquakes, but potentially could have occurred during as few as two or as many as four events. These preliminary results lead us to speculate that displacements in pre-2016 Kekerengu fault ruptures may have been significantly smaller than during the Kaikōura earthquake, with an average pre-2016 slip per event of ~5-6 m. This reinforces the singular nature of slip during the 2016 Kaikōura earthquake on the Kekerengu fault. Covid-related travel restrictions have thus far prevented us from completing our field work at this site, but our planned sampling and dating of several additional terraces will provide shorter-term incremental slip-rate values, tighter constraints on the longer-term incremental slip rate of the fault, and will resolve the ambiguity on the most recent three to four pre-2016 displacements along the Kekerengu fault.