Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

246 PATA Days 2024 Naturally-exposed post-earthquake slickenlines on the Kekerengu Fault following the 2016 earthquake We choose the Kekerengu Fault for this exercise, because we can benchmark our new (excavated) observations against previously- documentedslickenlineobservations thatwereobservedimmediately after the earthquake on natural scarp faces. By excavating below free faces that were formed during the 2016 earthquake (and later quickly eroded or degraded) we exposed new and fresh sections of the Kekerengu fault plane to compare any new slickenline data on those sections to those observed post-earthquake on the scarps. At the Glencoe Stream and Sawtooth Run sites (Fig. 1), we were able to make this comparison; however, at the Shag bend site, no post- earthquake slickenline data were documented. Many slickenlines inscribed during the 2016 earthquake were observed on natural scarps to be curved with an average length of 5-30 cm. Their shape recorded a transition from early dextral-reverse slip to pure strike- slip (Fig. 2) (Kearse et al., 2019). Offset of nearby landforms and cultural features such as fences indicated a net dextral-slip of 8-10 m and a reverse slip of 1-2 m. This corresponds to a net slip-vector pitch of <10°, yet the curved slickenlines, and many short straight ones, typically pitched more steeply than this. They transition downstream into shallow-plunging, more linear track segments that formed during the final ~90% of the slip-path. In a few cases, the downstream ends (i.e. later part of the slip) of the slickenline tracks recorded dextral-normal slip. Fig. 2: Example of curved slickenlines observed on naturally exposed free faces of the Kekerengu Fault following the 2016 Kaikōura earthquake. The fault surface is covered in a ~1 cm- thick layer of soft fault gouge. Slickenlines structures were engraved within this gouge layer. White arrows denote the local pitch angle of the slickenline track (and the motion of the near-side block relative to the far side block), and in this example show a transition in slip direction from dextral- reverse (20°) to strike-slip (0°). Fault is dipping towards the viewer at an angle of 65°. M E T H O D S To exhume the fault plane that hosted slip during the 2016 Kaikoura earthquake, we apply a modified approach based on Kearse & Kaneko (2020), which consists of 3 stages. Stage 1: using an excavator, we dig across the scarp of the Kekerengu Fault so that we can identify the location and orientation of the fault zone, in particular any splay which cuts upward all the way to the ground surface (Fig. 3). In stage two, we create a ~10-m-long, ~2 m-deep fault-parallel trench by removing material from one side of the fault. The edge of this trench is cut to within a 10-20 cm of the adjacent fault plane (Fig. 3). In stage 3, hand tools are used to carefully remove the remaining rock next to the fault plane. Often labour-intensive, this final stage was done in a piecemeal approach where small, approximately 25x25 cm windows of the fault were exposed in sequence. Slickenlines were photographed and measured on these small fault-plane