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

slickenlines on a freshly exposed 12 m-wide by 4 m-high outcrop of the subvertical strike-slip fault plane cutting gravel till and facing northwest (Fig. 3). The fault plane extends to the surface suggesting rupture in the most recent event (1717 AD). We observed a dominance of slickenlines (6) with convex- up curvature (rupture from the southwest) primarily as gouge-hosted striations on the principal slip surface. The convex-down curved slickenline tracks (3) observed are gouge-hosted features separated by 10 cm from the principal slip surface. We interpret that the convex-up slickenlines on the main fault surface likely record the most recent rupture and the convex-down slickenline tracks were survivors from a previous rupture. lf the earthquake gate occurs at the major change in kinematics at the Martyr River, then our results indicate the gate allows multi-section rupture from both the southwest and northeast (i.e. the gate swings both ways). Since the last three paleoearthquakes are known to bemulti-section ruptures (Howarth et al., 2021), if we attribute the observed slickenlines to be no older, then multi-section ruptures could nucleate both northeast and southwest of the earthquake gate. Based on our observations we tentatively suggest that the multi-section (~Mw 8) 1717 AD rupture of the Alpine Fault initiated from a location southwest of the earthquake gate (i.e. south of Martyr River). At our most northerly site of Chasm Creek, we documented a new rare bedrock-on-bedrock exposure of the Alpine Fault principal slip zone. The nature of the exposure limited the area of the principal slip surface we were able to uncover to ~200 cm 2 . Faint cm-length gouge-hosted slickenlines with differing rakes were observed but with no obvious curvature at this scale. Fig. 2: (A) Portion of the HC#2A principal slip surface at Hokuri Creek site with convex-down curved slickenlines with interpreted inset. (B) Example of fine-scale gouge-hosted convex-down slickenlines.