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

76 PATA Days 2024 The fast slip rate (~30 mm/yr), regular recurrence interval (249±58 years 95% confidence interval; coefficient of variation of 0.22), and time of last earthquake (1717 AD) add up to a staggering 75% (29-99% CI) probability of Alpine Fault surface rupture in the next 50 years, making this one of the world's most anticipated earthquakes and a discrete national-level hazard (Wells et al., 1999; Barth et al., 2014; Howarth et al., 2021). Related to this, considerable science and policy interest exists in anticipating the behavior and effects of the next Alpine Fault earthquake including magnitude, shaking intensity, nucleation location, surface rupture extent, surface slip distribution, and landscape effects like landsliding and aggradation (e.g. Robinson & Davies, 2013; Orchiston et al., 2016; Howarth et al., 2021). The more we can uncover about the properties of the last several paleoearthquakes, such as constraining surface slip distributions and paleoearthquake nucleation locations, the greater our potential to anticipate realistic earthquake scenarios far the Alpine Fault's 21st century earthquake(s). Rapid fault slip during earthquakes causes the two bodies of rock on either side of a natural (i.e. imperfectly planar) fault plane to abrade against each other, forming scratched striations known as slickenlines. Field observations of curved slickenlines on exposed fault free faces of the Kekerengu Fault generated during the 2016 Kaikoura Earthquake in New Zealand led to a model supported theoretical framework that curvilinear slip paths are expected during fault rupture in the shallow surface and that the sense of curvature (convex-up or convex- down far a strike-slip fault) can be used to record the direction of earthquake propagation (Kearse et al., 2019). Kearse & Kaneko (2020) expanded this framework to include expected slip path curvatures relative to a hypocenter far a range of fault kinematics (reverse, normal, sinistral, dextral), and supported this with eight historical surface- rupturing earthquakes far which curved slip paths, hypocenter locations, and focal mechanisms were known. This seemingly robust framework has led to a demonstration of methodology that curved slickenlines can be exhumed from beneath fault free surfaces and observed in fault trenches (see Kearse et al. abstract in these proceedings), thus presenting the opportunity to enhance paleoseismic records with information about prehistoric rupture directions. In this study we visited three remote sites (south to north: Hokuri Creek, Martyr River, Chasm Creek) spanning the central-southern Alpine Fault earthquake gate to (1) develop best practice methods to exhume and document slickenlines produced by prehistoric surface rupture, and (2) contribute paleoearthquake directivity constraints to the Alpine Fault paleoearthquake record. To our knowledge, our study is the first to deliberately document curved slickenlines on a major plate boundary structure lacking historie surface rupture and the first to document curved slickenlines from multiple earthquakes on the same fault plane. M E T H O D S All three sites are remote and required hand tools to excavate (shovel, pickaxe, trowel, knife, etc.). At each site we first documented ali naturally exposed fault planes and slickenlines with structural measurements, length subvertical HC#l we exhumed four unique curved slickenlines with 20º to 28º of curvature and ~20 cm length; these were ali convex- up indicating rupture from the southwest. The subvertical HC#2 plane proved more fruitful with a total of 17 curved slickenline tracks observed between two outcrops. The observed sense of convexity and slickenline formation mechanisms were spatially separated between the two outcrops such that crosscutting relationships between convex- up and convex-down tracks could not be determined. At HC#2A, a 13 m-long exposure recorded only convex-down curved slickenlines (rupture from the southwest) as wallrock-on gouge grooves to 3.5 m-length and finer gouge-hosted striations to 20 cm-length; curvature ranged from 4º to 12º (Fig. 2). At HC#2B, a 10 m-long exposure recorded only convex-up sense of curved slickenlines (rupture from the northeast) as gouge-hosted striations with 15º to 42º of curvature and lengths to 20 cm. In summary, at Hokuri Creek we observed both convex-up and convex-down curved slickenlines including on the same fault plane; curved slickenline population senses were spatially separate, suggesting that slight undulations in the fault plane surface may preferentially preserve patches of slickenlines from previous ruptures (i.e. in abrasion shadows). This highlights caution in assuming that any observed slickenlines relate to the most recent earthquake rupture without additional constraints. At Martyr River a single dextral-reverse principal slip surface is visible at river level juxtaposing indurated gravels and quartzofeldspathic fault core materials against overlying chloritic protocataclasite (e.g. Barth et al., 2013). Following this along strike and up slope we observe parallel partitioning of slip between a reverse fault plane and strike-slip fault plane that merge at the base of a prominent outcrop. Here we observed curved