Fault slip-rates and Coulomb stress interactions in the intersection zone of the Hope, Kelly and Alpine Faults, South Island, New Zealand

Jessie L. Vermeer , Mark Quigley, Rob Langridge, Brendan Duffy, Zoe K Mildon , Manuel-lukas Diercks

The Hope Fault is a major strike-slip plate boundary fault in the Marlborough Fault Zone of New Zealand’s South Island that transfers slip between the Alpine Fault and Hikurangi subduction zone. We use lidar-based geomorphic and fault mapping, and optically stimulated luminescence (OSL; quartz) and infrared stimulated luminescence (IRSL; feldspar) dating of fault-proximal sedimentary deposits to constrain post-last glacial slip-rates for the Taramakau section of the Hope Fault and the Kelly Fault. Dextral slip-rates on the central Hope Fault (12-15 mm/yr) decrease westward from 5.6 (+2.0/-0.8) mm/yr to 1.7 (+1.0/-0.5) mm/yr. Dextral slip-rates on the Kelly Fault range from 6.2 (+7.6/-1.4) mm/yr to 1.7 (+2.1/-0.4) mm/yr. Linking, incipient subsidiary faults have minimum slip-rates of 1.3 (+0.1/-0.4) mm/yr. Proposed causes of spatial variations in slip-rates include (i) complexities in diffusive slip localization and transfer across the deformation zone, (ii) undocumented slip on faults including buried or otherwise unrecognized traces, and (iii) possible transience in slip behaviours. Paleoseismic trenching and radiocarbon (14C) ages are used to constrain the timing of most recent surface rupture on the western Hope Fault (Taramakau section) to ca. 1680 and 1840 CE, with a preferred age of ca. 1800-1840 CE. Coulomb stress modelling of scenario earthquakes on individual faults in the Alpine-Hope-Kelly Fault network is used to explore physical drivers for understanding slip-rate variations across the network. Source fault ruptures on the central Alpine Fault impart positive stress changes on the Hope-Kelly receiver faults in excess of 5-10 bars, and vice versa. Northern Alpine Fault earthquakes reduce Coulomb stresses on the Hope-Kelly receiver faults, and vice versa. Earthquake spatio-temporal clustering is an important consideration in evaluating earthquake hazards in this region.