A multi-event paleoseismic record from the northern Alpine Fault at Marble Hill, Aotearoa New Zealand

Abstract

Multi-event paleoseismic records are essential for understanding fault behaviour, characterising seismic hazard, and testing earthquake simulator models. The reverse dextral Alpine Fault is the main Australian-Pacific plate boundary structure through the central and southern South Island of Aotearoa New Zealand. While the paleoseismic record of the southern and central sections is relatively well-understood, in contrast, the northern section of the Alpine Fault (AF-NS), adjacent to the distributed Marlborough Fault System (MFS) remains understudied. We present new on-fault paleoseismic data from Marble Hill, a well-known offset alluvial terrace site next to the Maruia River, including results from three trenches and four pits. Dating of deposits comes from radiocarbon and infra-red stimulated luminescence (IRSL) techniques. IRSL dates indicate that the degradational terrace sequence at Marble Hill spans the last ∼2000 years. Trenches reveal evidence for up to four recent paleoearthquake ruptures with timings of ∼1595–1834 (MRE, most recent event), 952–1406 (E2), 706–933 (E3), and 246–736 CE (E4). Preservation of this record has been possible due to avulsion with ensuing incision and degradation of the Maruia River toward its current north-westerly course. A local AF-NS recurrence interval of 413 ± 103 yr is derived from the three closed paleoearthquake intervals between the MRE and E4. The three youngest events are associated with ∼1.3 m (MRE), and 10.7 ± 1.3 m dextral displacement split between E2 and E3. This is the first multi-event timing and slip record from the AF-NS and our results highlight the inadequacies of earlier work that targeted only the MRE. A longer recurrence interval (than for the central and southern sections) and variable displacement per event at the Marble Hill site are indicative of its location along the Alpine Fault, implying a more complex rupture record along the AF-NS adjacent to the MFS and transitioning toward the Wairau Fault.