Refinements to the paleoseismic chronology of the eastern Awatere Fault from trenches near Upcot Saddle, Marlborough, New Zealand

Abstract

The Awatere Fault is one of four principal strike‐slip faults within the obliquely convergent plate boundary zone in northeastern South Island, New Zealand. The eastern section of the fault last ruptured in 1848, in the Mwc. 7.5 Marlborough earthquake, when surface rupture was traced by first‐hand accounts from the South Island's east coast inland to Barefell Pass (c. 110 km). We excavated two paleoseismic trenches near Upcot Saddle on the eastern section of the fault, c. 70 km inland from the coast. Interpretation of crosscutting relationships between sediments exposed in these trenches, in association with 21 radiocarbon dates, suggest at least 5, and possibly 6, events have ruptured this part of the Awatere Fault in the last 5600 yr. These events include a young rupture (<300 cal. yr BP) that is presumed to be the surface rupture of the 1848 Marlborough earthquake. The Upcot Saddle rupture history is combined with paleoseismic data from two previous trenches, located c. 55 km to the northeast at Lake Jasper, to infer a chronology of 9–10 events between AD 1848 and 8610 cal. yr BP on the eastern section. These 9–10 events define a mean recurrence interval of 820–950 cal. yr on that fault, which is within error of the mean recurrence interval calculated using available data on late Quaternary slip rates (6 ± 2 mm/yr) and mean coseismic slip during the 1848 earthquake (5.3 ±1.6 m). The intervals between individual paleoearthquakes on the eastern section are non‐uniform, and range from 220–620 to 1540–2120 yr.