A 6000-year record of surface-rupturing paleoearthquakes on the Wairau Fault, New Zealand

Abstract

ABSTRACTThe Wairau Fault is part of the Marlborough Fault System (MFS) in the northern South Island of New Zealand and is capable of generating future large-magnitude earthquakes. We use fault trenching and geomorphic mapping together with radiocarbon dating to constrain the timing, single-event displacement (SED), rupture length and magnitude of surface-rupturing earthquakes since 6000 yr BP. Data from two sites (Rainbow Station and Spring Creek) have been combined with published information to determine the timing of five large magnitude (e.g. MW ≥ 7) paleoearthquakes at: 1750–970 (event 1), 2110–1930 (event 2), 3200–2700 (event 3), 4600–3950 (event 4) and 5600–4900 (event 5) yr BP. These earthquakes have a mean recurrence interval and SED of c. 1000 yr and 5.8 ± 2 m (1σ), respectively. The average recurrence interval for events 1–5 is less than the c. 2200 yr estimated for the long-term (c. 18 kyr) offshore record and suggests that onshore parts of the fault could have experienced a period of increased earthquake frequency since 5600 yr BP. These five surface-rupturing earthquakes had rupture lengths of ≥ 6 to 140 km and moment magnitudes of at least MW 7–7.7. Co-rupture of the Wairau and Alpine faults during great earthquakes occurs rarely (if at all) on millennial or longer timescales.