Paleoseismicity of the eastern section of the Awatere fault, north-east South Island, New Zealand

Abstract

Stratigraphic and structural evidence from a newly excavated trench across the Awatere Fault, near Lake Jasper allow us to further to build upon and refine a previous paleoscismic chronology of the eastern section of the fault. The trench was excavated across a sag pond containing scarp derived colluvium, and fan-derived fluvial and lacustrine sediments including some organic-rich swamp deposits. We identify at least five ground rupturing events, four of which have occurred since 4550-5300 cal yr B.P. The youngest event is interpreted to be the 1848, ~Mw 7.5 Marlborough Earthquake. Data from our new trench constrain the timing of the penultimate event to be older than 310-510 cal yr B.P. and younger than 910-1180 cal yr B.P. At least two additional surface rupturing events took place between 700-1300 and 4550-5300 cal yr B.P. Radiocarbon dating and relationships between stratigraphic units and faults suggest that the younger of these events took place at 2000-3500 cal yr B.P. and the older at 4000-5100 cal yr B.P. Age constraints for the youngest two events overlap those in a previously published trench (Benson et al., 2001), and our new data add precision to constraints on the age of the penultimate event. Age bracketing of the third youngest surface rupture event inferred for the new trench (2000-3500 cal yr B.P.) supports data obtained in the previously published trench (Benson et al., 2001), strengthening our confidence in the timing of this third event. At least one of the two oldest events reported here may not have been recognized in the previously published work. The combined data set provides a more complete surface rupture chronology that embraces at least seven earthquakes younger than 8550-8945 cal yr B.P. This suggests a minimum mean recurrence interval of ~1390 years. Repeat intervals are not uniform, as individually they range from several hundred years to greater than 2000 years, varying by a factor of 2-3 about the mean.