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First application of 210Pb geo-chronology to paleo-seismology on the colluvial edge of the Heliki fault scarp

In its first application to palaeoseismic studies, we show that that 210Pb dating, supported by 137Cs dating, is potentially useful for estimating the ages of very young (less than 150 years) colluvial deposits associated with recent earthquake activity. Colluvial deposits exposed in two fault trench sections were sampled for laboratory assay and analysed to confirm interpretations that they post-dated the 1861 surface rupture. Only one of the two sites, the Eastern Eliki Fault Trench 1A, was found to provide an appropriate sediment sequence for dating via 210Pb and 137Cs. Here, a soil horizon was found to have been buried by alluvial sedimentation in the late AD 1800s, consistent with reactivation during the 1861 earthquake.

The precision of the dates obtained was found to depend largely on the date of occurrence of the earthquake event, and on the local sedimentary setting. The half-life of 210Pb (22.3 years) usually limits its dating range to 150 years (ca. 6-7 half lives), hence it is only useful in dating events which have occurred since ca. 1850. Extrapolation of the chronology to examine earlier events is not possible due to potential variations in sediment accumulation rate over time. Since errors on 210Pb dates generally increase as the excess 210Pb activity decays to low values (i.e. in older sediments), dating precision is likely to be better for more recent earthquake events, particularly those occurring after 1954 where the 210Pb ages can be corroborated by 137Cs dates.

Where possible, 210Pb and 137Cs dating should be used in combination to confirm dates, and fully assess variations in sediment input and erosional processes. The results suggest that this novel geo-chronological technique can be applied more widely to date colluvial deposits typically encountered in fault-trench studies.

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