Browsing by Author "Fitzsimons, SJ"

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    Identifying a reliable target fraction for radiocarbon dating sedimentary records from lakes
    (Elsevier, 2013-06-01) Howarth, JD; Fitzsimons, SJ; Jacobsen, GE; Vandergoes, MJ; Norris, RJ
    Lake basins that experience rapid rates of deposition act as high-resolution environmental archives because they produce sedimentary records that have centennial or even decadal resolution. However, identifying target fractions for radiocarbon dating of lake sediments remains problematic because reworked organic material from fluvial catchments can produce anomalously old radiocarbon ages. This study determines the extent to which reworked material from catchment soils impacts radiocarbon dates on pollen and other organic concentrates by comparing radiocarbon dates produced by these techniques against a chronostratigraphic marker in cores from Lake Mapourika, New Zealand. Pollen preferentially preserved and reworked from catchment soils was identified using soil palynology. A technique was then developed to remove reworked pollen types from pollen concentrates extracted from lake sediment. Identification and removal of reworked pollen from pollen concentrates produced ages that were consistently closer to the age of the chronostratigraphic horizon than other organic concentrates. However, these dates were still between 736 and 366 calendar years older than expected. The only organic fractions that reliably reproduced the age of the chronostratigraphic horizon were terrestrial leaf macrofossils, although terrestrial leaf macrofossils isolated from megaturbidite deposits, which are formed by high-energy depositional events, also provided anomalously old ages. The results indicate that leaf material extracted from hemipelagite, which accumulates gradually, is likely to be the only organic fraction to produce reliable chronology in lakes where a component of sedimentation is driven by the fluvial system. The results also demonstrate the importance of conducting a detailed investigation of physical sedimentology before selecting material for radiocarbon dating lake sediments. Crown Copyright © 2013, Elsevier Ltd.
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    Lacustrine paleoseismometers reveal spatial and temporal patterns of rupture during the last ten large earthquakes on the Alpine Fault, New Zealand
    (American Geophysical Union, 2014-12-19) Howarth, JD; Fitzsimons, SJ; Langridge, R; Clark, K; Cochran, UA; Norris, RJ; Jacobsen, GE
    The rarity of long, well-dated paleoseismic records from sites along plate boundary transform faults is a major constraint on the development and evaluation of conceptual models of fault rupture behaviour. This is the case for the 800 km long, high slip rate (27±5 mm yr-1), dextral strike-slip Alpine Fault at the boundary between the Pacific and Australian plates in southern New Zealand. We use lacustrine paleoseismology to evaluate the hypothesis that the Alpine Fault exhibits self-similar behaviour, that is, the fault always produces earthquakes at or near a maximum magnitude of Mw8. The hypothesis is tested using reconstructions of high intensity shaking from five lakes situated along 240 km of the Alpine Fault's Central section. Sedimentological investigation of lake cores shows that high intensity shaking events are recorded in the lake sediments as turbidites formed by subaqueous slumping. These turbidites are overlain by terrigenous sediment from co- and post-seismic landsliding on hillslopes in the lakes' mountainous catchments. Chronologies derived from Bayesian modelling of AMS 14C dates on terrestrial leaf macrofossils precisely constrain the timing of earthquakes at the lake sites, facilitating along-strike correlation. Shaking events correlate between the sites and with known ruptures of the Alpine Fault, confirming the seismic origin of the deposits and allowing thresholds of shaking intensity for deposit formation to be determined using isoseismal modelling. Modelled shaking intensities for the last two Alpine Fault earthquakes show that subaqueous slumping occurs when shaking intensities exceed Modified Mercalli scale (MM) VI-VII, and that increased fluvial sediment fluxes from earthquake-induced landslides occur when shaking intensities exceed MM IX. The record of synchronous MM IX shaking at the lake sites provides first order constraint on the rupture length of the last ten earthquakes on the central Alpine Fault. Rupture scenarios for these earthquakes are augmented by correlating event timing with long earthquake records from the South Westland section and geomorphic reconstructions of the slip distribution for the most recent ruptures to explore the best-fit model of fault behaviour and to test the hypothesis that the Alpine Fault always ruptures in great (Mw8) earthquakes.
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    Lake sediments record high intensity shaking that provides insight into the location and rupture length of large earthquakes on the Alpine Fault, New Zealand
    (Elsevier, 2014-10-01) Howarth, JD; Fitzsimons, SJ; Norris, RJ; Jacobsen, GE
    Understanding the seismic hazard posed by large earthquakes requires paleoseismic investigation because most faults have not ruptured repeatedly during the period of historic records. However, determining the location and length of fault ruptures using paleoseismic data remains challenging. Our study demonstrates that lake sediments record the high intensity shaking that occurs proximal to fault rupture, allowing the location and length of ruptures to be reconstructed. In two lakes adjacent to the Alpine Fault, New Zealand, seismic shaking is recorded as subaqueous mass-wasting derived turbidites formed by coseismic subaqueous slope failures, which are overlain by sets of hyperpycnites representing elevated fluvial sediment fluxes from earthquake-induced landslides. Precise radiocarbon age models show that shaking events are synchronous between the two lake sites and correlate with the timing of known Alpine Fault earthquakes. Modelled shaking intensities for the last two Alpine Fault earthquakes show that subaqueous mass-wasting occurs when shaking intensities exceed Modified Mercalli scale (MM) VI–VII, and that fluvial sediment fluxes from earthquake-induced landslides occur when shaking intensities exceed MM IX. The data demonstrate that lake records distinguish between strong (MM VI) and violent (MM IX) shaking at a lake site. The ability to map the spatial extent of MM IX shaking provides new insights into the timing and extent of rupture for the last five earthquakes on the Alpine Fault. The study demonstrates that lake deposits constrain the spatial extent of rupture during large earthquakes and may yield long records of the spatial and temporal patterns of fault rupture. © 2014, Elsevier B.V.