Detection of large,Holocene earthquakes using diatom analysis of coastal sedimentary sequences,Wellington, New Zealand |
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| Institution: | 1. Geology Department, University of Otago, Dunedin, New Zealand;2. Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia;1. Geomarine Research, 19 Debron Ave, Remuera, Auckland, New Zealand;2. Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand;3. Department of Education, Shimane University, Matsue, Japan |
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| Abstract: | Paleoenvironmental reconstructions of three coastal waterbodies in Wellington, New Zealand, reveal that sites were isolated from the sea within the last 7500 years through coseismic uplift and barrier growth. Evidence for coseismic uplift consists of distinct transitions in diatom assemblages representing large changes in relative sea-level or water-table level, commonly in association with sedimentological evidence for catchment disturbance or marine influx. Transitions are abrupt, laterally extensive and synchronous within each waterbody. Amount of change across transition horizons is assessed using quantitative estimates of paleosalinity and waterbody type as proxies for relative sea-level change. Seven transitions involve large paleoenvironmental changes and provide evidence for earthquakes occurring at approximately 5100, 3200 (recorded at two sites), 2300 (recorded at two sites), 1000 cal years BP and 1855 AD. Five other transitions involve smaller paleoenvironmental changes and are considered to be consistent with effects of earthquakes but do not provide independent evidence for earthquake occurrence. These smaller transitions occur at approximately 6800, 3600, 2200, 1000 (coincident with a large transition) and 500 cal years BP. The data refine ages and provide information about the extent and effects of past large earthquakes in the region. These are the first paleoecologically derived earthquake signatures for Wellington and they contribute to the sparse collection worldwide of off-fault sedimentary earthquake records for predominantly strike-slip faults. |
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