Tag Archives: Turbidity current

Assessing submarine great-earthquake statistics fails

Geologists who study turbidites assume that the distinctive graded beds from which they are constructed and a range of other textures represent flows of slurry down unstable steep slopes when submarine sediment deposits are displaced. Such turbidity currents were famously recorded by the severing of 12 transatlantic telecommunication cables off Newfoundland in 1929. This happened soon after an earthquake triggered 100 km hr-1 flows down the continental slope, which swept some 600 km eastwards.

Load structures on turbidite sandstones, Crook...

Typical structures in Upper Carboniferous turbidites near Bude, Cornwall, UK (credit: Flickr, Earthwatcher)

Sea beds at destructive margins provide the right conditions for repeated turbidity currents and it is reasonable to suppose that patterns should emerge from the resulting turbidite beds that in some way record the seismic history of the area. British and Indonesian geoscientists set out to test that hypothesis at the now infamous plate margin off Sumatra that hosted the great Acheh Earthquake and tsunamis of 26 December 2004 to kill 250 thousand people around the rim of the Indian Ocean (Sumner, E.J. et al. 2013. Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatra margin? Geology, v. 41, p.763-766).

Animation of 2004 Indonesia tsunami

Animation of Indonesian tsunami of 26 December 2004 (credit: Wikipedia)

Cores through turbidite sequences along a 500 km stretch of the margin formed the basis for this important attempt to test the possibility of recording long-term seismic statistics. To avoid false signals from turbidity currents stirred up by storms, floods and slope failure from rapid sediment build-up 17 sites were cored in deep water away from major terrestrial sediment supplies, which only flows triggered by major earthquakes would be likely to reach. To calibrate core depth to time involved a variety of radiometric  and stratigraphic methods

Disappointingly, few of the sites on the submarine slopes recorded turbidites that match events during the 150-year period of seismic records in the area, none being correlatable with the 2004 and 2005 great earthquakes. Indeed very little correlation of distinctive textures from site to site emerged from the study. Some sites on slopes revealed no turbidites at all from the last 150 years, whereas turbidites in others that could be accurately dated occurred when there were no large earthquakes. Only cores from the deep submarine trench consistently preserved near-surface turbidites that might record the 2004 and 2005 great earthquakes.

These are surprising as well as depressing results, but perhaps further coring will discover what kind of bathymetric features might yield useful and consistent seismic records from sediments.

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