Bouma Virtual Sampler Conference
20-22 April 2022 Day 1 - Flow Processes
Meg Baker | Passive seismic sensors record the longest runout sediment flows ever measured on Earth

Keywords: turbidity currents, seismic signals, direct monitoring Turbidity currents form some of the largest sediment accumulations on Earth, carry globally significant volumes of organic carbon, and can damage critical seafloor infrastructure. These fast and destructive events are notoriously challenging to measure in action, as they often damage any instruments anchored within the flow, making it difficult to record the frequency and flow behaviour of turbidity currents. Here we show for the first time that turbidity currents emit distinctive seismic signals that can be remotely sensed by instruments located outside the flow path, and thus out of harm’s way. Passive Ocean Bottom Seismograph (OBS) sensors, located on terraces of the Congo Canyon, offshore West Africa, recorded thirteen turbidity currents over an 8-month period. The occurrence and timing of these turbidity currents was confirmed by nearby moorings with acoustic Doppler current profilers. Results show that turbidity currents travelling over ~1.5 m/s produce a seismic signal concentrated below 10 Hz with a sudden onset and more gentle decay. Comparison of the seismic signals with information on flow velocities from the acoustic Doppler current profilers demonstrates that the seismic signal is generated at the flow front. This suggests that the flow front may contain a powerful and dense near-bed layer compared to the rest of the flow. The Ocean Bottom Seismographs measured two exceptionally powerful turbidity currents in January and March 2020. These flows broke seabed telecommunication cables and travelled more than 1,100 km offshore with transit speeds of up to 8 m/s, making these the longest runout sediment flows ever measured in action on Earth. The seismic signals show that these huge turbidity currents comprise a series of distinct pulses, and provides new insights into the internal structure and behaviour of large turbidity current events.

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