Internal tsunami waves transport sediment released by underwater landslides

Noel Brizuela,Anatoliy Filonov,Matthew H Alford

doi:10.1038/s41598-019-47080-0

Abstract

Accelerated by gravity, submarine landslides transfer energy to the marine environment, most notably leading to catastrophic tsunamis. While tsunamis are thought to use less than 15% of the total energy released by landslides, little is known about subsurface processes comprising the rest of their energy budgets. Here, we analyze the first set of observations depicting a lake’s interior response to underwater landslides and find that sediment transport is modulated by baroclinic waves that propagate along vertical gradients in temperature and sediment concentration. When traveling along a shallow thermocline, these waves can reach past topographic features that bound turbidity currents and thus expand the influence area of underwater landslides. With order of magnitude calculations, we estimate that observed thermocline internal waves received roughly 0.7% of available landslide energy and infer their contribution to homogenize the lake’s thermodynamical properties by means of turbulent mixing. Lastly, we show that landslides in our data set modified the lake’s intrinsic dynamical modes and thus had a permanent impact on its circulation. This suggests that measurements of subsurface wave propagation are sufficient to diagnose bathymetric transformations. Our experiment constitutes the first direct observation of both internal tsunami waves and turbidity current reflection. Moreover, it demonstrates that background density stratification has a significant effect on the transport of sediment after submarine landslides and provides a valuable reference for numerical models that simulate submarine mass failures.

Highlights

Tsunamis are reported at an average rate of one per year when surface waves generated by impulsive geophysical events flood coastal regions[1]
Until now, such observations of internal tsunami waves (IGWs generated by impulsive geophysical events) are lacking, as tsunamigenic processes are largely unpredictable and can wreck oceanographic instruments
Rapid spikes in bottom pressure are evidence of turbidity currents and underwater landslides, as large concentrations of suspended sediment exert an additional load www.nature.com/scientificreports on our sensor when they transit through P1

Summary

Introduction

Tsunamis are reported at an average rate of one per year when surface waves generated by impulsive geophysical events flood coastal regions[1]. IGWs are often measured with moored thermistors, and appear in time series as periodic temperature fluctuations whose magnitude is used to infer vertical displacements in the water column. Until now, such observations of internal tsunami waves (IGWs generated by impulsive geophysical events) are lacking, as tsunamigenic processes are largely unpredictable and can wreck oceanographic instruments. Linear (gray) and nonlinear (black) estimates of energy stored in thermocline oscillations ((C) see Methods) show that internal motions following the landslide of August 18 were many times more energetic than the lake’s wind-driven circulation. We analyze the resulting data and focus on the generation of IGWs by underwater landslides, the periodic behavior of a subsequent turbidity current, and the long-term dynamical effects of basin transformations

Methods

Results

Discussion

Conclusion