Volcanoes can make waves too: Tsunamis generated by pyroclastic density currents - Natalia Lipiejko, University of Auckland

We are pleased to welcome Natalia Lipiejko, University of Auckland, who will be our speaker at this weeks event.

Title - Volcanoes can make waves too: Tsunamis generated by pyroclastic density currents

Abstract - Although volcanic eruptions account for only 5% of all the documented tsunamis, they are responsible for almost 25% of all the fatalities associated directly with volcanic eruptions. One of the reasons for such high fatalities is that by transferring energy to the waves, tsunamigenic volcanic eruptions expand the potential hazard area well beyond the eruption itself. Moreover, volcanic tsunamis are rarely included in hazard maps because the source mechanisms, and hence the properties of the resultant waves, are poorly understood.
Volcanic tsunamis can be generated by source mechanisms that include pressure waves sent through the atmosphere, an entrance of pyroclastic density currents (PDCs) into the sea, a caldera collapse, slope instabilities, volcanic earthquakes, and underwater eruptions. A combination of different volcanic tsunami source mechanisms has often been involved in generating tsunamis, with a prime example being the 2022 eruption of the Hunga Tonga-Hunga Ha’apai volcano, where a variety of different volcanic sources caused destructive tsunamis.
Determining the main source mechanism and analysing tsunamis generated by complex sources is incredibly challenging. Therefore, a deeper understanding of the individual source mechanisms is necessary to analyse more complex events.
This research focuses on tsunami generation by the entrance of PDCs into water. A fluidised granular column was released from a reservoir; the particles flowed down a porous ramp into a water-filled flume and generated waves. The entrance of the fluidised flow into the water generated a solitary-like leading wave followed by a smaller trough and trailing waves. Upon impact, the flow separated into a part advected with the wave crest and a part which turbulently mixed with water and propagated along the bottom of the flume, forming an underwater particulate gravity current.
The granular material was made up of glass beads of diameters 63-90 µm and 600-850 µm and natural volcanic material from the AD233 Taupo eruption. The tsunamigenic potential of the fluidised flows was explored by varying several parameters, including the particle size and distribution of the granular material, the mass of the material and the water depth.
The experimental data were collected using high-speed cameras and processed in MatLab using a pixel thresholding approach and Particle Image Velocimetry analysis.