Comment on nhess-2022-31

Abstract

Monitoring for lahars on volcanoes can be challenging due to the ever-changing landscape which can drastically transform the properties and dynamics of the flow. These changes to the flows require the need for detection strategies and risk assessment that are tailored not only between different volcanoes, but at different distances along flow paths as well. Being able to understand how a flow event may transform in time and space along the channel is of utmost importance for hazard management. While visual observations and simple measuring devices in the past have shown how lahars transform along the flow path, these same features for the most part have not been described using seismological methods. On 18 March 2007 Mt. Ruapehu produced the biggest lahar in New Zealand in over 100 years. At 23:18 UTC the tephra dam holding the Crater Lake water back collapsed causing 1.3 x 106 m3 of water to flow out and rush down the Whangaehu channel. We describe here the seismic signature of a lake-breakout lahar over the course of 85 km along the Whangaehu river system using three 3-component broadband seismometers installed < 10 m from the channel at 7.4, 28, and 83 km from the crater lake source. Examination of 3-componennt seismic amplitudes, peak frequency content, and directionality combined with video imagery and sediment concentration data were used. The seismic data shows the evolution of the lahar as it transformed from a highly turbulent out-burst flood (high peak frequency throughout), to a fully bulked up multi-phase hyperconcentrated flow (varying frequency patterns depending on the lahar phase) to a slurry flow (bedload dominant). Estimated directionality ratios show the elongation of the lahar with distance down channel, where each recording station shows a similar pattern, but for differing lengths of time. Furthermore, using directionality ratios shows extraordinary promise for lahar monitoring and detection systems where streamflow is present in the channel.