Insights into the evolution of mass flow dynamics from the seismic analysis of the 18 March 2007 Mt. Ruapehu, New Zealand lake-breakout lahar

At 23:18 UTC on 18 March 2007 Mt. Ruapehu produced the biggest lahar in New Zealand in over 100 years when a tephra dam holding crater lake water back collapsed causing 1.3x106 m3 of water to flow out and rush down the Whangaehu channel. The outburst of water transformed into a hyperconcentrated streamflow, which traveled more then 200 km from source finally flowing into the Tasman Sea on the West coast of New Zealand. Here, we describe the seismic signature of the lake-breakout lahar over the course of 83 km along the Whangaehu river system using three 3-component broadband seismometers installed <10 m from the channel at distances of 7.4, 28, and 83 km from the crater lake source. Examination of 3-component seismic amplitudes, peak spectral frequency, and directionality combined with video imagery and sediment concentration data depicts the evolution of an ever transforming lahar 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 from source and extraordinary promise for mass flow monitoring and detection systems where streamflow is already present. Ultimately, the 3-component broadband seismic data for the 18 March 2007 lahar at Mt. Ruapehu may lead to more accurate and advanced real-time waring systems for mass flows through the use of seismic frequency and directionality analysis worldwide.