Seismic signals of snow‐slurry lahars in motion: 25 September 2007, Mt Ruapehu, New Zealand

Abstract

Detection of ground shaking forms the basis of many lahar‐warning systems. Seismic records of two lahar types at Ruapehu, New Zealand, in 2007 are used to examine their nature and internal dynamics. Upstream detection of a flow depends upon flow type and coupling with the ground. 3‐D characteristics of seismic signals can be used to distinguish the dominant rheology and gross physical composition. Water‐rich hyperconcentrated flows are turbulent; common inter‐particle and particle‐substrate collisions engender higher energy in cross‐channel vibrations relative to channel‐parallel. Plug‐like snow‐slurry lahars show greater energy in channel‐parallel signals, due to lateral deposition insulating channel margins, and low turbulence. Direct comparison of flow size must account for flow rheology; a water‐rich lahar will generate signals of greater amplitude than a similar‐sized snow‐slurry flow.