Analysis of phreatic events at Ruapehu volcano, New Zealand, using a new SOM approach

Abstract

We apply a Self-Organising Maps (SOM) method to assess the low level seismic activity prior to small scale phreatic ‘steam and gas’ events at Ruapehu volcano New Zealand. Two events were examined: a phreatic event on 4 October 2006 which displaced the crater lake producing a 4 m high wave on the lake edge, and the more energetic 25 September 2007 phreatic eruption.

The SOM analysis provides a classification of seismic tremor patterns that clusters into three characteristic types, that we can label by colours. The pattern for both eruptions is consistent with a pre-eruption pattern including enhanced spectral energy in the range 4 to 6 Hz – labelled ‘green tremor’ - giving way to spectra having broader spectra 2 to 6 Hz so called ‘red tremor’ just prior to the eruption, and a post eruption pattern including spectral peaks at generally lower frequencies 2 to 4 Hz - so called ‘blue tremor’.

The results may be explained by a pattern of slow pressurization of sealed volcanic system - ‘green’ - followed by failure of the seal - ‘red’ - and subsequent de-pressurisation - ‘blue’ – for the two events studied.

Technical Abstract

We apply Self-Organising Maps (SOM) to assess the low level seismic activity prior to small scale phreatic events at Ruapehu volcano New Zealand.

The SOM approach allows an automatic pattern recognition, virtually independent from a priori knowledge. Volcanic tremor spectra are randomly presented to the network in a competitive iterative training process, followed by a hierarchical clusterization of the SOM nodes. Spectra are then projected, ordered by time, to clusters on the map.  A coherent time evolution of the data through the clusters can highlight the existence of different regimes and the transitions between them.

Two Ruapehu events were examined: a phreatic event on 4 October 2006 which displaced the crater lake producing a 4 m high wave on the lake edge, and the more energetic 25 September 2007 phreatic eruption. The SOM analysis provides a classification of tremor spectral patterns that clusters into three regimes that we label by colours.

The pattern for both eruptions is consistent with a pre-eruption spectral pattern including enhanced spectral energy in the range of 4 to 6 Hz - labelled „green tremor‟. This gives way to spectra having broader energy between 2 to 6 Hz, so called „red tremor‟ just prior to the eruption.  The post eruption pattern including spectral peaks at generally lower frequencies 2 to 4 Hz - so called „blue tremor‟.

Clusterization into only three groups yields highly non- unique solutions which cannot explain the variety of processes operating at Ruapehu over long time periods. Regardless, the approach highlights noteworthy similarities that may be explained by a pattern of slow pressurization under a hydrothermal or magmatic seal – “green” - followed by seal failure - “red” - and subsequent de-pressurisation – “blue” - for the two events studied.  Although the application shown here is limited, we think it demonstrates the power of this classification approach.

Reference for full report:

Carnel, R; Jolly, AD; Barbui, L 2013 - Analysis of phreatic events at Ruapehu volcano, New Zealand using a new SOM approach, Journal of volcanology and geothermal research, 254: 69-79; doi: 10.1016/j.jvolgeores.2012.12.026