Investigating Temporal Variations in Seismic Anisotropy Associated with Volcanic Eruptions at Mt Ruapehu, New Zealand
Abstract
We examine past changes in anisotropy at Mount Ruapehu volcano using two methods that reduce the uncertainty of spatial variation masquerading as temporal changes. Mt. Ruapehu is an active andesitic stratovolcano in the centre of the North Island, New Zealand, and last produced a major magmatic eruption in 1995/1996 and minor phreatomagmatic eruptions in 2006 and 2007. Previous studies have reported that seismic anisotropy in the vicinity of Mt. Ruapehu, determined via shear wave splitting (SWS) analysis, changes throughout the eruptive cycle. The anisotropy is thought to be produced by either structural effects or stress in the crust aligning microcracks and creating an anisotropic medium. The polarisation of the fast quasi-shear wave, phi, is parallel to the maximum horizontal stress or strike of structure, and the delay time between the two quasi-shear waves, dt, is proportional to the strength of the anisotropy and the length of the raypath in the anisotropic medium. We identify the mechanism of anisotropy in regions around Ruapehu by comparison of phi to stress and structure. We examine past changes in anisotropy by firstly analysing clusters of earthquakes recorded at permanent stations, and secondly performing a simplified two-dimensional dt tomography with data from temporary deployments. The results are compared to our previously mapped seismic anisotropy to characterise the variation. Using these two methods, we have identified pronounced temporal variations of SWS parameters associated with volcanic eruptions in 1995 and 2006.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V53C2631J
- Keywords:
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- 7270 SEISMOLOGY / Tomography;
- 7280 SEISMOLOGY / Volcano seismology;
- 8419 VOLCANOLOGY / Volcano monitoring;
- 8494 VOLCANOLOGY / Instruments and techniques