Changes in attenuation related to eruptions of Mt. Ruapehu Volcano, New Zealand

Temporal variations of seismic attenuation have been observed at different volcanoes around the world and are often claimed to correlate with volcanic activity. Seismic attenuation and its variation in time at Mt. Ruapehu Volcano, New Zealand, was measured for the period October 1990 to May 2005 using local earthquakes originating from a nearby cluster of earthquakes called the Waiouru swarm, as recorded on 5 permanent seismometer stations. Cross-correlation of P and S waves reveals a similarity of events from this ongoing seismic source, which suggests that alterations due to source and path effects are minimized. Therefore, the swarm is suitable for the study of temporal changes in seismic attenuation. The attenuation (QC− 1) of coda waves following the direct P and S waves are calculated for five frequency bands centred at 1.5, 3.0, 6.0, 9.0, and 12.0 Hz using the single-scattering model. A best-fit equation of QC = Q0fα with Q0 = 54(± 7) and α = 1.02(± 0.06) shows an overall high frequency dependence of coda attenuation, which is characteristic for tectonically active regions. Small fluctuations of QC− 1 are observed at all stations over the entire time period and do not act as an apparent indicator of volcanic activity. Relative changes in integrated direct wave attenuation (δt*) along the whole path are obtained using a spectral-ratio method in which spectra of individual 1-second P wave windows are compared to a reference spectrum. During the time of volcanic activity the following changes were significant at the 95% confidence level: In the low-frequency band (1.5–6 Hz), a high attenuation anomaly with an abrupt increase of δt* by 0.23 s was detected in waveforms that have travelled through the volcano, and a shift from negative to positive δt* by 0.25 s was observed at a station next to the volcano. We suggest that these observations are related to variations detected in seismic anisotropy studies, which propose a model of a change in stress caused by pressurisation of magma dykes under the volcano.