Search for Temporal Changes in Seismic Attenuation under Mt. Ruapehu Volcano, North Island, New Zealand

Abstract

This study investigates temporal changes in seismic attenuation using regional earthquakes recorded at seismic stations around Mt. Ruapehu volcano, New Zealand. Temporal variation of attenuation has been observed at different volcanoes around the world and seems to correlate with volcanic activity. Recent studies at Mt. Ruapehu volcano have found a temporal variation of seismic anisotropy which was associated with a series of eruptions in 1995/96. This variation is presumably caused by a change in stress in the vicinity of the volcano caused by pressurisation of a filling magma dyke under the volcano with subsequent depressurisation after the eruptions. Changes in stress that alter the alignment of cracks and pore space are likely to influence seismic attenuation as well. Observations of variations in anisotropy and attenuation could provide a good monitoring tool for the state of stress around Mt. Ruapehu. Ultimately, this might allow the development of a warning tool for impending eruptions. Earthquakes originating from the persistent Waiouru swarm provide an ongoing seismic source at a distance of about 25 km from Mt. Ruapehu over the past 15 years. Cross-correlation of direct wave arrivals for different events shows a remarkable waveform similarity and reveals seismic families with similar source mechanisms over the entire time period analysed. Attenuation of coda waves is highly frequency dependent and gives a relation of QC = 53 f 1.02 around Mt. Ruapehu. Small temporal fluctuations of coda attenuation values are observed at all stations between 1990 and 2005 but there is no significant increase related to volcanic activity. Variations in the frequency dependence of coda attenuation and distinct changes in relative attenuation around 1995 are associated with the change in stress observed by seismic anisotropy. Stress changes are found to have an extent of at least 15 km around the summit of Mt. Ruapehu. Low intrinsic attenuation and the similarity of shear waves of well correlated events before and after the eruptions indicate a very small magma chamber under the volcano. The absolute attenuation measured from direct waves is poorly constrained due to a lack of data. Nevertheless, estimates from direct P and S waves suggest high attenuation in the vicinity Mt. Ruapehu, as expected due to the highly scattering shallow layers of recent volcanic deposits. Earthquakes from the nearby Waiouru swarm provide an excellent source of data for further studies of seismic properties around Mt. Ruapehu. The permanent swarm with very similar events over a long period of time can be used to monitor possible changes in seismic properties associated with volcanic activity, especially by means of the recently improved station coverage with modern instruments around the volcano.