Eruptions of Mt Ruapehu, 1995-1996: a comparison of eruptive and non-eruptive volcanic earthquakes

Abstract

During the eruptions of Mt Ruapehu in 1995 and 1996, many volcanic earthquakes occurred, some, but not all of which accompanied eruptions. Twenty-four events spread over the two years of activity that were recorded on the five station permanent network on and around Ruapehu and several temporary stations are analysed to determine if there are any differences between eruptive and non-eruptive events, using spectral techniques. The only significant difference between spectra and waveforms of the eruptive and non-eruptive events is a high frequency airwave accompanying eruptive events. Crater Lake attenuates this airwave when it is full. A generalised inversion to separate the source and site effects showed the source spectra of the eruptive and non-eruptive events are nearly identical, suggesting the source process generating the two types of events are similar. The source spectra had a peak between 0.5 and 2 Hz during 1995, which broadened to between 0.5 and 3.5 Hz in 1996. Station TUV contained relatively lower energy at 1 Hz than the other permanent stations, which explains the lack of low frequency recorded at that station. Station NGZ amplifies frequencies between 0.5 and 1 Hz. The events contained almost no high frequency energy above 4 Hz, and an attenuation coefficient Q has been estimated to have values ranging from 12 - 36 at 2 Hz.