New Zealand Journal of Geology and Geophysics abstracts

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Volcanic tremor at Ruapehu: characteristics and implications for the resonant source

A. W. HURST

Institute of Geological & Nuclear Sciences
P. O. Box 1320
Wellington, New Zealand

STEVEN SHERBURN

Institute of Geological & Nuclear Sciences
Private Bag 2000
Taupo, New Zealand

Abstract Many volcanoes produce volcanic tremor with consistent sharp peaks, which suggests that one or more resonators have been excited by a volcanic process. Such "harmonic" tremor can be classified as single-resonator or multiple-resonator. Multiple-resonator tremor has a number of peaks, not harmonically related, and the relative energy in the peaks varies rapidly with time. Single-resonator tremor, as observed on Ruapehu, has a few sharp peaks in the spectrum, and the tremor signal shows some coherence.
The main tremor observed on Ruapehu has a dominant frequency of c. 1.8-2.3 Hz, with very little second harmonic energy. Recordings within 1 km of Ruapehu Crater Lake sometimes show a strong third harmonic, but this is rapidly attenuated at greater distances. "Gliding" frequency changes are rarely seen. Tremor of c. 3 Hz also occurs, and this appears to come from another source, the intensity of which is independent of the main source.
"2 Hz" Ruapehu tremor recorded near the source shows a complex particle motion, but for distances >2 km, Rayleigh and other surface waves are dominant. The attenuation with distance of these surface waves at 2 Hz corresponds to that expected for a wave travelling along the surface of material with a Q of 17.
To explain the sharply peaked spectrum of typical volcanic tremor from Ruapehu, the "2 Hz" tremor must be produced in a high-Q resonator, with asymmetrical end conditions, to explain the lack of second harmonic. A possible mechanism for the resonator excitation is the "white noise" signal of high pressure gas flow.

Keywords volcanic tremor; resonator; Ruapehu; powerspectra

Received 8 January 1993; published 3 December 1993
New Zealand Journal of Geology & Geophysics, 1991, Vol. 36: 475—485
0028Ð8306/06/3604—0475 ©The Royal Society of New Zealand 1991

PDF file of entire paper: medium quality (1548K); (scanned from paper original: notes about this process). Digitisation of this article from the printed journal was kindly facilitated by the Geological Society of New Zealand

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