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

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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.

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