Modelling visco-elastic source processes at Ngauruhoe Volcano, Central North Island, New Zealand

Executive Summary

Ngauruhoe volcano has produced repeated earthquakes since early 2006 with each earthquake exhibiting similar waveform features. These earthquakes are known to have slowly evolving source properties through time with spectral frequencies increasing during the period of observation (2006-2009). We have conducted a limited inversion for moment tensor components of an example low-frequency earthquake occurring 17 January 2008. Our inversion method is completed in two ways: 1) inversion for a non evolving source process, and 2) inversion for a time varying source process. For the non-evolving source inversion, we find that the moment tensor components match either a vertical pipe geometry or a vertical crack oriented N-NE (along the volcanic axis). For earlier inversion runs, the best fit location was found at ~800m below the volcano summit area. The volumetric moment tensor components are about 1 to 5 x 10¹¹ Nm. The model fits the observations at the three closest stations (temporarily deployed seismometers) located within 1.5km of the source, but not at more distant seismic stations (OTVZ, NGZ) which are >3km from the source. This discrepancy is probably due to low velocities in the uppermost crust which are not represented in the velocity model. Additional model trials using an alternative low velocity surface layer (2.5km/s instead of 3.3km/s) yield improved fits to first arrivals at the further stations and underscore the uncertainties associated with the method. Alternative source models are not consistent with the waveform data. We also separately apply a new analytical approach to examine the role of damping in volcanic earthquake resonance. This approach shows that acoustic waves may be strongly damped within resonating cavities, while the associated interface was dominating the low-frequency earthquakes, can have relatively low damping values, as long as the viscosity within the cavity is high. This result has implications for the estimation of the resonator properties which control the low-frequency earthquakes at Ngauruhoe.