Monitoring unrest from ambient seismic noise recordings : results from Mount Ruapehu, New Zealand for 2022 : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Earth Science, Massey University, Palmerston North, New Zealand

Abstract

Ruapehu is an active andesitic composite volcano that had two significant eruptions on 4 October 2006 and on 25 September 2007 since the last major eruption in 1995–1996. These were mostly phreatic explosions that happened with minimal precursors. A significant unrest period occurred between March and June at Ruapehu volcano in 2022, with heating of the crater lake and tremor levels consistent with moderate to high levels of volcanic unrest. In this study, velocity changes during the 2022 unrest were explored using seismic interferometry. Analysis of one year of ambient noise data using moving window cross spectral analysis (MWCS) found a nearly 0.5% drop in the East-North edifice’s seismic velocity, while no reduction in velocity was noted in the West-North. This decrease began three weeks before the unrest signals of Crater Lake temperature and tremor were observed. This drop is a reversible process, and several factors, including fractures opening, fluid fluxes like water, gas, or magma, magmatic anomalies, magma intruding into the subsurface without reaching the surface, and environmental factors (such as rainfalls or changes in atmospheric pressure), could cause a low-velocity zone. There is, however, no evidence of the precise reason for the 2022 unrest.