The Erua earthquake cluster and seismic anisotropy in the Ruapehu region, New Zealand

Abstract

[1] We use seismicity generated from the Erua earthquake cluster (a consistently active area of seismicity about 20 km to the west of Mount Ruapehu) over the last 12 years to study seismic anisotropy in the Ruapehu region. In particular, we search for changes associated with two minor phreatic eruptions on the 4th of October 2006 and the 25th of September 2007. The seismicity rate, magnitude of completeness, focal mechanisms and b-value of the cluster are also examined to investigate whether the characteristics of the seismicity changed over the duration of the study. The hypocenters were relocated, which revealed a westward dip in the shallow seismicity. Shear wave splitting results revealed a decrease in delay time in the 2006-2007 period and a significant variation in the fast shear wave polarization in the same time period. The b-value also increased significantly from 1.0 ± 0.2 in 2004 to a peak of 1.8 ± 0.2 in 2007, but no other parameters were found to vary significantly over this time period. We attribute these changes to an increase in pore-fluid pressure in the Erua region due to fluid movement and suggest that this fluid movement may be associated with the eruptions in 2006 and 2007.

Seismic detection of increased degassing before Kīlauea's 2008 summit explosion

“…S1 and Supplementary Table S1). Although this result may seem surprising when considering the dramatic changes in f within the caldera, small changes in the magnitude of anisotropy can create large variations in f because f and dt are not coupled linearly, particularly when the mechanism of anisotropy varies 9,12 . At the times of the 2008 changes, therefore, subtle variations in dt may be masked by scatter in the measurements 11 .…”

“…The mechanism of aligned microcracks is thought to be the only one that allows seismic anisotropy to vary on observable time scales 7 , and temporal changes are traditionally interpreted as stemming from variations in the stress field due to large earthquakes 7 or magmatic intrusions 8 . There is mounting evidence, however, that the dominant mechanism for SWS can switch between a static condition, such as aligned fractures in fault zones, and a dynamic process, such as maximum horizontal compressive stress causing aligned microcracks to dilate 9,10 . In areas where there are strong changes in maximum compressive stress direction and magnitude on observable timescales, such as at active volcanoes, SWS analysis has proven a valuable tool when combined with ground deformation or other seismological observations for interpretation of volcanic processes such as magma migration [8][9][10][11][12][13] .…”

“…There is mounting evidence, however, that the dominant mechanism for SWS can switch between a static condition, such as aligned fractures in fault zones, and a dynamic process, such as maximum horizontal compressive stress causing aligned microcracks to dilate 9,10 . In areas where there are strong changes in maximum compressive stress direction and magnitude on observable timescales, such as at active volcanoes, SWS analysis has proven a valuable tool when combined with ground deformation or other seismological observations for interpretation of volcanic processes such as magma migration [8][9][10][11][12][13] . Such changes are always detected in hindsight, however, and seismologists struggle to employ monitoring of seismic anisotropy in real-or near-real-time to assist in eruption forecasting.…”

A review of retrospective stress-forecasts of earthquakes and eruptions

“…2 plots the durations of the observed variations of SWS time-delays above swarms of small earthquakes before and after three volcanic eruptions (listed in Table 2) where stress-accumulation and stress-relaxation in SWS time-delays have been observed retrospectively. Other observations of changes in SWS before and after volcanic eruptions have been reported (Miller and Savage, 2001;Gerst and Savage, 2004;Keats et al, 2011;Johnson and Poland, 2013;amongst others). Unfortunately, these other observations do not have the consistent source-to-recorder-geometry (within the shear-wave window at the free surface) required for analysis of the stress-accumulation and stress-relaxation as in Figs.…”

Shear wave automatic picking and splitting measurements at Ruapehu volcano, New Zealand

“…Figure (top left) contains an inset with the location of the box on a map of New Zealand, the names of all stations and regions, and regions defined in other studies. The boxes with red dashed lines are the Erua and Waiouru regions considered by Godfrey et al [], and the black dashed lines are those considered by Keats et al [] and Johnson and Savage [].…”