Browsing by Author "Savage, Martha"
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Item Restricted ESCI305: Antarctic Research Centre: Environmental and Applied Geophysics(2023) Savage, MarthaItem Restricted ESCI305: Earth Science: Environmental and Applied Geophysics(Te Herenga Waka—Victoria University of Wellington, 2024) Chamberlain, Calum; Savage, MarthaItem Restricted ESCI305: Earth Sciences: Exploration Geophysics(2018) Savage, MarthaItem Restricted ESCI407: Earth Sciences: Global Tectonics(Victoria University of Wellington, 2011) Savage, MarthaItem Restricted ESCI407: Earth Sciences: Tectonics(Victoria University of Wellington, 2007) Savage, MarthaItem Restricted ESCI411: Earth Sciences : Advanced Applied Geophysics(Te Herenga Waka—Victoria University of Wellington, 2023) Savage, MarthaItem Restricted GPHS441: Geophysics: Solid Earth and Geophysics(Victoria University of Wellington, 2016) Savage, MarthaItem Restricted GPHS441: Geophysics: Solid Earth Geophysics(2018) Savage, MarthaItem Restricted GPHS445: Geophysics: Introduction to Seismology(Victoria University of Wellington, 2005) Savage, MarthaItem Restricted GPHS445: Geophysics: Observational Earthquake Seismology(Victoria University of Wellington, 2016) Savage, MarthaItem Restricted GPHS445: Geophysics: Observational Earthquake Seismology(Victoria University of Wellington, 2017) Savage, MarthaItem Restricted GPHS445: Geophysics: Observational Earthquake Seismology(2018) Savage, MarthaItem Restricted GPHS445: Geophysics: Observational Earthquake Seismology(Victoria University of Wellington, 2012) Savage, MarthaItem Restricted GPHS446: Geophysics: Advanced Seismology(Victoria University of Wellington, 2010) Savage, MarthaItem Restricted GPHS446: Geophysics: Advanced Seismology(Victoria University of Wellington, 2011) Savage, MarthaItem Restricted GPHS446: Geophysics: Advanced Seismology(Victoria University of Wellington, 2008) Savage, MarthaItem Restricted GPHS446: Geophysics: Advanced Seismology(Victoria University of Wellington, 2005) Savage, MarthaItem Restricted Search for Temporal Changes in Seismic Attenuation under Mt. Ruapehu Volcano, North Island, New Zealand(Te Herenga Waka—Victoria University of Wellington, 2006) Kuehler, Tanja; Savage, Martha; Hurst, TonyThis study investigates temporal changes in seismic attenuation using regional earthquakes recorded at seismic stations around Mt. Ruapehu volcano, New Zealand. Temporal variation of attenuation has been observed at different volcanoes around the world and seems to correlate with volcanic activity. Recent studies at Mt. Ruapehu volcano have found a temporal variation of seismic anisotropy which was associated with a series of eruptions in 1995/96. This variation is presumably caused by a change in stress in the vicinity of the volcano caused by pressurisation of a filling magma dyke under the volcano with subsequent depressurisation after the eruptions. Changes in stress that alter the alignment of cracks and pore space are likely to influence seismic attenuation as well. Observations of variations in anisotropy and attenuation could provide a good monitoring tool for the state of stress around Mt. Ruapehu. Ultimately, this might allow the development of a warning tool for impending eruptions. Earthquakes originating from the persistent Waiouru swarm provide an ongoing seismic source at a distance of about 25 km from Mt. Ruapehu over the past 15 years. Cross-correlation of direct wave arrivals for different events shows a remarkable waveform similarity and reveals seismic families with similar source mechanisms over the entire time period analysed. Attenuation of coda waves is highly frequency dependent and gives a relation of QC = 53 f 1.02 around Mt. Ruapehu. Small temporal fluctuations of coda attenuation values are observed at all stations between 1990 and 2005 but there is no significant increase related to volcanic activity. Variations in the frequency dependence of coda attenuation and distinct changes in relative attenuation around 1995 are associated with the change in stress observed by seismic anisotropy. Stress changes are found to have an extent of at least 15 km around the summit of Mt. Ruapehu. Low intrinsic attenuation and the similarity of shear waves of well correlated events before and after the eruptions indicate a very small magma chamber under the volcano. The absolute attenuation measured from direct waves is poorly constrained due to a lack of data. Nevertheless, estimates from direct P and S waves suggest high attenuation in the vicinity Mt. Ruapehu, as expected due to the highly scattering shallow layers of recent volcanic deposits. Earthquakes from the nearby Waiouru swarm provide an excellent source of data for further studies of seismic properties around Mt. Ruapehu. The permanent swarm with very similar events over a long period of time can be used to monitor possible changes in seismic properties associated with volcanic activity, especially by means of the recently improved station coverage with modern instruments around the volcano.Item Restricted Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone(Te Herenga Waka—Victoria University of Wellington, 2017) Mroczek, Stefan; Savage, MarthaIn order to investigate the cracks/fractures in the geothermal fields of Rotokawa and Ngatamariki, we measure seismic anisotropy across both fields and interpret the results in the context of stress aligned microcracks. Cracks aligned perpendicular to the direction of maximum horizontal stress close and their fluid is forced into cracks aligned with maximum horizontal stress (SHmax). Seismic anisotropy is the directional dependence of a seismic wave's velocity and provides a measure of crack orientation and density. To measure seismic anisotropy we conduct shear wave splitting measurements on 52,000 station-earthquake pairs across both Rotokawa and Ngatamariki from earthquakes recorded during 2015. Both fields are the subject of other geophysical and geological studies. Thus they are excellent subjects for studying seismic anisotropy. We cluster our measurements by their station-event path and fit the parameters from these clusters to those from theoretical crack planes. We also apply 2-D tomography to shear wave splitting time delays (𝛿t) and spatial averaging to shear wave splitting fast polarisations (∅). In addition, we compare time delays with P-wave to S-wave velocity ratios (νP / vS) . Local measurements of stress within Rotokawa and regional measures of stress within the Taupo Volcanic Zone provide a comparison for the shear wave splitting measurements. We measure ∅ which agrees with the NE-SW regional direction of SHmax across Ngatamariki and parts of Rotokawa. Within Rotokawa, we observe a rotation of ∅ away from NE-SW toward N-S that agrees with borehole measurements of direction of SHmax of 023° and 030°. Spatial averaging of ∅ reveals mean orientations close to the strike of nearby active faults. The theoretical crack planes, that fit best to the shear wave splitting measurements, correspond to aligned cracks striking 045° outside of both fields, 035° within Ngatamariki, and 035° through to 0° within Rotokawa. The average percent anisotropy for the full dataset, approximately 4%, is close to the upper bound for an intact rock. Delay time tomography shows regions of higher delay time per kilometre of path length (s=km) within both fields and possibly associated with the production field fault in Rotokawa. vP =vS shows a wide range of normally distributed values, from 1.1 through to 2.4 with a mean of 1.6, indicating a mixture of gas filled and saturated cracks. A positive correlation between delay time per kilometre (𝛿tpkm) and νP /νS indicates that the majority of the cracks are saturated.Item Restricted A Seismological Investigation of the Kawerau Geothermal Field(Te Herenga Waka—Victoria University of Wellington, 2014) Keats, Brook; Savage, Martha; Townend, John; Bannister, StephenWe use a recently installed seismometer network to investigate the seismicity of the Kawerau Geothermal Field. Our dataset consists of 401 shallow earthquakes reported by GeoNet between 1 January 2008 and 1 July 2013 with an average magnitude (ML) of 1.85. P and S phase arrivals and rst motions were manually picked and the events relocated using NonLinLoc and HYPODD. The resulting set of locations showed a domed seismicity distribution centred on the central part of the geothermal eld, with the base of seismicity increasing from around 4 km in the centre to 6‒7 km around the fringes, with the depth increase most notable to the East. This provides support for the conceptual model of the eld and suggests that higher temperatures may be located directly beneath the eld rather than below Putauaki as previously thought. Focal mechanism solutions were obtained for 14 events and revealed a combination of strike-slip and dip slip faulting. Closely spaced similar mechanisms suggested we see repeated activity on the same fault structures within a 10 month time frame. Inverting these focal mechanisms for stress revealed horizontal maximum and minimum stress vectors with SHmax oriented at 65°, slightly clockwise from the 43° rift axis of the Whakatane Graben. A low stress ratio of 0.2 suggested the extension associated with the rifting of the Whakatane Graben is the dominant stress regime at Kawerau.
