Automatic Shear Wave Splitting Measurements at Mt. Ruapehu Volcano, New Zealand

<p>This thesis presents an automatic shear wave splitting measurement tool and the results from its application to data recorded in the vicinity of Mt. Ruapehu volcano on the North Island of New Zealand. The best methodology and parameters for routine automatic monitoring are determined and approximately 10,000 events are processed. About 50% of all S-phases lead to measurements of acceptable quality. Results obtained with this technique are reproducible and objective, but more scattered than results from manual measurements. The newly developed automatic measurement tool is used to measure shear wave splitting for previously analysed data and for new data recorded in 2003-2007. In contrast to previous studies at Mt. Ruapehu, we have a larger and continuous data set from numerous three-component seismic stations. No major temporal changes are found within the new data, but results vary for di erent station locations. I</p>

Automatic Shear Wave Splitting Measurements at Mt. Ruapehu Volcano, New Zealand

<p>This thesis presents an automatic shear wave splitting measurement tool and the results from its application to data recorded in the vicinity of Mt. Ruapehu volcano on the North Island of New Zealand. The best methodology and parameters for routine automatic monitoring are determined and approximately 10,000 events are processed. About 50% of all S-phases lead to measurements of acceptable quality. Results obtained with this technique are reproducible and objective, but more scattered than results from manual measurements. The newly developed automatic measurement tool is used to measure shear wave splitting for previously analysed data and for new data recorded in 2003-2007. In contrast to previous studies at Mt. Ruapehu, we have a larger and continuous data set from numerous three-component seismic stations. No major temporal changes are found within the new data, but results vary for di erent station locations. I</p>

Geology and Petrology of Ruapehu Volcano and Related Vents

<p>Ruapehu Volcano is an active, multiple-vent, andesite composite volcano at the southern terminus of the Taupo Volcanic Zone, central North Island, New Zealand. The present-day volume of Ruapehu is estimated at 110 km3, and construction of the massif probably occurred during the past 0.5 m.y. Geologic mapping and stratigraphic studies have led to the recognition of four periods of cone construction, each occurring over 104-105 year time intervals. On the basis of lithologic/petrographic differences, and conspicuous unconformities which separate the deposits of each cone-building period, four new formations are defined, comprises the Ruapehu Group. Te Herenga formation (new formation name) comprises the oldest deposits of Ruapehu (upper lavas ca. 0.23 Ma) and is exposed as planeze surfaces and aretes on N and NW Ruapehu. The formation includes lava flows, tuff breccias, and small intrusive bodies surrounded by zones of hydrothermal alteration. There is little petrographic and compositional diversity; most lavas are porphyritic titanomagnetite- augite- hypersthene- plagioclase basic andesites. Wahiance Formation (new formation name) is younger than Te Herenga Fm,. but of unknown age. It is well exposed on SE Ruapehu, and comprises mostly lava flows and tuff breccias. The lavas comprise acid and basic andesites. Mangewhero Formation (new formation name) is well exposed everywhere except SE Ruapehu, and the upper lavas and pyroclastics (ca. 0.02 Ma) form the present high peeks and main cone of Ruapehu. The lavas are petrographically and geochemically diverse, ranging from basalt to decite in bulk composition. Some of the lower lavas are olivine-beering andesites of hybrid orgin. Whakapapa Formation (new formation name; ca 15,000 years to present) comprises conspicuously young lava flows, tuff breccias, airfall pyroclastics and minor pyroclastic flows of acid- and basic andesite. The deposits of these post-glacial summit and flank eruptions are subdivided into the lwikau, Rangataua, Tama and Crater Lake Members. 'Related vents' produced Heuhungatahi Andesite Fm. (> 0.5 Ma?), and Holocene deposits of basalt and basic andesite at isolated, monogenetic centres comprising Ohakune Andesite Fm., Pukeonake Andesite Fm., and Waimarino Basalt Fm. (new formation name). Most Ruapehu lavas are medium-K acid and basic andesites (mean of 144 bulk rock analyses is 57.8 wt % SiO2), but rare basalt and minor decite are present. Nearly all lavas are porphyritic in plagioclase, augite and hypersthene [plus or minus] olivine, with titanomagnetite micro- phenocrysts, and contain abundant metamorphic and igneous rock inclusions. Petrography, mineral chemistry and bulk rock chemistry indicate fractional crystallization series from parental basalts (52-53 % SiO2, Q-normative, low-alumina) to medium-K basic- and acid andesites (58-59 % SiO2). Early fractionating minerals are olivine and clinopyroxene with minor chrome spinel and plagioclase, followed by plagioclase, orthopyroxene, clinopyroxene and minor titanomagnetite in later stages of differentiation. Thus, basalt differentiation to produce andesites involves 'POAM-type' (Gill, 1981) fractional crystallization. Three second-order differentiation processes operate concurrently with frational crystallization: (1) Crystal accumulation involves addition of co-genetic plutonic rock fragments and crystals derived from them. These inclusions are common and few rocks represent liquid compositions. (2) Magma mixing involves mingling of magmas in repeatedly-occupied conduits. End members are as diverse as basalt and decite, yielding petrogaphically and chemically distinctive high-Mg andesites of the upper cone complex and parasitic centres. (3) Selective crustal assimilation is suggested by partially fused metamorphic inclusions, positive correlation of 87Sr/86Sr with SiO2, and failure of simple 'POAM' fractionation to explain decites (63-65 % SiO2). Petrogenesis of Ruapehu andesites takes place under open-system condition, involving production of parental Q-normative basalts in the mantle wedge, concurrent fractional crystallization and crustal contamination, entrainment of co-genetic plutonic rocks, and mixing of magmas in common conduits.</p>

Geology and Petrology of Ruapehu Volcano and Related Vents

<p>Ruapehu Volcano is an active, multiple-vent, andesite composite volcano at the southern terminus of the Taupo Volcanic Zone, central North Island, New Zealand. The present-day volume of Ruapehu is estimated at 110 km3, and construction of the massif probably occurred during the past 0.5 m.y. Geologic mapping and stratigraphic studies have led to the recognition of four periods of cone construction, each occurring over 104-105 year time intervals. On the basis of lithologic/petrographic differences, and conspicuous unconformities which separate the deposits of each cone-building period, four new formations are defined, comprises the Ruapehu Group. Te Herenga formation (new formation name) comprises the oldest deposits of Ruapehu (upper lavas ca. 0.23 Ma) and is exposed as planeze surfaces and aretes on N and NW Ruapehu. The formation includes lava flows, tuff breccias, and small intrusive bodies surrounded by zones of hydrothermal alteration. There is little petrographic and compositional diversity; most lavas are porphyritic titanomagnetite- augite- hypersthene- plagioclase basic andesites. Wahiance Formation (new formation name) is younger than Te Herenga Fm,. but of unknown age. It is well exposed on SE Ruapehu, and comprises mostly lava flows and tuff breccias. The lavas comprise acid and basic andesites. Mangewhero Formation (new formation name) is well exposed everywhere except SE Ruapehu, and the upper lavas and pyroclastics (ca. 0.02 Ma) form the present high peeks and main cone of Ruapehu. The lavas are petrographically and geochemically diverse, ranging from basalt to decite in bulk composition. Some of the lower lavas are olivine-beering andesites of hybrid orgin. Whakapapa Formation (new formation name; ca 15,000 years to present) comprises conspicuously young lava flows, tuff breccias, airfall pyroclastics and minor pyroclastic flows of acid- and basic andesite. The deposits of these post-glacial summit and flank eruptions are subdivided into the lwikau, Rangataua, Tama and Crater Lake Members. 'Related vents' produced Heuhungatahi Andesite Fm. (> 0.5 Ma?), and Holocene deposits of basalt and basic andesite at isolated, monogenetic centres comprising Ohakune Andesite Fm., Pukeonake Andesite Fm., and Waimarino Basalt Fm. (new formation name). Most Ruapehu lavas are medium-K acid and basic andesites (mean of 144 bulk rock analyses is 57.8 wt % SiO2), but rare basalt and minor decite are present. Nearly all lavas are porphyritic in plagioclase, augite and hypersthene [plus or minus] olivine, with titanomagnetite micro- phenocrysts, and contain abundant metamorphic and igneous rock inclusions. Petrography, mineral chemistry and bulk rock chemistry indicate fractional crystallization series from parental basalts (52-53 % SiO2, Q-normative, low-alumina) to medium-K basic- and acid andesites (58-59 % SiO2). Early fractionating minerals are olivine and clinopyroxene with minor chrome spinel and plagioclase, followed by plagioclase, orthopyroxene, clinopyroxene and minor titanomagnetite in later stages of differentiation. Thus, basalt differentiation to produce andesites involves 'POAM-type' (Gill, 1981) fractional crystallization. Three second-order differentiation processes operate concurrently with frational crystallization: (1) Crystal accumulation involves addition of co-genetic plutonic rock fragments and crystals derived from them. These inclusions are common and few rocks represent liquid compositions. (2) Magma mixing involves mingling of magmas in repeatedly-occupied conduits. End members are as diverse as basalt and decite, yielding petrogaphically and chemically distinctive high-Mg andesites of the upper cone complex and parasitic centres. (3) Selective crustal assimilation is suggested by partially fused metamorphic inclusions, positive correlation of 87Sr/86Sr with SiO2, and failure of simple 'POAM' fractionation to explain decites (63-65 % SiO2). Petrogenesis of Ruapehu andesites takes place under open-system condition, involving production of parental Q-normative basalts in the mantle wedge, concurrent fractional crystallization and crustal contamination, entrainment of co-genetic plutonic rocks, and mixing of magmas in common conduits.</p>

Temporal Changes in Seismic Anisotropy as a New Eruption Forecasting Tool

<p>The orientation of crustal anisotropy changed by ~80 degrees in association with the 1995/96 eruption of Mt. Ruapehu volcano, New Zealand. This change occurred with a confidence level of more than 99.9%, and affects an area with a radius of at least 5 km around the summit. It provides the basis for a new monitoring technique and possibly for future mid-term eruption forecasting at volcanoes. Three deployments of seismometers were conducted on Mt. Ruapehu in 1994, 1998 and 2002. The fast anisotropic direction was measured by a semi-automatic algorithm, using the method of shear wave splitting. Prior to the eruption, a strong trend for the fast anisotropic direction was found to be around NW-SE, which is approximately perpendicular to the regional main stress direction. This deployment was followed by a moderate phreatomagmatic eruption in 1995/96, which ejected material with an overall volume of around 0.02-0.05 km3. Splitting results from a deployment after the eruption (1998) suggested that the fast anisotropic direction for deep earthquakes (>55 km) has changed by around 80 degrees, becoming parallel to the regional stress field. Shallow earthquakes (<35 km) also show this behaviour, but with more scatter of the fast directions. Another deployment (2002) covered the exact station locations of both the 1994 and the 1998 deployments and indicates further changes. Fast directions of deep events remain rotated by 80 degrees compared to the pre-eruption direction, whereas a realignment of the shallow events towards the pre-eruption direction is observed. The interpretation is that prior to the eruption, a pressurised magma dike system overprinted the regional stress field, generating a local stress field and therefore altering the fast anisotropic direction via preferred crack alignment. Numerical modelling suggests that the stress drop during the eruption was sufficient to change the local stress direction back to the regional trend, which was then observed in the 1998 experiment. A refilling and pressurising magma dike system is responsible for the newly observed realignment of the fast directions for the shallow events, but is not yet strong enough to rotate the deeper events with their longer delay times and lower frequencies. These effects provide a new method for volcano monitoring at Mt. Ruapehu and possibly at other volcanoes on Earth. They might, after further work, serve as a tool for eruption forecasting at Mt. Ruapehu or elsewhere. It is therefore proposed that changes in anisotropy around other volcanoes be investigated.</p>

Temporal Changes in Seismic Anisotropy as a New Eruption Forecasting Tool

<p>The orientation of crustal anisotropy changed by ~80 degrees in association with the 1995/96 eruption of Mt. Ruapehu volcano, New Zealand. This change occurred with a confidence level of more than 99.9%, and affects an area with a radius of at least 5 km around the summit. It provides the basis for a new monitoring technique and possibly for future mid-term eruption forecasting at volcanoes. Three deployments of seismometers were conducted on Mt. Ruapehu in 1994, 1998 and 2002. The fast anisotropic direction was measured by a semi-automatic algorithm, using the method of shear wave splitting. Prior to the eruption, a strong trend for the fast anisotropic direction was found to be around NW-SE, which is approximately perpendicular to the regional main stress direction. This deployment was followed by a moderate phreatomagmatic eruption in 1995/96, which ejected material with an overall volume of around 0.02-0.05 km3. Splitting results from a deployment after the eruption (1998) suggested that the fast anisotropic direction for deep earthquakes (>55 km) has changed by around 80 degrees, becoming parallel to the regional stress field. Shallow earthquakes (<35 km) also show this behaviour, but with more scatter of the fast directions. Another deployment (2002) covered the exact station locations of both the 1994 and the 1998 deployments and indicates further changes. Fast directions of deep events remain rotated by 80 degrees compared to the pre-eruption direction, whereas a realignment of the shallow events towards the pre-eruption direction is observed. The interpretation is that prior to the eruption, a pressurised magma dike system overprinted the regional stress field, generating a local stress field and therefore altering the fast anisotropic direction via preferred crack alignment. Numerical modelling suggests that the stress drop during the eruption was sufficient to change the local stress direction back to the regional trend, which was then observed in the 1998 experiment. A refilling and pressurising magma dike system is responsible for the newly observed realignment of the fast directions for the shallow events, but is not yet strong enough to rotate the deeper events with their longer delay times and lower frequencies. These effects provide a new method for volcano monitoring at Mt. Ruapehu and possibly at other volcanoes on Earth. They might, after further work, serve as a tool for eruption forecasting at Mt. Ruapehu or elsewhere. It is therefore proposed that changes in anisotropy around other volcanoes be investigated.</p>

Rapid assembly of high-Mg andesites and dacites by magma mixing at a continental arc stratovolcano

Studies of pre-eruptive processes at active volcanoes require precise petrochronological constraints if they are to contribute to hazard assessment during future eruption events. We present petrological and geochemical data and orthopyroxene diffusion time scales for samples from Late Pleistocene high-Mg andesite-dacite lavas (Mg# 53–69) at Ruapehu volcano, New Zealand, as a case study of rapid magma genesis and eruption at a continental arc stratovolcano. Assembly of Ruapehu high-Mg magmas involved the mixing of primitive magmas plus entrained mantle-equilibrated olivines with mid-crustal felsic mush bodies, yielding hybridized magmas with ubiquitous pyroxene reverse-zoning patterns. Orthopyroxene Fe-Mg interdiffusion time scales linked to quantitative crystal orientation data show that most lavas erupted &lt;10 days after resumption of crystal growth following magma mixing events. The eruption of lavas within days of mixing events implies that pre-eruptive warnings may be correspondingly short.

Glaciovolcanic emplacement of an intermediate hydroclastic breccia-lobe complex during the penultimate glacial period (190–130 ka), Ruapehu volcano, New Zealand

An intermediate-composition hydroclastic breccia deposit is exposed in the upper reaches of a deep glacial valley at Ruapehu volcano, New Zealand, indicating an ancient accumulation of water existed near the current summit area. Lobate intrusions within the deposit have variably fluidal and brecciated margins, and are inferred to have been intruded while the deposit was wet and unconsolidated. The tectonic setting, elevation of Ruapehu, and glacial evidence suggest that the deposit-forming eruption took place in meltwater produced from an ancient glacier. The breccia-lobe complex is inferred to have been emplaced at &gt; 154 ± 12 ka, during the penultimate glacial period (190–130 ka) when Ruapehu’s glaciers were more extensive than today. This age is based on overlying radiometrically dated lava flows, and by correlation with a well-constrained geochemical stratigraphy for Ruapehu. Field relations indicate that the glacier was at least 150 m thick, and ubiquitous quench textures and jigsaw-fit fracturing suggest that the clastic deposit was formed from non-explosive fragmentation of lava in standing water. Such features are unusual for the high flanks of a volcanic edifice where steep topography typically hinders accumulation of water or thick ice, and hence the formation and retention of hydroclastic material. Although not well-constrained for this time, the vent configuration at Ruapehu is inferred to have contributed to an irregular edifice morphology, allowing thick ice to locally accumulate and meltwater to be trapped.