Seismic Attenuation Anisotropy in the Southernmost Part of the Taupo Volcanic Zone, North Island, New Zealand

<p>We investigate the mechanisms of seismic anisotropy and attenuation (1/Q) beneath the southernmost part of the Taupo Volcanic Zone (TVZ) by computing variations in S-wave attenuation factors with the direction of wave polarization. We rotate pairs of horizontal components in steps of 22.5◦ from 0◦ to 67.5◦ and into the radial and transverse directions to search for the optimal separation of the attenuation curves and thereby determine an anisotropy symmetry system. The frequency dependence of Q for the rotated S-waves is estimated by means of the non-parametric generalized inversion technique (GIT) of Castro et al. (1990) using shallow earthquakes (< 40 km depth) recorded by GeoNet within 100 km of Mt. Ruapehu. To analyze the effects on computed attenuation properties of source locations, we divide our dataset into two groups: a “TVZ” group containing earthquakes within the TVZ in a distance range of 5–55 km and a “non-TVZ” group containing earthquakes outside the TVZ in a distance range of 5–50 km. To measure Q, we compute the spectral amplitude decay with distance in terms of empirical functions at 20 separate frequencies in the frequency bands 2–10 Hz and 2– 12 Hz for the TVZ and non-TVZ datasets respectively. We construct homogeneous and two-layer Q models for the TVZ dataset based on characteristic features of the attenuation function, while for outside TVZ we only analyse a homogeneous Q model. The homogeneous Q models obtained for the two datasets indicate that S-waves are more attenuated within the TVZ than outside. The homogeneous Q model for the TVZ dataset reveals that the S-wave is anisotropic at high frequencies ( f > 6 Hz) along N–S/E– W directions with the relation QSE ( f ) = (6.15±1.22) f (1.73±0.12) and QSN ( f ) = (4.14± 1.26) f (2.06±0.14), while the non-TVZ dataset shows a weak frequency dependence of attenuation anisotropy at low frequencies in NE–SW/SE–NW directions giving the power law function QSNE ( f ) = (50.93±1.18) f (0.20±0.10) and QSSE ( f ) = (22.60±1.10) f (0.53±0.06). Here, the uncertainty estimates are 95% confidence intervals. To investigate the variation of attenuation anisotropy with depth within the TVZ, we first calculate Q along propagation paths (< 25 km, which corresponds to a maximum turning point depth of 9 km ) and then using paths of 25–55 km length. Small attenuation anisotropy with low attenuation in the N–S direction for the upper crust of TVZ may be related to heterogenous structure as reported by previous studies. Attenuation anisotropy in the northwest direction yielding lower attenuation inferred for the deeper crust suggests the presence of connected melt aligned with the extension direction of TVZ .</p>

Seismic Attenuation Anisotropy in the Southernmost Part of the Taupo Volcanic Zone, North Island, New Zealand

<p>We investigate the mechanisms of seismic anisotropy and attenuation (1/Q) beneath the southernmost part of the Taupo Volcanic Zone (TVZ) by computing variations in S-wave attenuation factors with the direction of wave polarization. We rotate pairs of horizontal components in steps of 22.5◦ from 0◦ to 67.5◦ and into the radial and transverse directions to search for the optimal separation of the attenuation curves and thereby determine an anisotropy symmetry system. The frequency dependence of Q for the rotated S-waves is estimated by means of the non-parametric generalized inversion technique (GIT) of Castro et al. (1990) using shallow earthquakes (< 40 km depth) recorded by GeoNet within 100 km of Mt. Ruapehu. To analyze the effects on computed attenuation properties of source locations, we divide our dataset into two groups: a “TVZ” group containing earthquakes within the TVZ in a distance range of 5–55 km and a “non-TVZ” group containing earthquakes outside the TVZ in a distance range of 5–50 km. To measure Q, we compute the spectral amplitude decay with distance in terms of empirical functions at 20 separate frequencies in the frequency bands 2–10 Hz and 2– 12 Hz for the TVZ and non-TVZ datasets respectively. We construct homogeneous and two-layer Q models for the TVZ dataset based on characteristic features of the attenuation function, while for outside TVZ we only analyse a homogeneous Q model. The homogeneous Q models obtained for the two datasets indicate that S-waves are more attenuated within the TVZ than outside. The homogeneous Q model for the TVZ dataset reveals that the S-wave is anisotropic at high frequencies ( f > 6 Hz) along N–S/E– W directions with the relation QSE ( f ) = (6.15±1.22) f (1.73±0.12) and QSN ( f ) = (4.14± 1.26) f (2.06±0.14), while the non-TVZ dataset shows a weak frequency dependence of attenuation anisotropy at low frequencies in NE–SW/SE–NW directions giving the power law function QSNE ( f ) = (50.93±1.18) f (0.20±0.10) and QSSE ( f ) = (22.60±1.10) f (0.53±0.06). Here, the uncertainty estimates are 95% confidence intervals. To investigate the variation of attenuation anisotropy with depth within the TVZ, we first calculate Q along propagation paths (< 25 km, which corresponds to a maximum turning point depth of 9 km ) and then using paths of 25–55 km length. Small attenuation anisotropy with low attenuation in the N–S direction for the upper crust of TVZ may be related to heterogenous structure as reported by previous studies. Attenuation anisotropy in the northwest direction yielding lower attenuation inferred for the deeper crust suggests the presence of connected melt aligned with the extension direction of TVZ .</p>

How Academics and the Public Experienced Immersive Virtual Reality for Geo-Education

Immersive virtual reality can potentially open up interesting geological sites to students, academics and others who may not have had the opportunity to visit such sites previously. We study how users perceive the usefulness of an immersive virtual reality approach applied to Earth Sciences teaching and communication. During nine immersive virtual reality-based events held in 2018 and 2019 in various locations (Vienna in Austria, Milan and Catania in Italy, Santorini in Greece), a large number of visitors had the opportunity to navigate, in immersive mode, across geological landscapes reconstructed by cutting-edge, unmanned aerial system-based photogrammetry techniques. The reconstructed virtual geological environments are specifically chosen virtual geosites, from Santorini (Greece), the North Volcanic Zone (Iceland), and Mt. Etna (Italy). Following the user experiences, we collected 459 questionnaires, with a large spread in participant age and cultural background. We find that the majority of respondents would be willing to repeat the immersive virtual reality experience, and importantly, most of the students and Earth Science academics who took part in the navigation confirmed the usefulness of this approach for geo-education purposes.

Tectonic controls on geomorphology and spatial distribution of monogenetic volcanoes in the Central Southern Volcanic Zone of the Andes (Argentina)

Monogenetic volcanoes are among the most common volcanic landforms on Earth. The morphology and distribution of small volcanoes can provide important information about eruption dynamics and tectonics. The Southern Volcanic Zone of the Andes (CSVZ) comprises one of the most active magmatic regions on Earth. Characterized by the presence of polygenetic volcanoes and calderas in a complex tectonic setting, this region also hosts hundreds of small, back-arc monogenetic volcanoes. In this contribution, we apply a Geographic Information System (GIS) that combines imagery data and digital elevation models to establish the first comprehensive dataset of monogenetic volcanoes in the CSVZ (38° to 40° S), exploring their eruption dynamics and relationship to tectonic and structural processes. Combining spatial analysis and geomorphological observations, we identify the presence of 356 monogenetic volcanoes distributed into nine clusters, now grouped in the Zapala Volcanic Field (ZVF). The ZVF is marked by the predominance of cinder cones (80%) followed by phreatomagmatic volcanoes (20%), suggesting some influence of external water in the eruption dynamics. Generally, monogenetic vents present a clear association with local and regional lineaments, suggesting a strong structural control on the occurrence of the monogenetic deposits. The higher vent densities are observed in the southern Loncopué Though, an important extensional feature related to tearing of the subducted Nazca plate underneath the South American Plate. Morphometric parameters of cinder cones indicate variable stress orientations in the CSVZ that possibly result from the oblique tectonics in the region. From north to south, the maximum principal stress rotates from NE-SW to E-W and becomes progressively less constrained as it distances from the current magmatic arc. Based on the relative ages, we map the evolution of monogenetic volcanism through time. Our results suggest a waning in the monogenetic activity in ZVF over time. When compared to monogenetic fields in the Central Andes, the ZVF is marked by higher vent densities and number of phreatomagmatic landforms, with the absence of lava domes. This ultimately reflects the contrasting crustal structure and climate conditions of these two regions.

Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone

<p>In 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.</p>

Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone

<p>In 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.</p>

Palaeomagnetic secular variation recorded by lavas from the Taupo Volcanic Zone, New Zealand

<p>In order to understand the origin, temporal behaviour and spatial characteristics of Earth’s magnetic field, globally distributed records of the palaeomagnetic direction and absolute palaeointensity are required. However a paucity of data from the southern hemisphere significantly limits the resolution of global field models, particularly on short time-scales. In this thesis new, high quality palaeomagnetic data from volcanic materials sampled within the Taupo Volcanic Zone, New Zealand are presented, with a focus on the Tongariro and Okataina Volcanic Centre. New palaeomagnetic directions were obtained from 19 andesitic or rhyolitic lavas, of which 10 also produced successful palaeointensity results. Palaeointensity experiments were conducted using a combination of traditional Thellier-type thermal, and microwave techniques. Detailed magneto-mineralogical investigations carried out alongside these experiments helped to characterise the primary remanence carriers and to justify the reliability of the results. The study also revises the age controls and results from earlier palaeomagnetic studies on Holocene volcanic materials from the area. All new or revised data are summarized into a new data compilation for New Zealand, which includes 24 directions and ten palaeointensities dated between 1886 AD and 15,000 yrs BP. The new directional data reproduces the features of the most recently published continuous record from Lake Mavora (Fiordland, New Zealand), although with directions ranging in their extremes from 321° (west) to 26° (east) declination and -82 to -49° in inclination, the discrete dataset describes somewhat larger amplitude swings. With few exceptions, the new palaeointensity dataset describes a steady increase in the palaeointensity throughout the Holocene, from 37.0 ± 5.7 μT obtained from a pre-8 ka lava to 70.6 ± 4.1 μT from the youngest (≤ 500 yrs BP) flows sampled. A similar trend is also predicted by the latest global field model pfm9k. Furthermore, the data falls within the range of palaeointensity variation suggested by the Mavora record. The dataset roughly agrees with a global VADM reconstruction in the early Holocene (> 5000 yrs BP), but yields values significantly above the global trend in the late Holocene (< 1000 yrs BP) which supports the presence of significant non-dipolar components over the SW Pacific region in the time period, visible in global field models and from continuous PSV records. A comparison of the directional records with the Mavora Curve provided refinement of age estimates of five lava flows from the Tongariro Volcanic Centre, from uncertainties in the range of 2-3000 years. The new palaeomagnetic emplacement age estimates for these flows have age brackets as short as 500 years and thus highlight different phases of the young cone building eruptive activity on Ruapehu volcano.</p>

Palaeomagnetic secular variation recorded by lavas from the Taupo Volcanic Zone, New Zealand

<p>In order to understand the origin, temporal behaviour and spatial characteristics of Earth’s magnetic field, globally distributed records of the palaeomagnetic direction and absolute palaeointensity are required. However a paucity of data from the southern hemisphere significantly limits the resolution of global field models, particularly on short time-scales. In this thesis new, high quality palaeomagnetic data from volcanic materials sampled within the Taupo Volcanic Zone, New Zealand are presented, with a focus on the Tongariro and Okataina Volcanic Centre. New palaeomagnetic directions were obtained from 19 andesitic or rhyolitic lavas, of which 10 also produced successful palaeointensity results. Palaeointensity experiments were conducted using a combination of traditional Thellier-type thermal, and microwave techniques. Detailed magneto-mineralogical investigations carried out alongside these experiments helped to characterise the primary remanence carriers and to justify the reliability of the results. The study also revises the age controls and results from earlier palaeomagnetic studies on Holocene volcanic materials from the area. All new or revised data are summarized into a new data compilation for New Zealand, which includes 24 directions and ten palaeointensities dated between 1886 AD and 15,000 yrs BP. The new directional data reproduces the features of the most recently published continuous record from Lake Mavora (Fiordland, New Zealand), although with directions ranging in their extremes from 321° (west) to 26° (east) declination and -82 to -49° in inclination, the discrete dataset describes somewhat larger amplitude swings. With few exceptions, the new palaeointensity dataset describes a steady increase in the palaeointensity throughout the Holocene, from 37.0 ± 5.7 μT obtained from a pre-8 ka lava to 70.6 ± 4.1 μT from the youngest (≤ 500 yrs BP) flows sampled. A similar trend is also predicted by the latest global field model pfm9k. Furthermore, the data falls within the range of palaeointensity variation suggested by the Mavora record. The dataset roughly agrees with a global VADM reconstruction in the early Holocene (> 5000 yrs BP), but yields values significantly above the global trend in the late Holocene (< 1000 yrs BP) which supports the presence of significant non-dipolar components over the SW Pacific region in the time period, visible in global field models and from continuous PSV records. A comparison of the directional records with the Mavora Curve provided refinement of age estimates of five lava flows from the Tongariro Volcanic Centre, from uncertainties in the range of 2-3000 years. The new palaeomagnetic emplacement age estimates for these flows have age brackets as short as 500 years and thus highlight different phases of the young cone building eruptive activity on Ruapehu volcano.</p>