2021 US National Seismic Hazard Model for the State of Hawaii

The 2021 US National Seismic Hazard Model (NSHM) for the State of Hawaii updates the previous two-decade-old assessment by incorporating new data and modeling techniques to improve the underlying ground shaking forecasts of tectonic-fault, tectonic-flexure, volcanic, and caldera collapse earthquakes. Two earthquake ground shaking hazard forecasts (public policy and research) are produced that differ in how they account for declustered catalogs. The earthquake source model is based on (1) declustered earthquake catalogs smoothed with adaptive methods, (2) earthquake rate forecasts based on three temporally varying 60-year time periods, (3) maximum magnitude criteria that extend to larger earthquakes than previously considered, (4) a separate Kīlauea-specific seismogenic caldera collapse model that accounts for clustered event behavior observed during the 2018 eruption, and (5) fault ruptures that consider historical seismicity, GPS-based strain rates, and a new Quaternary fault database. Two new Hawaii-specific ground motion models (GMMs) and five additional global models consistent with Hawaii shaking data are used to forecast ground shaking at 23 spectral periods and peak parameters. Site effects are calculated using western US and Hawaii specific empirical equations and provide shaking forecasts for 8 site classes. For most sites the new analysis results in similar spectral accelerations as those in the 2001 NSHM, with a few exceptions caused mostly by GMM changes. Ground motions are the highest in the southern portion of the Island of Hawai’i due to high rates of forecasted earthquakes on décollement faults. Shaking decays to the northwest where lower earthquake rates result from flexure of the tectonic plate. Large epistemic uncertainties in source characterizations and GMMs lead to an overall high uncertainty (more than a factor of 3) in ground shaking at Honolulu and Hilo. The new shaking model indicates significant chances of slight or greater damaging ground motions across most of the island chain.

Post-Oruanui supereruption recovery, reconstruction and evolution of Taupo volcano, New Zealand

<p>This thesis research presents geochemical perspectives on the magmatic recovery of Taupo volcano (New Zealand) in the aftermath of the 25.4 ka Oruanui supereruption. Following the Oruanui, and after only ~5 kyr of quiescence, Taupo erupted three small volume (~0.1 km3) dacitic units, followed by another ~5 kyr break, and then the modern sequence from ~12 ka onwards of 25 rhyolitic units organised into 3 geochemically distinct subgroups (SG1-SG3). The eruptive units are stratigraphically constrained over exceptionally short time intervals, providing fine-scale temporal snapshots of the magma system. In this thesis I compare and contrast whole-rock, mineral and glass compositions of Oruanui and post-Oruanui magmas through time to investigate the post-supereruption reconstruction and evolution of Taupo through to the latest eruption. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U/Th disequilibrium model-ages in zircons from Taupo SG1 rhyolites (erupted 12 ka-10 ka) and SG2 rhyolites (erupted 7 ka-2.6 ka) imply the presence of only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains. U-Pb dating of these equiline grains shows that both 300-450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units which were vented within the published Whakamaru caldera outline. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolites have notably high concentrations of U, Th, P, Y+ (REE)3+ and Nb but with only minor changes in Hf and Ti. SG2 zircons typically have higher Sc, reflecting large-scale changes in melt chemistry and crystallising mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U-Th ages from individual post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallisation, acting within a crustal protolith chemically independent of that which built the Oruanui. Cooling periods alternate with times of rejuvenation and eruption, in some cases demonstrably accompanying syn-eruptive regional rifting and mafic injection. Not only were the processes that developed the supersized Oruanui magma body unusually rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. Major and trace element whole rock, glass and mineral chemistry of post-Oruanui eruptive products indicate how the host magma system re-established and evolved. The dacite units show wide variations in melt inclusion compositions and strongly zoned minerals consistent with interaction of less-evolved mafic magmas at a depths of >8 km, overlapping with the inferred base of the old Oruanui mush system. The dacites reflect the first products of the rebuilding silicic magma system, as most of the Oruanui mush was reconfigured or significantly modified in composition following thermal fluxing accompanying post-caldera collapse readjustment. The first (SG1) rhyolites erupted from 12 ka formed through shallow fractionation (4-5 km depth) and cooling of a parental melt similar in composition to the earlier dacite melts, with overlapping melt inclusion and crystal core compositions between the two magma types. For the younger rhyolite units, fine-scale temporal changes in melt chemistry and mineral phase stability occur over time, which are closely linked to the development, stabilisation and maturation of a new and likely unitary rhyolite mush system at Taupo. The new mush system is closely linked to and sometimes physically interacts with the underlying mafic melts, which are similar in composition to those involved in the Oruanui eruption and provide the long-term thermal and chemical driving force for magmatism. We consider that the new mush body has expanded to >250 km3 (and possibly up to 1000 km3) but has not yet been located by geophysical investigations. For the most recent SG3 eruptions, the system once again underwent widespread destabilisation, resulting in increased levels of melt extraction from the silicic mush. Trends in whole-rock chemistry and close links between melt inclusions and mineral zoning with earlier units indicates that the 35 km3 Unit Y (Taupo eruption) melt dominant body formed in response to mafic disruption of the silicic mush pile. Associated Fe-Mg diffusion timescales in orthopyroxene suggest that Taupo is capable of changing behaviour and generating large eruptible melt bodies on timescales as short as decades to centuries. The 232 AD Unit Y eruption culminated from a critical combination of high differential tectonic stress build up, and increased potency in the silicic magma system resulting from elevated levels of mafic magma input, resulting in one of the largest and most violent worldwide Holocene eruptions. The post-Y magma system then responded to further disruption with the eruption of sub-lacustrine dome(s). Taupo is considered to be capable of rapidly recovering in its modern form to continue its hyperactive eruptive behaviour on timescales that are of human interest and concern.</p>

Post-Oruanui supereruption recovery, reconstruction and evolution of Taupo volcano, New Zealand

<p>This thesis research presents geochemical perspectives on the magmatic recovery of Taupo volcano (New Zealand) in the aftermath of the 25.4 ka Oruanui supereruption. Following the Oruanui, and after only ~5 kyr of quiescence, Taupo erupted three small volume (~0.1 km3) dacitic units, followed by another ~5 kyr break, and then the modern sequence from ~12 ka onwards of 25 rhyolitic units organised into 3 geochemically distinct subgroups (SG1-SG3). The eruptive units are stratigraphically constrained over exceptionally short time intervals, providing fine-scale temporal snapshots of the magma system. In this thesis I compare and contrast whole-rock, mineral and glass compositions of Oruanui and post-Oruanui magmas through time to investigate the post-supereruption reconstruction and evolution of Taupo through to the latest eruption. Despite overlapping vent sites and crustal source domains between the Oruanui and post-Oruanui eruptions, U/Th disequilibrium model-ages in zircons from Taupo SG1 rhyolites (erupted 12 ka-10 ka) and SG2 rhyolites (erupted 7 ka-2.6 ka) imply the presence of only minor inheritance of crystals from the Oruanui magma source. Post-Oruanui model-age spectra are instead typically centred close to eruption ages with subordinate older pre-300 ka equiline grains. U-Pb dating of these equiline grains shows that both 300-450 ka plutonic-derived and pre-100 Ma greywacke basement-derived zircons are present. The former largely coincide in age with zircons from the 350 ka Whakamaru eruption products, and are dominant over greywacke in young units which were vented within the published Whakamaru caldera outline. Despite multiple ages and vent sites, trace element compositions are broadly similar in zircons, regardless of their ages. However, a small subset of zircons analysed from SG1 rhyolites have notably high concentrations of U, Th, P, Y+ (REE)3+ and Nb but with only minor changes in Hf and Ti. SG2 zircons typically have higher Sc, reflecting large-scale changes in melt chemistry and crystallising mineral phases with time. The age spectra indicate that most Oruanui zircons were removed by thermally induced dissolution immediately following the supereruption. U-Th ages from individual post-Oruanui eruptions show consistent inheritance of post-Oruanui grains with model ages that centre between the temporally separated but geographically overlapping eruption groups, generating model-age modes. Within the statistical limitations of the isotopic measurements, we interpret these repeated modes to be significant, resulting from incorporation of crystal populations from cyclic post-Oruanui periods of magmatic cooling and crystallisation, acting within a crustal protolith chemically independent of that which built the Oruanui. Cooling periods alternate with times of rejuvenation and eruption, in some cases demonstrably accompanying syn-eruptive regional rifting and mafic injection. Not only were the processes that developed the supersized Oruanui magma body unusually rapid, but this huge magma system was effectively reset and rebuilt on a comparably short timescale. Major and trace element whole rock, glass and mineral chemistry of post-Oruanui eruptive products indicate how the host magma system re-established and evolved. The dacite units show wide variations in melt inclusion compositions and strongly zoned minerals consistent with interaction of less-evolved mafic magmas at a depths of >8 km, overlapping with the inferred base of the old Oruanui mush system. The dacites reflect the first products of the rebuilding silicic magma system, as most of the Oruanui mush was reconfigured or significantly modified in composition following thermal fluxing accompanying post-caldera collapse readjustment. The first (SG1) rhyolites erupted from 12 ka formed through shallow fractionation (4-5 km depth) and cooling of a parental melt similar in composition to the earlier dacite melts, with overlapping melt inclusion and crystal core compositions between the two magma types. For the younger rhyolite units, fine-scale temporal changes in melt chemistry and mineral phase stability occur over time, which are closely linked to the development, stabilisation and maturation of a new and likely unitary rhyolite mush system at Taupo. The new mush system is closely linked to and sometimes physically interacts with the underlying mafic melts, which are similar in composition to those involved in the Oruanui eruption and provide the long-term thermal and chemical driving force for magmatism. We consider that the new mush body has expanded to >250 km3 (and possibly up to 1000 km3) but has not yet been located by geophysical investigations. For the most recent SG3 eruptions, the system once again underwent widespread destabilisation, resulting in increased levels of melt extraction from the silicic mush. Trends in whole-rock chemistry and close links between melt inclusions and mineral zoning with earlier units indicates that the 35 km3 Unit Y (Taupo eruption) melt dominant body formed in response to mafic disruption of the silicic mush pile. Associated Fe-Mg diffusion timescales in orthopyroxene suggest that Taupo is capable of changing behaviour and generating large eruptible melt bodies on timescales as short as decades to centuries. The 232 AD Unit Y eruption culminated from a critical combination of high differential tectonic stress build up, and increased potency in the silicic magma system resulting from elevated levels of mafic magma input, resulting in one of the largest and most violent worldwide Holocene eruptions. The post-Y magma system then responded to further disruption with the eruption of sub-lacustrine dome(s). Taupo is considered to be capable of rapidly recovering in its modern form to continue its hyperactive eruptive behaviour on timescales that are of human interest and concern.</p>

The Interrelationships between Faulting and Volcanism in the Okataina Volcanic Centre, New Zealand

<p>Continental rifts show close spatial relations between faulting and volcanism, however the interrelations between each process and their roles in the accommodation of regional extension are not well understood. The geometric and kinematic relations between an active silicic caldera complex and active faults in the upper 3-4 km of the crust (i.e. Taupo Rift) are investigated using regional gravity data, digital elevation models, outcrop mapping, seismic reflection lines, focal mechanisms and an historical account of the 1886 AD Tarawera eruption adjacent to, and within, the Okataina Volcanic Centre, New Zealand.The location and geometry of the Okataina Caldera were influenced by pre-existing faults. The caldera is elongate north-south, has a maximum subsidence of 3 +/- 0.5 km at the rift axis and occupies a 10 km hard-linked left step in the rift. The principal rift faults (55-75 degrees dip) define the location and geometry of the northwest and southeast margins and locally accommodate piecemeal caldera collapse. Segments of the east and west margins of the caldera margin are near vertical (70-90 degrees dip), trend north-south, and are inferred to be faults formed by the reactivation of a pervasive Mesozoic basement fabric (i.e. bedding, terrane boundaries, and/or faults). Measured displacements along the Paeroa and Whirinaki Fault zones in, and adjacent to, the Okataina Volcanic Centre took place over time periods ranging from 60 to 220 ka (together with historical accounts of the 1886 AD Tarawera eruption). These indicate that neither dike intrusion nor caldera collapse have a measurable influence on fault displacement rates outside the volcanic complex. Within the volcanic complex, vertical displacement along the Whirinaki Fault zone increases by up to 50% between the caldera topographic margin and inner collapse boundary. This increase in vertical displacement is predominantly due to the collapse of the caldera 60 ka ago. In the Okataina Volcanic Centre, extension is accommodated by a combination of tectonic faulting, dike intrusion, and gravitational caldera collapse. Gravitational caldera collapse is however, superimposed on regional extension without contributing to it. Rift-orthogonal extension dominates across the Taupo Rift with a minor (</= 20 degrees) component of right-lateral slip increasing northwards. The regional principal horizontal extension direction rotates 30 degrees clockwise south to north along the rift. The modal principal horizontal extension direction for the Okataina Volcanic Centre trends ~145 degrees, approximately normal to northeast striking rift faults and intra-caldera linear vent zones, and oblique to north-south faults. Zones of crustal weakness, brittle deformation, and dilation at the intersections of northeast-southwest dip slip and north-south oblique slip active fault sets are inferred to locally promote the ascent of magma. Preliminary examination of volcanism outside the Okataina Volcanic Centre suggests that intersecting northeast-southwest and north-south fault sets may also play a role in defining the geometry of calderas and locations of volcanic centres throughout the Taupo Volcanic Zone. Outside these volcanic centres (e.g. Taupo and Okataina) active extension is primarily accommodated by normal faulting which is driven by tectonic processes (e.g. far-field plate motions) and is not attributed to dike intrusion. The Taupo Rift has not yet reached the stage where it is dominated by magma-assisted extension and is primarily a young tectonic rift in an arc environment.</p></p>

The Interrelationships between Faulting and Volcanism in the Okataina Volcanic Centre, New Zealand

<p>Continental rifts show close spatial relations between faulting and volcanism, however the interrelations between each process and their roles in the accommodation of regional extension are not well understood. The geometric and kinematic relations between an active silicic caldera complex and active faults in the upper 3-4 km of the crust (i.e. Taupo Rift) are investigated using regional gravity data, digital elevation models, outcrop mapping, seismic reflection lines, focal mechanisms and an historical account of the 1886 AD Tarawera eruption adjacent to, and within, the Okataina Volcanic Centre, New Zealand.The location and geometry of the Okataina Caldera were influenced by pre-existing faults. The caldera is elongate north-south, has a maximum subsidence of 3 +/- 0.5 km at the rift axis and occupies a 10 km hard-linked left step in the rift. The principal rift faults (55-75 degrees dip) define the location and geometry of the northwest and southeast margins and locally accommodate piecemeal caldera collapse. Segments of the east and west margins of the caldera margin are near vertical (70-90 degrees dip), trend north-south, and are inferred to be faults formed by the reactivation of a pervasive Mesozoic basement fabric (i.e. bedding, terrane boundaries, and/or faults). Measured displacements along the Paeroa and Whirinaki Fault zones in, and adjacent to, the Okataina Volcanic Centre took place over time periods ranging from 60 to 220 ka (together with historical accounts of the 1886 AD Tarawera eruption). These indicate that neither dike intrusion nor caldera collapse have a measurable influence on fault displacement rates outside the volcanic complex. Within the volcanic complex, vertical displacement along the Whirinaki Fault zone increases by up to 50% between the caldera topographic margin and inner collapse boundary. This increase in vertical displacement is predominantly due to the collapse of the caldera 60 ka ago. In the Okataina Volcanic Centre, extension is accommodated by a combination of tectonic faulting, dike intrusion, and gravitational caldera collapse. Gravitational caldera collapse is however, superimposed on regional extension without contributing to it. Rift-orthogonal extension dominates across the Taupo Rift with a minor (</= 20 degrees) component of right-lateral slip increasing northwards. The regional principal horizontal extension direction rotates 30 degrees clockwise south to north along the rift. The modal principal horizontal extension direction for the Okataina Volcanic Centre trends ~145 degrees, approximately normal to northeast striking rift faults and intra-caldera linear vent zones, and oblique to north-south faults. Zones of crustal weakness, brittle deformation, and dilation at the intersections of northeast-southwest dip slip and north-south oblique slip active fault sets are inferred to locally promote the ascent of magma. Preliminary examination of volcanism outside the Okataina Volcanic Centre suggests that intersecting northeast-southwest and north-south fault sets may also play a role in defining the geometry of calderas and locations of volcanic centres throughout the Taupo Volcanic Zone. Outside these volcanic centres (e.g. Taupo and Okataina) active extension is primarily accommodated by normal faulting which is driven by tectonic processes (e.g. far-field plate motions) and is not attributed to dike intrusion. The Taupo Rift has not yet reached the stage where it is dominated by magma-assisted extension and is primarily a young tectonic rift in an arc environment.</p></p>