The Auckland Volcanic Field (AVF) consists of ~53 volcanoes, distributed over an area of ~ 360 km2. Located within the AVF is Auckland City, New Zealand’s largest population centre (~1.7M people), highlighting the need to adequately model future pre-eruption scenarios for enhanced preparedness. Geological evidence shows that past eruptions were variably explosive and formed maars, lava shields, and tuff rings, largely controlled by the extent of magma-water interaction. Any future eruption from a new vent location within the AVF would cause significant socio-economic impacts, extensive evacuations, and national-scale impacts. Our work aims to constrain the next probable vent location using novel physics-based approaches. Current approaches to identifying the next AVF vent location use statistical analysis to define probability maps. Here we use a novel approach based on physical understanding of dyke propagation and newly developed 3D numerical codes that consider crustal stresses as the main controls on the orientation of dyke pathways. We estimate regional stresses based on GNSS data and consider surface mass redistributions at the Hauraki Rift and other volcano-tectonic structures in the wider area to constrain the overall elastic stress field. We backtrack magma pathways from known vent locations downward through the crust. Pathways are oblique and reach below the Hauraki Gulf or Firth of Thames (~30 km E of Auckland City) at the inferred depth of magma dyke release (35-50 km). We infer a common magma source is physically plausible in that location. Further work will improve the robustness of the model, constrain the spatial spread of vents over time, dyke propagation velocity, and implications for early identification of future volcanic unrest.

The crustal and upper mantle structure of the Beishan orogenic collage, which serves as the southern part of the middle Central Asian Orogenic Belt, provides crucial in- sights into the history of the multiple open- ings and closings of the Paleo-Asian Ocean during the Paleozoic. There is considerable dispute over the eventual closure position, timing, and subduction polarity of the Paleo- Asian Ocean, particularly in the southern Beishan orogenic collage. The main cause of these controversies is the lack of a high resolution lithospheric structure in this area. In this study, we first present a 140-km-long, high-resolution seismic reflection profile taken across the northern Dunhuang Block and the southern Beishan orogenic collage. The seismic imaging provides new con- straints on the structure of the lower crust, the Moho, and the upper mantle beneath the southernmost Central Asian Orogenic Belt. A subhorizontal reflector in the middle crust, two sets of north-dipping reflectors from the lower crust to the upper mantle, and several south-dipping reflectors in the upper crust in the northern part of the profile were im- aged. Based on our study and other geologi- cal, chronological, and geophysical data, we propose that the two sequences of north dipping reflectors from the lower crust to the upper mantle represent two stages of northdipping subduction of the southern branch of the Paleo-Asian Ocean. The first stage in the southern Beishan orogenic collage is Late Silurian-Early Devonian, and the second stage is late Carboniferous-Early Permian. The two-stage subduction process gave rise to the Huaniushan arc and the subsequent Shibanshan arc, respectively. These findings provide new constraints on the controversial subduction polarity and the multistage amalgamation of the microcontinental blocks and arcs in the southern Central Asian Orogenic Belt.

The spatiotemporal characteristics of infrequent back‐arc rifting events and their relationships to volcanic unrest as well as other transient processes within the subduction system are not well known. We report 10 spatio‐temporal swarms of earthquakes that occurred along ∼175 km of the northern and central Taupō Volcanic Zone (TVZ) March–September 2019. The swarms lack clear mainshock‐aftershock distributions, suggesting involvement of pressurized fluids. The most energetic swarms occurred beneath the southwestern flank of Whakaari/White Island volcano (WI) and were accompanied by elevated SO 2 emissions and ∼8 mm southwest displacement of the edifice. Focal mechanisms of the WI swarms suggest horizontal dilation in the direction of tectonic extension achieved by slip along networks of closely spaced, NE‐striking normal faults linked by oblique‐slip to strike‐slip faults. Given the small GNSS displacements and upward‐migrating swarms, we favor the interpretation that magmatic volatiles were released along faults in response to changes in crustal stress. Based on the punctuated, cascading nature of swarms along the northern TVZ, we hypothesize that faults and magmatic systems are fluid‐rich, experiencing aseismic creep, and critically stressed, raising the possibility that small changes in crustal stress from slow slip along the Hikurangi subduction zone caused a reduction in the minimum horizontal stress. This change occurred along the entire northern TVZ over ∼5 months, potentially contributing to widespread volcanic unrest. The probable release of magmatic fluids from shallow magma bodies in the vicinity of WI between May and June argues against a causal relation to the 9 December 2019 WI eruption.

The Vanuatu Trench hosts tsunamigenic earthquakes exceeding magnitude 7. For these source locations, current tsunami early warning systems in Aotearoa - New Zealand are based on earthquake point-source parameters and as tsunamis propagate, initial forecasts are refined by DART (Deep-ocean Assessment and Reporting of Tsunamis) sea-level analysis. This study, which is part of the R-CET (The Rapid Characterization of Earthquakes and Tsunami) project led by GNS Science, aims to seismologically characterize the spatio-temporal behavior of the regional tsunamigenic ruptures in near real-time. This transition from basic point sources to 4-D rupture propagation could enhance the accuracy of initial tsunami threat maps and hazard response. A new array of broadband seismic stations, designated as the R-CET array, has been strategically deployed in New Zealand to support this analysis for events in the South Pacific. In this proof-of-concept study, we applied a beamforming array seismological technique to analyze the R-CET array recordings from a recent tsunamigenic earthquake (Mw 7.7) in the Vanuatu region, the Loyalty Islands, on May 19, 2023. We use sliding window fk-analysis beamforming, which can simultaneously measure backazimuth and slowness. Incorporating the sliding window, in conjunction with the fk diagrams, helps to observe temporal azimuthal changes, which facilitates tracking the rupture length and direction over time. Preliminary results showed the azimuthal and temporal variation of the rupture is in alignment with post-processing estimates of the finite fault solution for this event reported by USGS. We test the utility of this analysis in tsunami forecasts by comparing threat maps generated from the beamforming source and initial response maps used in real-time response on the day. We further compare our results to actual measured coastal cancellation gauges (tide gauges) and DART observations. We show that this analysis has the potential to improve initial tsunami forecasts prior to the onset of tsunami waves at deep ocean tsunamimeters. We further present the technique as an enticing path to meet UN Ocean Decade tsunami warning goals within our framework of ensemble and time-dependent forecasting.

The New Zealand Community Fault Model (NZ CFM) is a publicly available representation of New Zealand fault zones that have the potential to produce damaging earthquakes. Compiled through collaborative engagement between New Zealand earthquake-science experts, this first edition (version 1.0) of the NZ CFM builds upon previous compilations of earthquake-source active fault models with the addition of new and modified information. Developed primarily to support an update of the New Zealand National Seismic Hazard Model, the NZ CFM comprises two principal components. The first dataset is a two-dimensional map representation of the surface traces of 880 generalised fault zones. Each fault zone is assigned specific geometric and kinematic attributes, including uncertainties, supplemented with a subjective quality ranking focused primarily on the confidence in assigned slip rates. The second component is a three-dimensional representation of the fault zones as triangulated mesh surfaces that are projected down-dip from the two-dimensional mapped traces to a geophysically-defined maximum fault rupture depth. This article summarises the compilation and parameterisation of the NZ CFM, along with background on its relation to predecessor datasets, and forward applications to probabilistic seismic hazard assessment and physics-based earthquake models currently being developed for Aotearoa New Zealand.

The complex geology of Timor orogen is here addressed with an integrated Geological and Geophysical interpretation. At the surface, the aeroradiometric maps are correlated with the geological data to address the complex geology map. At the subsurface, the aeromagnetic and aerogravity maps are spatially and vertically interpreted and integrated with the geological data to address the geological units, structures, tectonic, and basin evolution. A case study from the central part of onshore Timor-Leste favors the overthrust model, where the upper plate is interpreted as a terrane association between Asian and Australian rock affinities, while the lower plate is the Australian Continental Margin. This complex geology is resulted from the pre-Cenozoic and Cenozoic tectonic developments, and the Miocene/Pliocene collision between the Banda Arc and the Australian plate. This interpretation is now proven with the recent Lafaek-1 exploration drilling in onshore Timor-Leste.

In small-volume volcanism, pre-existing crustal structures can influence mag-ma ascent processes. Rangitoto volcano in the Auckland Volcanic Field, New Zealand, provides a possible example of magma-structure interaction as this volcano was emplaced adjacent to an outcrop of a regional-scale basement fault, the Islington Bay Fault. In this study, we evaluated the magmatic plumbing system of Rangitoto using gravity and magnetic data acquired over the volcano and the adjacent, non-volcanic Motutapu Island. We modelled the Rangitoto internal architecture and magma plumbing using 2.5D forward and 3D inverse modelling methods. Both models are constrained by petrophysical data, while drill hole logs are only used to constrain the 2.5D model. Our model endmembers suggest that parallel magma pathways are present below the Rangitoto summit cones. In 3D magnetic models, this is evidenced by a fault-aligned pair of high-susceptibility bodies. Interpreting our models in conjunction with previously published geological and geophysical models allows us to hypothesise that the fault-parallel alignment of Rangitoto magma pathways reflects the primary influence of the Islington Bay Fault over the Rangitoto magma ascent. Magma diversion at shallow levels by other finer structures intersecting and adjacent to the fault could explain why Rangitoto erupted 3.5 km west of the Islington Bay Fault surface trace.

The Alpine Fault at the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aimed to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth. Here we present results from a seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole almost reached the fault plane. This unique 3D seismic survey includes several 2D lines and a 3D array at the surface as well as borehole recordings. Within the borehole, the unique option to compare two measurement systems is used: conventional three-component borehole geophones and a fibre optic cable (heterodyne Distributed Vibration Sensing system (hDVS)). Both systems show coherent signals but only the hDVS system allowed a recording along the complete length of the borehole.Despite the challenging conditions for seismic imaging within a glacial valley filled with sediments and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. The pre-processing of the seismic data also includes wavefield separation for the zero-offset borehole data. Seismic images are obtained by prestack depth migration approaches.Within the glacial valley, particularly steep valley flanks are imaged directly and correlate well with results from the P-wave velocity model obtained by first arrival travel-time tomography. Additionally, a glacially over-deepened trough with nearly horizontally layered sediments is identified about 0.5 km south of the DFDP-2B borehole.With regard to the expected Alpine fault zone, a set of several reflectors dipping 40-56° to the southeast are identified in a ~600 m wide zone between depths of 0.2 and 1.2 km that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at 40°, which is interpreted to be the main Alpine Fault reflector. This reflector is only ~100 m ahead from the lower end of the borehole. At shallower depths (z<0.5 km), additional reflectors are identified as fault segments and generally have steeper dips up to 56°. About 1 km south of the drill site, a major fault is identified at a depth of 0.1-0.5 km that might be caused by the regional tectonics interacting with local valley structures. A good correlation is observed among the separate seismic data sets and with geological results such as the borehole stratigraphy and the expected surface trace of the fault.In conclusion, several structural details of the fault zone and its environment are seismically imaged and show the complexity of the Alpine Fault at the Whataroa Valley. Thus, a detailed seismic characterization clarifies the subsurface structures, which is crucial to understand the transpressive fault’s tectonic processes.

ABSTRACT The glacio-fluvial sediments of the Whataroa Valley on the west coast of New Zealand’s South Island contain a record of environmental change since the Last Glacial Maximum. The valley is cut by the Australia–Pacific plate-bounding Alpine Fault, the position of which is obscured by recent glacio-fluvial outwash deposits. Five seismic profiles collected across the inferred surface trace of the Alpine Fault between 2011 and 2018, using a variety of source types, are presented here; these profiles provide constraints on the fault’s location in the upper few hundred metres of the near surface. The sedimentary strata in the Whataroa Valley have been classified into seismic facies based on their reflectivity characteristics. Two reflective seismic facies, recognisable in all profiles, have been interpreted to correspond to recent fluvial outwash gravels (seismic facies 1) and older post-glacial glaciofluvial or marine sediments (seismic facies 2). These reflective packages are cut by Alpine Fault rupture surfaces with reverse motion dips of 60° to 80° and total throw of up to 50 m. Faults imaged in three of the profiles are interpreted to correspond to the most recent surface rupture of the Alpine Fault, due to their close proximity to a recently trenched surface scarp.

The upper North Island of New Zealand has large concentrations of population and infrastructure that make it vulnerable to earthquakes on the Kerepehi and Wairoa North faults. Using a physics-based simulator, we modelled 0-50 Hz ground motions for M-w 7.3 and M-w 6.6 characteristic earthquakes on these structures. We considered the effects of low-velocity basins beneath the Hauraki Rift and the city of Hamilton that can amplify ground shaking. For a Kerepehi Fault earthquake, long period (>1s) shaking was amplified a by factor of two to three in Hamilton and towns near the Firth of Thames. Severe to violent, long duration shaking would occur close to the source with the potential to trigger liquefaction that damages flood defence networks and farmland in the Hauraki Depression. Auckland, Hamilton and Tauranga could experience moderate to very strong shaking. Impacts in Auckland are larger for a Wairoa North Fault earthquake, which could generate peak ground accelerations of 0.5g at reservoir dams in the Hunua Ranges, 0.2g at the international airport, and 0.1-0.2g at the CBD and port. Road, rail and transmission networks are vulnerable to disruption where they converge at infrastructure hotspots 10 km from the fault in South Auckland.

The Alpine Fault zone in New Zealand marks a major transpressional plate boundary that is late in its typical earthquake cycle. Understanding the subsurface structures is crucial to understand the tectonic processes taking place. A unique seismic survey including 2D lines, a 3D array, and borehole recordings, has been performed in the Whataroa Valley and provides new insights into the Alpine Fault zone down to ∼2 km depth at the location of the Deep Fault Drilling Project (DFDP)‐2 drill site. Seismic images are obtained by focusing prestack depth migration approaches. Despite the challenging conditions for seismic imaging within a sediment filled glacial valley and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged. A set of several reflectors dipping 40°–56° to the southeast are identified in a ∼600 m wide zone that is interpreted to be the minimum extent of the damage zone. Different approaches image one distinct reflector dipping at ∼40°, which is interpreted to be the main Alpine Fault reflector located only ∼100 m beneath the maximum drilled depth of the DFDP‐2B borehole. At shallower depths ( z < 0.5 km), additional reflectors are identified as fault segments with generally steeper dips up to 56°. Additionally, a glacially over‐deepened trough with nearly horizontally layered sediments and a major fault ( z < 0.5 km) are identified 0.5–1 km south of the DFDP‐2B borehole. Thus, a complex structural environment is seismically imaged and shows the complexity of the Alpine Fault at Whataroa.

Auckland Volcanic Field (AVF) is a basaltic intraplate volcanic field in North Island, New Zealand, upon which >1.6 million people live. Seismic velocity tomography and geochemistry suggest a primary mantle source region at a depth of 70-90 km. Geochemical analysis indicates a range of magma compositions, and that melts ascend with little crustal interaction. Eruptions generally began with a phreatomagmatic phase forming maar and tuff rings with tephra fall, base surges, and ballistic projectiles as the main hazards. Subsequent magmatic phases formed scoria cones, and sometimes produced lava flows. Ages of 47 of the 53 volcanic centres reveal that the AVF first erupted similar to 193 ka, and last erupted similar to 500 yrs. BP. These geochronological constraints indicate repose periods <= 0.1-13 kyr, which have decreased since similar to 60 ka. From known geological and exposure information, and using an interdisciplinary approach, eight future eruption scenarios have been developed for planning processes. Outstanding questions for the AVF concern the cause of mantle melting, the structure of the underlying lithosphere, magma ascent rates, controls on repose periods and eruptive volumes. Answering these questions may improve our understanding of warning periods, monitoring strategies, spatiotemporal risk profiles, and socio-economic impacts of volcanism on New Zealand's largest city.

The identification and characterisation of faults in urban environments is important to inform seismic and landslide hazard, yet urban development often obscures geological and geomorphological evidence of fault traces. On the other hand, urban development also generates a wealth of borehole data, which, when combined with geophysical surveys, can enable a view into the subsurface. Here we combine geomorphological and geological mapping, gravity surveying, and 3D geological modelling to identify, map and characterise several faults in Beachlands, Auckland, some of which have large offsets. Our work has identified one new fault, the Motukaraka Fault, and confirmed the presence of two proposed faults, the Waikopua North and Te Puru faults. The Motukaraka and Waikopua North faults are both steeply dipping normal faults, which strike NNW and downthrow Mesozoic basement to the west. The Motukaraka Fault has an offset of 250 m (±100 m) and the Waikopua North Fault a combined offset of 240 m (±50 m) across two parallel fault segments. The Te Puru fault strikes northeast near the northern extent of the Waikopua Fault, and downthrows Mesozoic basement to the northwest by 60–100 m. Further investigations are required to determine whether these buried faults are active.

The Alpine Fault along the West Coast of the South Island (New Zealand) is a major plate boundary that is expected to rupture in the next 50 years, likely as a magnitude 8 earthquake. The Deep Fault Drilling Project (DFDP) aims to deliver insight into the geological structure of this fault zone and its evolution by drilling and sampling the Alpine Fault at depth.Here we present results from a 3D seismic survey around the DFDP-2 drill site in the Whataroa Valley where the drillhole penetrated almost down to the fault surface. Within the glacial valley, we collected 3D seismic data to constrain valley structures that were obscured in previous 2D seismic data. The new data consist of a 3D extended vertical seismic profiling (VSP) survey using three-component receivers and a fibre optic cable in the DFDP-2B borehole as well as a variety of receivers at the surface.The data set enables us to derive a reliable 3D P-wave velocity model by first-arrival travel time tomography. We identify a 100-460 m thick sediment layer (average velocity 2200±400 m/s) above the basement (average velocity 4200±500 m/s). Particularly on the western valley side, a region of high velocities steeply rises to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3D structures implied by the velocity model on the upthrown (Pacific Plate) side of the Alpine Fault correlate well with the surface topography and borehole findings.A reliable velocity model is not only valuable by itself but it is also required as input for prestack depth migration (PSDM). We performed PSDM with a part of the 3D data set to derive a structural image of the subsurface within the Whataroa Valley. The top of the basement identified in the P-wave velocity model coincides well with reflectors in the migrated images so that we can analyse the geometry of the basement in detail.

In this companion study to Simons et al. (Bull Volcanol 82, 2020a), we examine Strombolian style explosive activity at Yasur (Vanuatu) over 11 weeks via seismic, thermal-infrared and visual observations. In part 1 of this study (Simons et al., Bull Volcanol 82, 2020a), we investigated the link between variations in the surface expression and style of volcanism at Yasur in relation to vent, shallow conduit and shallow magma reservoir processes. Now, in part 2, based on the database of 4998 explosions, we describe, define and classify Yasur’s Strombolian eruption styles and magnitudes, and compare and contrast to Stromboli and other similar centres. The styles of most of Yasur’s explosions fit with other known Strombolian eruptions, but they are 2–3 times more frequent and with higher average eruption heights than at Stromboli. Observed activity at Yasur also produced more ash than at Stromboli. The most powerful eruptive periods are dominated by type 2a explosions (ballistics and ash) and type 2b (ash-dominated) explosions. A new type of Strombolian style explosion was observed at Yasur and is here termed a type 3 explosion. They occur when vents are buried by a thick cap of loose, poorly sorted pyroclastic breccia. Type 3 explosions begin as a series of emergent ash-rich jets that rupture and fountain from the breccia cover, generating prolonged jetting of ash to form vigorous low plumes. The loose breccia cover apparently dissipates the violent early phase of the explosion. The greatest hazard of type comes from their occurrence with little warning from a smooth crater floor without an apparent vent and after longer than typical intervals between eruptions. The findings extend the observed range of Strombolian explosion styles and imply that surficial vent cover and burial may influence gas slug stability and contribute to variations in ash and ballistic output.

The New Zealand Alpine Fault is a major plate boundary that is expected to be close to rupture, allowing a unique study of fault properties prior to a future earthquake. Here we present 3‐D seismic data from the DFDP‐2 drill site in Whataroa to constrain valley structures that were obscured in previous 2‐D seismic data. The new data consist of a 3‐D extended vertical seismic profiling (VSP) survey using three‐component and fiber optic receivers in the DFDP‐2B borehole and a variety of receivers deployed at the surface. The data set enables us to derive a detailed 3‐D P wave velocity model by first‐arrival traveltime tomography. We identify a 100–460 m thick sediment layer (mean velocity 2,200 ± 400 m/s) above the basement (mean velocity 4,200 ± 500 m/s). Particularly on the western valley side, a region of high velocities rises steeply to the surface and mimics the topography. We interpret this to be the infilled flank of the glacial valley that has been eroded into the basement. In general, the 3‐D structures revealed by the velocity model on the hanging wall of the Alpine Fault correlate well with the surface topography and borehole findings. As a reliable velocity model is not only valuable in itself but also crucial for static corrections and migration algorithms, the Whataroa Valley P wave velocity model we have derived will be of great importance for ongoing seismic imaging. Our results highlight the importance of 3‐D seismic data for investigating glacial valley structures in general and the Alpine Fault and adjacent structures in particular.

This study details a case of transition of volcanic activity between two closely spaced (20-40 m) vents within the southern crater of Yasur Volcano, Vanuatu, over a period of 12 days. The transition from one vent to another newly emerged one was captured by a multi-instrument array, with broadband seismic, thermal-IR camera, and visual recordings presented. The results describe systematic changes in explosive parameters (e.g., style, power, and frequency) at both vents that reflect the transfer of magma and volatile gas flux within a shallow branching conduit system. The decline of activity at the initial vent corresponds with a shift from bomb-rich to ash-rich behavior indicating a progressive decrease in magma and volatile gas flux and increasing shallow conduit magma viscosity. The concurrent increase in activity at the emerging vent accompanies more bomb-rich behavior that indicates increased flux and lower magma viscosity. During the transition period, most explosions were systematically paired between the two vents, occurring within <1 to up to 45 s of each other. Broadband seismic data show that paired explosions produce a common seismic signal that resembles that of a single, nonpaired explosion. The presence of a common low-frequency (0.06-1 Hz) signal indicates that multiple shallow conduits may transmit the same gas and impulsive energy differently from a single gas rise and burst event. Temporal offsets between paired explosions reflect the different conduit pathways and viscosity states of the magma in each conduit, with the lagging vent always displaying greater ash-rich activity indicating higher magma viscosity and gas path tortuosity.

This study details a case of transition of volcanic activity between two closely spaced (20–40 m) vents within the southern crater of Yasur Volcano, Vanuatu, over a period of 12 days. The transition from one vent to another newly emerged one was captured by a multi‐instrument array, with broadband seismic, thermal‐IR camera, and visual recordings presented. The results describe systematic changes in explosive parameters (e.g., style, power, and frequency) at both vents that reflect the transfer of magma and volatile gas flux within a shallow branching conduit system. The decline of activity at the initial vent corresponds with a shift from bomb‐rich to ash‐rich behavior indicating a progressive decrease in magma and volatile gas flux and increasing shallow conduit magma viscosity. The concurrent increase in activity at the emerging vent accompanies more bomb‐rich behavior that indicates increased flux and lower magma viscosity. During the transition period, most explosions were systematically paired between the two vents, occurring within <1 to up to 45 s of each other. Broadband seismic data show that paired explosions produce a common seismic signal that resembles that of a single, nonpaired explosion. The presence of a common low‐frequency (0.06–1 Hz) signal indicates that multiple shallow conduits may transmit the same gas and impulsive energy differently from a single gas rise and burst event. Temporal offsets between paired explosions reflect the different conduit pathways and viscosity states of the magma in each conduit, with the lagging vent always displaying greater ash‐rich activity indicating higher magma viscosity and gas path tortuosity.