Calum John Chamberlain

Eruptions in the Auckland Volcanic Field in Aotearoa New Zealand have not occurred since seismic monitoring began in Aotearoa New Zealand, but it is generally presumed that future eruptions from the Auckland Volcanic Field will be preceded by elevated rates of seismicity. High levels of anthropogenic seismic noise, generally low numbers of earthquakes and relatively high earthquake detection thresholds used for nationwide earthquake monitoring by the Aotearoa New Zealand national earthquake monitoring system (GeoNet) mean that the national earthquake catalogue used for volcanic monitoring contains few earthquakes in the Auckland Volcanic Field. Moreover, variations in cataloguing methods throughout the history of seismic monitoring mean that the quality and completeness of the GeoNet earthquake catalogue in Auckland are strongly heterogeneous in time precluding robust analysis of earthquake occurrence variability within the Auckland Volcanic Field and hence inhibit the identification of possible precursory changes from that background state. We apply a semi‐automated workflow, incorporating modern machine‐learning earthquake detection methods validated by manual review, to develop a self‐consistent catalogue of seismicity in the Auckland Volcanic Field spanning 2011–2022 which we use to evaluate the background state of seismicity within the Auckland Volcanic Field in a time of quiescence. This catalogue contains 328 earthquakes within the Auckland Volcanic Field monitoring region, more than five times as many earthquakes as catalogued by GeoNet. The new catalogue is composed mostly of background seismicity with some discrete clusters. We confirm that the background rate of seismicity in Auckland is low (approximately 21 earthquakes annually above magnitude 0.6: the magnitude of completeness of this new catalogue) and provide a statistically useful indication of that background state, both in time and space, from which to characterise possible deviations prior to a future eruption within the Auckland Volcanic Field.

Distributed acoustic Sensing (DAS) data collected along a 30 km length of telecommunications fiber crossing the Alpine Fault near Haast enable analysis of interactions between fluvioglacial and seismotectonic processes. Here we use DAS recordings of 25 earthquakes to probe near‐surface structure beneath the Haast river valley. For each earthquake, delayed P‐ and S‐wave arrivals incompatible with a regional velocity model are observed at common locations. We show using a planar slab model that these delayed arrivals are proportional to the thickness of subjacent low‐velocity structures. 3D ray‐tracing reveals basin‐like low‐velocity features of approximately ∼1–4 km width and 200–650 m depth, consistent with independent seismic and gravity measurements collected along partly co‐located lines. We infer these low‐velocity structures to be sediment‐filled erosional basins that may exacerbate ground shaking in large earthquakes. The method developed here is general and highlights the potential of dense DAS measurements to provide high‐resolution subsurface characterization.

The early aftershock period can provide information on the mainshock source and on the time-varying hazard during an aftershock sequence. However, early aftershock catalogs are often highly incomplete and inaccurate due to elevated rates of aftershocks and high-background seismic amplitudes from overlapping event waveforms. Template-matching (matched-filter) methods, based on cross-correlation of known earthquake waveforms with continuous data, have been shown to significantly improve the completeness of early aftershock catalogs and can generate automatic high-quality phase-arrival information. Until recently, applying matched-filters in real time was not tractable. By applying and updating recently developed robust and efficient near-real-time open-source matched-filter workflows and tools (RT-EQcorrscan), we provide improved real-time location and magnitude calculation processes to characterize the earthquake source during earthquake rapid responses. Here, we outline the development, testing, and application of our methods and demonstrate the capability of RT-EQcorrscan and new associated workflows to enhance rapid aftershock catalogs and source characterization. We provide examples of these methods as applied to simulated real-time scenarios of recent M 6+ sequences across Aotearoa New Zealand: the M 6.2 Eketahuna, M 6.5 Cook Strait, and M 7.8 Kaikōura earthquakes. Our results show that the distribution of detected and relocated aftershocks, estimated in near-real time, can be used to estimate source fault geometry (strike, dip, length, and width) and scaled magnitude within a few minutes to an hour of a trigger event, depending on source complexity and background seismicity rates (available templates). We also discuss several nuances that distinguish real-time applications of matched-filters from more commonplace retrospective analyses, thus highlighting considerations for applying those real-time tools during a response. Our outputs—both updated plug-ins for RT-EQcorrscan, alongside tools for quantification of fault geometry from aftershock distributions—provide valuable resources for use alongside other source characterization methods during earthquake response.

The Alpine Fault (Aotearoa New Zealand) is a major plate boundary transform fault, that quasi‐periodically hosts large (M7‐8) to great (M8+) earthquakes. The fault is thought to be segmented with sections rupturing both individually and in combination. Surface mapping indicates the fault changes from near‐vertically orientated in the South‐Westland Section, to dipping ∼45–60° SE in the Central Section, but it is unclear how this change is accommodated in the subsurface. We use receiver functions generated from long‐term seismic stations and a temporary array to image the subsurface at this section boundary. Using velocity inversion and common conversion point stacking, we find there are distinct crustal structures on either side of the fault trace. The subvertical fault likely extends northeastward past the section boundary and two fault structures may co‐exist beyond the section boundary. This geometry is consistent with microseismicity and previous tectonic studies, and helps to explain through‐going fault ruptures.

The Alpine fault is a major transpressive plate boundary in southwestern New Zealand known on paleoseismological grounds to produce great (Mw∼8) earthquakes on <300 yr timescales. The fault is now late in its typical and well-determined interseismic period, having last ruptured in a great earthquake in 1717 C.E. Between October 2021 and November 2023, the Southern Alps Long Skinny Array (SALSA) consisting of 43 broadband and 4 short-period seismometers, was installed with ∼10 km interstation spacing along a 450 km stretch of the Alpine fault, between Maruia and Milford Sound/Piopiotahi. SALSA’s geometry was designed to enable the synthesis of virtual earthquakes representing slip at arbitrary points on the Alpine fault as part of research into the range of far-field ground motions that might result from geologically plausible rupture scenarios. Virtual earthquakes provide a means of evaluating ground motions from a much larger number of kinematic rupture scenarios than is computationally tractable using full-waveform simulations. In addition to providing closely spaced noise measurements, SALSA has yielded novel recordings of microseismicity and tremor along most of the onshore extent of the Alpine fault and will enable ongoing research into the present-day state of the fault and future rupture behavior. Here, we describe the rationale for deploying SALSA and present initial results and analysis that illustrate the dataset and indicate avenues for future research.

Geodetic observations are uncovering increasing numbers of slow-slip events (SSEs), prompting research into the relationships between slow slip and varying forms of seismicity. Such studies are often limited to retrospectively searching for seismicity near to individual SSEs in time and space, disregarding similar activity without SSEs occurring nearby. We perform a systematic examination of the relationship between seismicity and slow slip at the Hikurangi subduction zone (Aotearoa New Zealand) using new and more complete catalogs of earthquakes and SSEs spanning a period of 14 yr. A novel clustering algorithm utilizing a spatiotemporally varying magnitude threshold is proposed to detect diverse seismic sequences. Subsequent k-means clustering in high dimensions classifies each identified sequence as seismic swarm or mainshocks–aftershocks type. We then assess the mutual information between slow slip and seismicity in spatially defined macroclusters (clusters of clusters), revealing significant temporal correlations that vary in nature by spatial region and type of seismic sequence. The diversity in spatial and temporal correlations highlights a complex relationship between slow slip and seismicity, which is likely related to heterogeneity in slow-slip occurrence and geological conditions, and possibly to variations in the driving forces behind slow slip along the Hikurangi.

Understanding the conditions and mechanisms that produce earthquakes is of high scientific relevance. The central Southern Alps/Kā Tiritiri o te Moana of New Zealand is an active orogen that offers a unique opportunity to study the processes that drive seismogenesis. A new high‐quality, matched‐filter‐based microseismicity catalog has been constructed by following a fully automated detection and location process. The catalog spans from 2009 to 2020 with local magnitudes between −1.81 and 3.42. Our results indicate a seasonal pattern in earthquake occurrence, with a notable correlation between shallow‐depth earthquakes (6 km hypocentral depths) and large, mostly summer, rainfall events beneath three separate glaciers. We hypothesize that a combination of seasonal snowmelt and heavy rainfall events raises the water table during spring and summer, changing pore‐fluid pressure in the upper crust and triggering earthquakes. Our findings highlight the role of extreme rainfall and glacier dynamics in triggering shallow earthquakes, which can improve hazard assessment in the central Southern Alps/Kā Tiritiri o te Moana and similar alpine regions worldwide.

©2019. American Geophysical Union. All Rights Reserved. The 14 November 2016 Mw7.8 Kaikōura earthquake in the northern South Island, New Zealand, involved highly complex, multifault rupture. We combine data from a temporary network and the permanent national seismograph network to repick and relocate ~2,700 aftershocks of M≥3 that occurred between 14 November 2016 and 13 May 2017. Automatic phase-picking is carried out using REST, a newly developed hybrid method whose pick quality is assessed by comparing automatic picks for a subset of 138 events with analysts' picks. Aftershock hypocenters computed from high-quality REST picks and a 3-D velocity model cluster almost exclusively in the shallow crust of the upper plate and reveal linkages at depth between surface-rupturing fault segments. Only eight aftershocks are relocated on a deeper structure positioned between patches of geodetically detected afterslip. This indicates that afterslip has not triggered significant earthquake activity on the subduction interface during the period of aftershock activity analyzed.

©2018. American Geophysical Union. All Rights Reserved. The Alpine Fault is an oblique strike-slip fault that is known to fail in large magnitude (M7–8) earthquakes, yet it is currently seismically quiescent. We examine the low-magnitude earthquake activity occurring along the central portion of the Alpine Fault using seismic data from five temporary seismic networks deployed for various lengths of time between late 2008 and early 2017. Starting from continuous seismic data, we detect earthquake arrivals and construct the longest and most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes. This enables us to study the distribution and characteristics of the seismicity in unprecedented detail. Earthquake locations are constrained by high-quality automatic and manual picks, and we perform relocations using waveform cross-correlation to better constrain hypocenters. We have derived a new local magnitude scale calibrated by M w values. Magnitudes range between M L −1.2 and 4.6, and our catalog is complete above M L 1.1. Earthquakes mainly occur southeast of the Alpine Fault (in the hanging wall) and exhibit low magnitudes. We observe a lack of seismicity beneath Aoraki/Mount Cook, which we associate with high uplift rates and high heat flow. Seismogenic cutoff depths vary along the strike of the Alpine Fault from 8 km, beneath the highest topography, to 20 km in the adjacent areas.

We report the first catalog of low‐frequency earthquakes in the Hikurangi subduction zone, located beneath the Kaimanawa Range of the North Island at 50 km depth, downdip of regularly recurring (every 4–5 years) deep M7 slow slip events. To systematically detect low‐frequency earthquakes within the regional continuous seismic data, we utilized a matched‐filter approach with template waveforms derived from previous observations of tectonic tremor. We built our catalog of 36 low‐frequency earthquake sources, that produced almost 21,000 events over more than a decade, with two matched‐filter search iterations. In each iteration, the detections were gathered into families and their coherent waveforms processed and stacked to extract high‐quality waveforms, allowing us to pick seismic phase arrivals to locate the low‐frequency earthquakes. We highlight three characteristic features to validate that our detected events are indeed low‐frequency earthquakes: the eponymous deficit of high frequencies in their seismic waveforms, the episodic swarms of activity that define their activity through time, and their location at the plate boundary with a double‐couple source mechanism and geometry consistent with the subduction interface. Considering the observed low‐frequency earthquakes' relationship to neighboring slow slip, we observe the event swarms to occur much more frequently than the M7 slow slip events located just updip. Similar to other deep low‐frequency earthquakes in other subduction zones, we suggest that this characteristic clustering in time is driven by more frequent, smaller slow slip events that are not clearly observable at the surface.