Pascal Audet

In November 2021, the Canadian National Facility for Seismological Investigations launched its first long-term test of the new Güralp Aquarius broadband ocean-bottom seismometers (OBS). The study targeted the lower Laurentian Fan seismic zone (∼4.5 km water depth), a seismically active zone of oceanic lithosphere downslope from the epicenter of the 1929 Grand Banks earthquake (Ms 7.2) offshore Atlantic Canada. A total of 12 OBS were deployed, of which eight were recovered during an August 2022 recovery cruise, with three months of data suitable for analysis. Because of the absence of Global Positioning System clock synchronization, we performed ambient noise cross-correlation to assess clock drift; seven OBS displayed negligible drift, but one OBS clock had a relative drift of 1.14 s over the three months and was corrected before further analyses. We estimated the orientations of horizontal OBS components using particle-motion analysis of P waves from four teleseismic earthquakes (Mw ≥ 6.8) and their tilt angles using a noise-based transfer-function technique. We then scanned the data for local-to-regional earthquakes using machine learning-based phase picking and automated association tools. Seventeen earthquakes were identified, including two detected by Earthquakes Canada, with estimated local magnitudes up to 2.2 and depths reaching ∼39 km relative to the seafloor. Seismic data also revealed baleen whale calls in the 15–25 Hz range. Using the double-spectrogram method, we detected fin and blue whale vocalizations. Applying a hybrid double-pair double-difference method with cross-correlation delay times, we provide one example 24-hr blue whale track, spanning ∼58 km.

Divergent plate boundaries drive magmatism and plate spreading, yet their role in whole-mantle convection remains unclear, as most systems are submarine. When spreading centres subduct, slab windows open pathways for asthenospheric and potentially deeper mantle upwelling. Here we use 20 years of teleseismic P-to-S receiver functions from 429 stations to map the 410 and 660 km discontinuities beneath western Canada. Common-conversion-point stacking reveals coherent double depressions across a slab window formed by the subducted Resurrection/Farallon Ridge system, in contrast to the adjacent craton. Mineral physics and seismic tomography constraints indicate mantle transition zone temperature anomalies of 100-300 K that follow the slab-window geometry. Monte Carlo estimates of constrained whole-mantle geotherms further suggest a warmer-than-average lower mantle. Together, these observations suggest that the fossil slab window provides a conduit for lower-mantle heat into the upper mantle and lithosphere, contributing to long-term uplift of the Canadian Cordillera.

Divergent plate boundaries are major sites of magmatism and plate creation, yet their role in whole-mantle convection remains poorly constrained because most mid-ocean ridges lie beneath the oceans. When spreading centres are subducted, slab windows open pathways for asthenospheric and potentially deeper mantle upwelling. Here we use 20 years of teleseismic P-to-S receiver function data from 429 broadband stations to image the 410 km and 660 km discontinuities beneath western Canada. Common-conversion-point stacking reveals coherent depressions of both d410 and d660 across a well-documented slab window generated by subduction of the Resurrection/Farallon ridge, in sharp contrast to the adjacent craton. Using mineral-physics constraints on Clapeyron slopes and an ensemble of global and regional tomography models, we infer MTZ temperature anomalies of order 100–300 K and show that these correlate closely with the reconstructed slab-window geometry. A representative P-wave model (DETOX-P01) further indicates a sharp transition from this hot MTZ to a colder, high-velocity body in the mid–lower mantle, consistent with a detached segment of the subducted Resurrection plate embedded in a warmer-than-average deep mantle. Monte Carlo geotherm calculations suggest that the lower mantle beneath the Cordillera is on average ~150–200 K hotter than global reference profiles. Together, these observations suggest that a fossil divergent boundary, expressed as a slab window, provides a conduit for lower-mantle heat into the upper mantle and lithosphere, and that this deep thermal input contributes to the long-term uplift of the Canadian Cordillera.

The absence of plate tectonics and the young surface age (0.3-1 billion years) of Venus have led to diverse geodynamic models for Venus. The energetics of the Venusian interior drives these models; however, the lack of direct constraints on surface heat flow hampers their quantitative assessment. Here we present a global heat flow map for Venus, as well as estimates of the total heat loss, obtained from an inversion of geophysical data, including lithospheric effective elastic thickness, crustal thickness, and radioactive heat production. Heat flow on Venus is lower and less geographically structured than on Earth, with an average of 31 mW m ^—2 , but with highs associated to rifts systems reaching values typical of active terrestrial areas. The obtained total heat loss is 11-17 TW, similar to estimates of the total radioactive heat production. Therefore, at present, Venus proportionally dissipates much less heat than Earth.

The geology of New Zealand has been shaped by tectonic plate interactions driven by mantle convection over the past 60 million years, but the effects of these interactions on the transition to the lower mantle are not yet well understood. We analyze 10 years of teleseismic ‐wave receiver functions using common conversion point stacking to investigate mantle transition zone discontinuities. The resulting topography of the 410‐km and 660‐km discontinuities, smoothed using kriging interpolation, reveals localized thermal anomalies. We observe more than 10 km of thinning in the mantle transition zone beneath central North Island coincident with the Hikurangi slab, accompanied by a significantly thickened and hydrous melt‐rich layer above the 410‐km discontinuity. In northern South Island, the 410‐km discontinuity is uplifted by approximately 15 km, likely reflecting subducted materials from the oblique Hikurangi slab material reaching mantle transition zone depths. These observations are consistent with patterns of seismicity and seismic velocity anomalies from tomographic models. Additionally, we identify significant thinning of the mantle transition zone beneath both the Northland region (10 km) and the Great South Basin (15 km), indicating the influence of potential local thermal upwellings. The different slab polarities and transpressional boundary may drive lateral mantle flow, shaping the complex transition zone topography beneath New Zealand.

Earthquakes abruptly release tectonic stress that builds slowly over time through the coupled evolution of faults and the surrounding crust. Seismic wavespeeds track crustal deformation and stress changes, but typical monitoring methods are most sensitive to shallow depths. Using receiver functions, we tracked rupture-zone wavespeed and anisotropy changes throughout the crust during the 2019 Ridgecrest earthquake sequence. Shallow coseismic wavespeed reductions recovered within months, whereas a deeper postseismic wavespeed drop persisted without measurable recovery over several years. The deep, persistent wavespeed drop likely reflects accumulating damage driven by postseismic deformation, suggesting two possible scenarios: (i) a slow interseismic recovery where wavespeed and anisotropy track long-term stress evolution; or (ii) permanent deformation of an immature fault zone. Both scenarios affect the dynamics and energy budget of the seismic cycle.

Slow-slip events at global subduction zones relieve tectonic stress over days to years. Through slow-slip cycles, high fluid pressures observed at the top of subducting plates are thought to fluctuate, potentially due to the valving action of an impermeable layer near the plate interface. We model teleseismic scattering data at the Manawatu deep slow-slip patch at the Hikurangi margin in New Zealand and find high seismic P-to-S wave velocity ratios, V_P/V_S, in the upper ~5 km of the subducting Pacific Plate, reflecting sustained elevated fluid pressures that decrease during slow-slip and increase during inter-slow-slip periods. Within a ~ 3 km thick lower crustal layer of the overriding Australian Plate, decreasing V_P/V_S during inter-slow-slip periods reflects permeability reduction due to mineral precipitation. Increasing V_P/V_S during slow-slip reflects increasing permeability and crack density, facilitating upward fluid transfer through this layer. Our results suggest it acts as a valve to relieve high fluid pressures in the subducting slab.

The Canadian Cordillera marks a transition region from the current plate boundary through the Phanerozoic Cordilleran orogen to the Precambrian cratons. Knowledge of the subsurface structure of western Canada has been greatly advanced by seismological investigations during the past two decades, pioneered by the Lithoprobe project and, more recently, by regional passive seismic arrays. In this study, we construct a new model (WCANM22) of crustal thickness and P‐ to S‐wave velocity ratio, or V_p/V_s, by compiling receiver function data from 473 stations and existing constraints from over 2,600‐km long active source experiments. Our model covers a broad swath (about 1/4) of the land area of North America (105°–140°W, 48°–72°N) and shows an overall flat Moho beneath the Cordillera with an average depth of ∼36 km and a standard deviation of 3 km across orogenic belts. This study provides a comprehensive catalog of V_p/V_s in western Canada and reveals a moderate correlation between Poisson's ratio and the age of crustal domains. The average V_p/V_s values are 1.72, 1.79, and 1.82 for the Phanerozoic Cordillera, Proterozoic Cratons, and Archean‐aged Medicine Hat Block, respectively, suggesting continued modifications to crustal composition through episodic tectonothermal events. This distinct trend in western Canada sheds new light on the debated role of secular changes in the composition of continental crust.

SUMMARY: Seismic velocity models provide important constraints on Greenland’s deep structure, which, in turn, has profound implications for our understanding of the tectonic history of this region. However, the resolution of seismic models has been limited by a sparse network, particularly in northern and central Greenland. We address these limitations by generating new high-resolution Rayleigh-wave phase velocity maps encompassing Greenland and northeastern Canada by processing over three decades of teleseismic earthquake records and incorporating recently added stations in Greenland and Arctic Canada. These phase velocity maps are sensitive to structure from the lower crust down to the sub-lithospheric mantle (25–185 s period). We find significant heterogeneity and a strong correlation between isotropic and anisotropic seismic velocities with inferred geological structure. High seismic velocities associated with cratonic lithosphere are broadly divided into two regions, with a belt of reduced velocity spanning central Greenland, which we interpret as lithospheric erosion resulting from interaction between the Greenland continental keel and the Iceland plume. Within each region, we identify tectonic subdivisions that suggest fundamental differences between the blocks that make up Precambrian Greenland. In the south, the North Atlantic craton (NAC) has a high-velocity keel exhibiting anisotropic stratification. Between the NAC and the cratonic lithosphere further north, the Proterozoic Nagssugtoqidian orogenic belt shows a distinct signature of reduced seismic velocity to $\sim$75 s period, but then appears to pinch out at depth. The northern Greenland lithosphere exhibits significant isotropic heterogeneity, with a distinct core of high velocities in the northwest ($\sim$55–75 s period) giving way to a set of distinct east-west trending high-velocity belts at longer periods. At all periods sensitive to the lithospheric mantle in this region, anisotropic fast orientations are E–W, consistent with a north–south Precambrian assembly of the Greenland shield. In contrast to the NAC, there is no evidence of anisotropic stratification in the northern part of the cratonic keel. Based on both isotropic and anisotropic phase-velocity anomalies, we suggest that the Phanerozoic Caledonian and Ellesmerian-Franklinian fold belts are relatively thin-skinned features onshore Greenland, though the Caledonian belt may have a stronger signature off the east coast. At the longest periods, a prominent low-velocity anomaly initially centred on Iceland migrates northwards and spreads beneath central-eastern Greenland. Coupled with NW–SE trending anisotropy, this feature is interpreted as the effect of mantle flow radiating outward from the Iceland plume and interacting with the eroded Greenland lithosphere.