The fast-slipping (~ 14 cm/yr) Gofar transform fault (GTF) East Pacific Rise has three active segments G1 to G3 from east to west. These fault segments produce large earthquakes (MW ~ 6.0) quasi-periodically every five to six years (Figure 1). Interestingly, large earthquakes rupture the same 20-km long fault patches separated by a ~ 10-km rupture barrier zone, implying along-strike variations in fault zone material properties (McGuire et al. 2012). This further suggests that rupture patches and barrier zones remain stable over multiple seismic cycles. Here, we jointly invert P- and S-wave travel times, recorded at an ocean-bottom-seismometer (OBS) experiment along G1 between January 2019 and February 2020, to determine the 3-D seismic velocity structure of the area and relocate the local earthquakes. Our velocity models reveal a large low-velocity anomaly extending through the entire oceanic crust along G1 with some along-strike variations. We identify a 10 km-long rupture barrier zone, which is interpreted to be highly fractured with enhanced fluid circulation. We further suggest that the observed deep seismicity underlying the rupture barrier zone may indicate sea-water infiltration.  Lastly, we apply the same approach to the westernmost segment of the GTF and observe some similarities and differences between the two fault zone seismic velocity structures.

The recent deployment of temporary broadband seismic networks, notably the EarthScope USArray-Transportable Array (TA), has drastically improved the station coverage across northwestern Canada over the last ten years, enabling application of high-resolution passive-source seismic methods (i.e., seismic tomography, receiver functions and core phase shear wave splitting). This review highlights the main discoveries pertaining to the seismic velocity structure, origin and deformation of the lithosphere in the northern Canadian Cordillera (NCC). High-resolution seismic tomography models reveal that the lower crust in the NCC is marked by low velocity anomalies extending from the Gulf of Alaska to the Cordilleran deformation front, which are interpreted to reflect elevated temperatures that buoyantly support regional high elevations and potentially represent the seismic signature of strain transfer from the Yakutat collision zone to the Mackenzie Mountains. The Moho is relatively flat and shallow across the NCC, and is underlain by a thin layer of mantle lithosphere. Seismic velocity models further unveiled large-scale mantle structures associated with the unexposed Mackenzie craton in the north, and the Liard Transfer Zone in the south, which appear to buttress the NCC and further focus deformation in the eastern NCC. Seismic anisotropy and tomography provide evidence that the Tintina and Denali faults penetrate into the lithospheric mantle and played a first order role in shaping the present-day NCC. We propose that future studies should aim to: 1) resolve the shape of the Cordillera-craton boundary at upper mantle depths; 2) accurately estimate the lithosphere thickness in the NCC; and 3) improve coverage in the Beaufort Sea to understand the controls on convergent tectonics in the northern NCC.

The Schwechat depression, in the Vienna Basin (VB), is currently the main target area for deep geothermal exploration in eastern Austria. Knowledge of the subsurface heavily relies on active seismic reflection profiling experiments that are expensive and logistically demanding. Affordable geophysical prospecting methods are needed to reduce subsurface uncertainty over large spatial areas. Over recent years, seismic ambient noise tomography (ANT) has proven to be a cost-effective and environment-friendly exploration technique fulfilling this need. Here, we present an ANT study of the central Vienna Basin revealing the shear-wave velocity, and shear-wave radial anisotropy, structure down to 5 km beneath the surface. We deployed an array of 100 seismic nodal instruments during 5 weeks over summer 2023. We measured fundamental-mode Rayleigh and Love-wave group velocity dispersion from seismic noise correlations, and employed transdimensional Bayesian tomography to invert for isotropic Rayleigh and Love group velocity maps at periods ranging from 0.8 to 5.5 s. We then extracted Rayleigh and Love group velocity dispersion curves from the maps at all locations, and jointly inverted them for shear-wave velocity and radial anisotropy as a function of depth using a transdimensional Bayesian framework. Our shear-wave velocity model reveals a basin-like low-velocity feature, interpreted as the seismic signature of the Schwechat depression. Another low-velocity feature is observed beneath the city of Vienna, which could be of great interest for geothermal exploration. The shear-wave velocity radial anisotropy structure indicates a thin negative anisotropy layer in the top 150 meters, likely associated with water-saturated open cracks. Between 150 meters and 1.5 km depth, we observe widespread positive radial anisotropy across the entire study area, corresponding to sub-horizontal layering within the Neogene basin. At greater depths, the Schwechat depression is characterized by positive radial anisotropy, while the edges of the Schwechat depression exhibit negative radial anisotropy due to steeply dipping strata and normal faults responsible for the formation of this major depocenter in the Vienna Basin.