Seismic P- and S-wave velocity Tomography in Scandinavia

2020 ◽  
Author(s):  
Nevra Bulut ◽  
Valerie Maupin ◽  
Hans Thybo
Keyword(s):  
Travel Time ◽  
North Atlantic ◽  
Upper Mantle ◽  
Body Waves ◽  
Significant Heterogeneity ◽  
Finite Frequency ◽  
S Wave ◽  
The North ◽  
The North Atlantic ◽  
High Topography

<p>The causes of the high topography in Scandinavia along the North Atlantic passive continental margins are enigmatic, and two end-member models have been proposed. One opinion is that the high topography has been maintained since the Caledonian orogeny, because isostatic rebound has compensated for most of the erosion over >400 My. The other opinion is that the topography is Cenozoic and that it is related to plate tectonic or deep thermal / geodynamic processes. Onshore uplift is related to simultaneous offshore subsidence, and the rapid topographic changes may be the combined result of a series of complementary processes.</p><p>Here, we provide new evidence for the upper mantle structure by calculating a tomographic model for Fennoscandia (Scandinavia and Finland) by teleseismic inversion of finite-frequency P- and S- wave travel-time residuals. We use seismic signals from earthquakes at epicentral distances between 30° and 104° and with magnitudes larger than 5.5, gathered on 200 broad-band seismic stations installed by the ScanArray project in Norway, Sweden and Finland, which operated during 2012-2017, together with data from earlier projects and stationary stations..</p><p>We measure relative travel-time residuals of direct body waves in high- and low-frequency bands, and carry out an appropriate frequency-dependent crustal correction. The average residuals vary over the region, and show clear trends depending on location and and back-azimuthal directions. This demonstrates the presence of significant heterogeneity of seismic velocities in the upper mantle across the region. Based on the travel-time residuals<strong>,</strong> we carry out finite-frequency body-wave tomographic inversion to determine the P and S wave seismic velocity structure of the upper-mantle. By use of “relative kernels” we reduce problems related to station coverage with asynchronous datasets, which allows the use of data from different deployments for the inversion. The resulting seismic model is compared to the existing and past topography in order to contribute to the understanding of mechanisms responsible for the topographic changes in the Fennoscandian region, which we relate to the general tectonic and geological evolution of the North Atlantic region. The models provide basis for deriving high-resolution models of temperature and compositional anomalies that may contribute to the understanding of the observed, enigmatic topography.</p>

Is High Topography Around the North Atlantic Supported From the Upper Mantle?

2020 ◽  
Vol 125 (9) ◽  
Author(s):  
Babak Hejrani ◽  
Niels Balling ◽  
Bo Holm Jacobsen ◽  
Søren Bom Nielsen
Keyword(s):  
North Atlantic ◽  
Upper Mantle ◽  
The North ◽  
The North Atlantic ◽  
High Topography

Source locations of microseisms in the North Atlantic from Matched Field Processing using full Green's Functions.

2021 ◽  
Author(s):  
Sven Schippkus ◽  
Céline Hadziioannou
Keyword(s):  
Wave Propagation ◽  
Travel Time ◽  
North Atlantic ◽  
Seismic Noise ◽  
Green's Functions ◽  
Green’S Functions ◽  
Regional Scale ◽  
Matched Field Processing ◽  
The North ◽  
The North Atlantic

<p>Precise knowledge of the sources of seismic noise is fundamental to our understanding of the ambient seismic field and its generation mechanisms. Two approaches to locating such sources exist currently. One is based on minimizing the misfit between estimated Green's functions from cross-correlation of seismic noise and synthetically computed correlation functions. This approach is computationally expensive and not yet widely adopted. The other, more common approach is Beamforming, where a beam is computed by shifting waveforms in time corresponding to the slowness of a potentially arriving wave front. Beamforming allows fast computations, but is limited to the plane-wave assumption and sources outside of the array.</p><p>Matched Field Processing (MFP) is Beamforming in the spatial domain. By probing potential source locations directly, it allows for arbitrary wave propagation in the medium as well as sources inside of arrays. MFP has been successfully applied at local scale using a constant velocity for travel-time estimation, sufficient at that scale. At regional scale, travel times can be estimated from phase velocity maps, which are not yet available globally at microseism frequencies.</p><p>To expand MFP’s applicability to new regions and larger scales, we replace the replica vectors that contain only travel-time information with full synthetic Green's functions. This allows to capture the full complexity of wave propagation by including relative amplitude information between receivers and multiple phases. We apply the method to continuous recordings of stations surrounding the North Atlantic and locate seismic sources in the primary and secondary microseism band, using pre-computed databases of Green's functions for computational efficiency. The framework we introduce here can easily be adapted to a laterally homogeneous Earth once such Green’s function databases become available, hopefully in the near future.</p>

Upper mantle conditions during the opening of the North Atlantic Ocean

2020 ◽  
Author(s):  
Zsófia Zalai ◽  
Jenny Collier ◽  
Gareth Roberts ◽  
Thomas Funck
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Upper Mantle ◽  
North Atlantic Ocean ◽  
Temperature Anomaly ◽  
Potential Temperature ◽  
The North ◽  
The North Atlantic ◽  
Break Up ◽  
Mantle Potential Temperature

<p>Mantle conditions during the opening of the North Atlantic Ocean and specifically the presence or otherwise of a deep mantle plume have been much debated. Current models fall into two groups: the plume impingement and the plate-driven models. The plume impingement model associates the arrival of the Icelandic plume with continental break-up of the North Atlantic and the observed excess magmatism is associated with passive upwelling and elevated mantle potential temperatures. However, the plate-driven model associates this excess magmatism with increased mantle fertility due to inherited lithospheric structure and/or small-scale convection induced by sub-lithospheric topography.</p><p>We examine the spatial and temporal variation of upper mantle conditions at the time of continental break-up using an inventory of 40 published seismic refraction velocity-depth profiles acquired between the Charlie Gibbs and the East Greenland Fracture Zones. We make use of the Hc-Vp method to estimate mantle potential temperature and the ratio of active to passive upwelling by extracting igneous crustal thickness, Hc, and its mean p-wave velocity, Vp. Finally, we compare the spatial and temporal patterns obtained to those predicted by previously proposed models of mantle conditions around the time of break-up.</p><p>Our results indicate an asymmetry in mantle potential temperature between the Greenland and the European side, the latter being 100°C hotter. The temperature anomaly also varies on a wavelength of 300-500 km along strike both margins. In most profiles, the mantle potential temperature decreases with time, with normal temperatures of 1300°C being reached 5-10 Ma after the onset of seafloor spreading at 55 Ma. This temperature appears to be “steady state” once reached. The exception to this is the Greenland-Iceland-Faroes Ridge where the “steady state” temperature is 100°C higher. However, the decreasing trend of mantle potential temperature with time is not uniform across the whole North Atlantic region: the temperature decreases by a 60°C/Ma rate at the Hatton margin, while at the Møre and Vøring margins it is considerably slower, at only 20°C/Ma. A 100°C lower than normal mantle potential temperature anomaly was found at the now extinct Aegir Ridge spreading centre even though it was located less than 300 km away from the proposed reconstructed position of the Icelandic plume. Nevertheless, the plume’s position coincides well with the highest calculated upwelling ratios. The NE Greenland margin is also characterised by moderate upwelling compared to the purely passive European side.</p><p>Overall the spatial distribution of high active upwelling ratios and widespread elevated mantle potential temperatures support the plume impingement model for the opening of the North Atlantic Ocean. This thermal anomaly was exhausted at a varying rate on the different margins in 5-10 Ma. Furthermore, the 300-500 km wide localised thermal anomalies and the proximity of the proposed plume location to a low temperature anomaly indicate moderation by local complexities that might be a manifestation of upper mantle flow induced by structural inheritance or plate tectonic processes.</p>

scholarly journals Ambient seismic noise imaging of the lowermost mantle beneath the North Atlantic Ocean

2020 ◽  
Vol 222 (2) ◽  
pp. 1339-1351 ◽  
Author(s):  
Lise Retailleau ◽  
Pierre Boué ◽  
Lei Li ◽  
Michel Campillo
Keyword(s):  
North Atlantic ◽  
Lower Mantle ◽  
North Atlantic Ocean ◽  
Synthetic Data ◽  
Body Waves ◽  
P Wave ◽  
Eastern United States ◽  
The North ◽  
Seismic Records ◽  
The North Atlantic

SUMMARY Body waves can be extracted from correlation functions computed from seismic records even at teleseismic distances. Here we use P and PcP waves from the secondary microseism frequency band that are propagating between Europe and the Eastern United States to image the core–mantle boundary (CMB) and D” structure beneath the North Atlantic. This study presents the first 3-D image of the lower mantle obtained from ocean-generated microseism data. Robustness of our results is evaluated by comparing images produced by propagation in both directions. Our observations reveal complex patterns of lateral and vertical variations of P-wave reflectivity with a particularly strong anomaly extending upward in the lower mantle up to 2600 km deep. We compare these results with synthetic data and associate this anomaly to a Vp velocity increase above the CMB. Our image aims at promoting the study of the lower mantle with microseism noise excitations.

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