Optimal resolution tomography with error tracking and the structure of the crust and upper mantle beneath Ireland and Britain
https://doi.org/10.1002/2017GC007347
https://academic.oup.com/gji/article/226/3/2158/6247624
Explained to a child
This study looks at what is happening deep beneath the area of Ireland and Britain and surroundings using seismic waves, vibrations caused by earthquakes that travel through the Earth. We use these waves like a special kind of X-ray to create pictures of the underground layers.
What makes this study special is that the we used new and improved methods to analyze the seismic waves more accurately than before. We combined different types of seismic data and advanced computer models to get a clearer and more detailed image of the Earth's mantle lithosphere (the hard shell outermost layer of the Earth).
Our results show traces of a hot plume of rock rising from deep inside the Earth, linked to the Iceland hotspot. This plume caused the Earth's outer shell in the region to become thinner millions of years ago, which explains past volcanic activity. It also helps understand why the land is slowly rising and why there are small earthquakes today.
By using better tools and combining different kinds of data, we are able to better connect what is happening deep underground with what we see on the surface, helping us understand how Earth's inside affects the land above.
Explained to a friend
This study explores how the Iceland plume has influenced the geology of the British Isles by examining changes in the Earth’s lithosphere, the rigid outer shell, over millions of years. The plume's heat and rising mantle material caused the lithosphere to thin, which in turn led to volcanic activity (e.g., the Giant's Causeway), uplift, and long-term patterns of earthquakes in the region.
A crucial part of our research was achieving optimal resolution. This means we worked carefully to produce the clearest and most detailed images possible of the Earth's structure beneath the surface. By analyzing seismic waves recorded across many stations, we tracked how these waves travel through different layers, allowing us to build a detailed 3D model of the lithosphere and the underlying mantle.
Having such high-resolution images helps us better understand how the plume's heat and dynamics shaped the region's geology. We can identify where the lithosphere is thinnest and see how seismic activity corresponds to these zones.
Overall, this work sheds light on the complex interactions between mantle plumes, tectonic forces, and earthquakes, providing valuable insights into how processes deep within the Earth influence surface geology and natural hazards.
Explained to an expert
The classical Backus–Gilbert method seeks localized Earth-structure averages at the shortest length scales possible, given a data set, data errors, and a threshold for acceptable model errors. The resolving length at a point is the width of the local averaging kernel, and the optimal averaging kernel is the narrowest one such that the model error is below a specified level. This approach is well suited for seismic tomography, which maps 3-D Earth structure using large sets of seismic measurements. The continual measurement-error decreases and data-redundancy increases have reduced the impact of random errors on tomographic models. Systematic errors, however, are resistant to data redundancy and their effect on the model is difficult to predict. Here, we develop a method for finding the optimal resolving length at every point, implementing it for surface-wave tomography.
As in the Backus–Gilbert method, every solution at a point results from an entire-system inversion, and the model error is reduced by increasing the model-parameter averaging. The key advantage of our method stems from its direct, empirical evaluation of the posterior model error at a point. We first measure inter-station phase velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Numerous versions of the maps with varying smoothness are then computed, ranging from very rough to very smooth. Phase-velocity curves extracted from the maps at every point can be inverted for shear-velocity (VS) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. We evaluate the error by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure and determine the optimal resolving length at a point such that the error of the local phase-velocity curve is below a threshold.
A 3-D VS model is then computed by the inversion of the composite phase-velocity maps with an optimal resolution at every point. The estimated optimal resolution shows smooth lateral variations, confirming the robustness of the procedure. Importantly, the optimal resolving length does not scale with the density of the data coverage: some of the best-sampled locations display relatively low lateral resolution, probably due to systematic errors in the data.
We apply the method to image the lithosphere and underlying mantle beneath Ireland and Britain. Our very large data set was created using new data from Ireland Array, the Irish National Seismic Network, the UK Seismograph Network and other deployments. A total of 11 238 inter-station dispersion curves, spanning a very broad total period range (4–500 s), yield unprecedented data coverage of the area and provide fine regional resolution from the crust to the deep asthenosphere. The lateral resolution of the 3-D model is computed explicitly and varies from 39 km in central Ireland to over 800 km at the edges of the area, where the data coverage declines. Our tomography reveals pronounced, previously unknown variations in the lithospheric thickness beneath Ireland and Britain, with implications for their Caledonian assembly and for the mechanisms of the British Tertiary Igneous Province magmatism.
Optimal resolution tomography with error tracking and the structure of the crust and upper mantle beneath Ireland and Britain
Published in Geophysical Journal International, 2021.
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Raffaele Bonadio, Sergei Lebedev, Thomas Meier, Pierre Arroucau, Andrew J Schaeffer, Andrea Licciardi, Matthew R Agius, Clare Horan, Louise Collins, Brian M O’Reilly, Peter W Readman, Ireland Array Working Group, Optimal resolution tomography with error tracking and the structure of the crust and upper mantle beneath Ireland and Britain, Geophysical Journal International, Volume 226, Issue 3, September 2021, Pages 2158–2188, https://doi.org/10.1093/gji/ggab169