1 research outputs found
Optimal resolution tomography with error tracking and the structure of the crust and upper mantle beneath Ireland and Britain
The classical BackusâGilbert 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 BackusâGilbert 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 (V S ) 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 V S 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 (4â500 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