Resolving the lithosphere-asthenosphere boundary with seismic Rayleigh waves

Abstract

S U M M A R YSeismic surface wave measurements offer tight constraints on shear wave speed values within the lithosphere and asthenosphere. It is still a matter of debate, however, how accurately and under what conditions surface waves can resolve the depth and thickness of the lithosphere-asthenosphere boundary (LAB). We investigate the sensitivity of Rayleigh waves to LAB properties and find that if the LAB is associated with a 2-3 per cent shear speed reduction at a depth less than 100 km, then 10-20 km variations in its depth translate into phase-velocity changes of up to 1 per cent. Variations in the thickness of the LAB from zero to a few tens of kilometres cause much smaller phase-velocity changes (∼0.1 per cent), although the signal of the LAB-thickness variations is enhanced by a distinct frequency-dependent pattern of phase-velocity increases and decreases. For LAB depths increasing beyond 100 km, larger absolute variations in the LAB depth and thickness are needed to generate response of the same amplitude in phase velocities.We introduce a grid-search inversion of phase-velocity curves for the LAB depth and thickness, defining them as the middle and the width, respectively, of a depth interval with a linear shear-speed decrease in it. The inversion comprises dense sampling of the LAB depth-LAB thickness parameter plane and non-linear, gradient-search inversions at every point; it accounts for any trade-offs of the LAB parameters with shear speed variations above and below the LAB. Inversions of phase-velocity curves for the LAB depth and thickness show that the two parameters have uncorrelated uncertainties, with the LAB depth constrained better than the LAB thickness. Random errors in phase-velocity measurements have a limited effect on LAB-depth measurements, causing errors of only a few kilometres. Measurements of the LAB thickness are less robust and are not possible in the presence of large (∼1 per cent) random errors. Substantial systematic biases in the measurements (errors that persist over broad period ranges of tens of seconds) can affect both LAB-depth and LAB-thickness estimates; phasevelocity curves with such biases are not suitable for inversions for LAB properties. Resolving the LAB, however, in particular the LAB depth, should already be possible with some of the accurate, broad-band, phase-velocity measurements available today.Applying our method to phase-velocity data measured in Phanerozoic west-central Germany, we find best-fitting LAB depths in the 60-80 km range (the LAB depth always understood to be the middle of the shear-speed reduction depth range), with minimum misfits achieved near the 70-km LAB depth. Beneath the Archean Guyana Shield, we determine a 140-175 km LAB-depth range, with a 160 km best-fitting (mid-)LAB depth.

Mesozoic and Cenozoic evolution of the Central European lithosphere

“…Bartzsch et al, 2011). The depth of the LAB can be estimated by inversion of accurate phase velocity measurements with an uncertainty of 10 km to 20 km for LAB depths lower than 100 km (Bartzsch et al, 2011). Differences in LAB depth are easier to resolve than absolute LAB depths by comparing the depth of the transition towards low velocities in the asthenosphere between models.…”

“…Bartzsch et al, 2011). The depth of the LAB can be estimated by inversion of accurate phase velocity measurements with an uncertainty of 10 km to 20 km for LAB depths lower than 100 km (Bartzsch et al, 2011).…”

Deep Earth Structure - Transition Zone and Mantle Discontinuities

“…Therefore, seismologists sometimes call this discontinuity the 'Gutenberg' discontinuity (often abbreviated as 'G' discontinuity). The concept of a high-viscosity lithospheric layer overlying a lowviscosity asthenospheric layer was introduced to explain observations such as the postglacial uplift of Scandinavia (e.g., Barrell, 1914). The Lehmann discontinuity (Lehmann, 1961) is considered as the bottom of the asthenosphere (mostly observed in continental regions; Deuss and Woodhouse, 2002;Gu et al, 2001a).…”

Lithospheric structure in the Baikal–central Mongolia region from integrated geophysical‐petrological inversion of surface‐wave data and topographic elevation

“…In this paper, we test a coupled geophysical‐petrological inversion of fundamental‐mode, Rayleigh‐ and Love‐wave dispersion curves and topography data, with supplementary constraints from surface heat flow, crustal seismic models and xenolith data, for one‐dimensional structural profiles. The signal of the lithosphere‐asthenosphere structure in phase‐velocity data is very subtle [ Bartzsch et al , 2011]. An accuracy of a few tenths of per cent in the phase velocity value is required in both the measurements and the inversion.…”