Resolving a low-velocity zone with surface-wave data

S U M M A R YWe present a method for the retrieval of the phase velocities of surface-wave overtones. The 'single-station' method is successful for several Love and Rayleigh overtone branches (up to at least four) in mode-specific period ranges between 40 and 200 s. It uses mode-branch cross-correlation functions and relies on adjusting the phase and amplitude of the mode branches one at a time. A standard statistical optimization technique is used. We discuss in detail the apriori information that is added t o stabilize the retrieval procedure. In addition, we present a technique to estimate the reliability of individual phase and amplitude measurements. The retrieval method and the technique to estimate reliabilities can be used together in a highly automated way, making the methods especially suited for studying the large volume of digital data now available.We include several applications to synthetic a n d recorded waveforms. We will discuss in detail an experiment with 90 waveforms that have travelled along very similar paths from Vanuatu to California. For this path, we will present average overtone phase velocities and a n average 1-D velocity structure.

Section: Similar Pathssupporting

confidence: 92%

Measuring surface-wave overtone phase velocities using a mode-branch stripping technique

Heijst¹,

Woodhouse²

1997

“…1970). Moreover, fundamental‐mode surface wave dispersion data alone has a limited resolving power when a low‐velocity zone is present (van Heijst et al .1994).…”

Section: Introductionmentioning

confidence: 99%

Global high-resolution phase velocity distributions of overtone and fundamental-mode surface waves determined by mode branch stripping

Heijst

Woodhouse

1999

105

Summary We apply the mode branch stripping (MBS) technique of van Heijst & Woodhouse (1997) to approximately 110 000 three‐component seismograms. We select high‐quality data with the reliability estimate of van Heijst & Woodhouse (1997). We assess the influence of different selection criteria and remove outliers using smooth degree‐12 phase velocity maps. We present final results in terms of Rayleigh‐ and Love‐wave fundamental‐mode and overtone global high‐resolution (l=40) phase velocity maps. We determine the optimum damping of the high‐resolution maps with a method based on cross‐validation. Our fundamental‐mode phase velocity maps are generally in good agreement with previous studies, especially with Ekström et al.(1997). They do, however, contain more short‐wavelength structure than previous studies as we apply relatively little damping. The Rayleigh wave overtone phase velocity measurements made with MBS are of high quality in broad frequency ranges. The measurements are generally well explained by phase velocity maps, and variance reductions for some modes, after rejecting outliers, are as high as 85 per cent. We compare our global phase velocity distributions to the previous results of Stutzmann & Montagner (1994) and to phase velocity maps predicted by 3‐D tomographic mantle models. Agreement of the model predictions with our Rayleigh wave phase velocity maps, in terms of both amplitude and observed structures, is good. For Love waves, the quality of the measurements made with MBS is not as high as for Rayleigh waves. The variance reductions achieved are lower and the agreement between model predictions and our phase velocity maps is less, especially in the frequency ranges where interference between different mode branches is strong. Finally, as an additional check on the quality of our overtone measurements, we present a comparison of the fundamental‐mode Rayleigh phase velocity distribution at 40 s and our fourth Rayleigh wave overtone phase velocity distribution at 62 s. These two modes are similarly sensitive to velocity anomalies in the top of the upper mantle. We show that the two phase velocity maps are in very close agreement.

“…We note, however, that an addition of Love wave data, where available, will increase the resolving power of surface waves in relation to the LAB, both because complementary constraints on LAB properties will be added and because radial anisotropy of the lithosphere and asthenosphere will be constrained (e.g. Van Heijst et al 1994; Gung et al 2003; Nettles & Dziewonski 2008; Yoshizawa et al 2010). Radial anisotropy is caused by fabric within the lithosphere; once it is known, we can determine isotropic‐average shear speeds that reflect temperature and composition at depth.…”

Section: Discussionmentioning

confidence: 99%

“…In addition to phase velocities of the fundamental mode, those of the higher modes can also be measured (e.g. Lebedev et al 2005; Visser et al 2007) and used as additional constraints on mantle structure (Visser et al 2008; Yoshizawa & Ekström 2010), including the LAB properties (Van Heijst et al 1994).…”

Section: Discussionmentioning

confidence: 99%

“…Perhaps surprisingly, it has remained somewhat unclear to this day how sensitive surface waves are to the LAB depth and thickness and whether and how they can be used to constrain them. Van Heijst et al (1994) considered a related question of surface wave sensitivity to the presence of a sublithospheric low‐velocity zone (LVZ) and argued that the signal of LVZ properties in phase‐velocity data did not exceed errors in the measurements, which they estimated at 1 per cent, random or systematic. Eaton et al (2009) reviewed the multitude of definitions used to identify the LAB on shear‐speed profiles computed by means of surface wave inversions.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Resolving the lithosphere-asthenosphere boundary with seismic Rayleigh waves

Bartzsch

Lebedev

Meier

2011

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.