Teleseismic shear wave splitting measurements in noisyenvironments

Abstract

High noise levels hamper teleseismic shear wave splitting measurements, which bandpass filtering does not always help. To investigate how robust splitting measurements are to noise, we analysed a set of synthetic records with known splitting parameters and added fixed levels of noise. In the presence of weak anisotropy, single‐waveform splitting measurements are unreliable when operating with noisy data sets. A practical rule in terms of S/N ratio and splitting delay time parameters is that splitting is confidently detectable at S/N > 8, regardless of the wave’s original polarization orientation. However, for the evidence of weak anisotropy to be detectable and measurable at an S/N value of 4, the backazimuth separation of the phases from the fast polarization direction needs to be higher than 20°. Stacks of individual measurements consistently yield reliable results down to S/N values of 4. Applying stacking to data from DSB (Dublin, Ireland), the fast polarization direction φ and lag time δt are 58° and 0.95 s. This orientation reflects surface trends of deformation in the area, as found elsewhere in the UK. Our result thus reinforces the proposed model that the detected anisotropy in the British Isles originates from lithospheric coherent deformation preserved from the last main tectonic episode.

Spatial and temporal constraints on sources of seismic anisotropy: Evidence from the Scottish highlands

“…However, more often in the noisy environment of Scotland, this resulted in splitting parameters with large associated errors (≥0.5 s in δ t, ≥20° in ϕ) for individual earthquakes. We instead carry out our analysis on un‐filtered seismograms and adopt the stacking procedure of Restivo and Helffrich [1999]. In the stacking procedure, high signal‐to‐noise ratio measurements are given more weight.…”

“…Our aim is to constrain better the spatial and temporal origins of anisotropy beneath the area. Existing studies of shear wave splitting in the British Isles suggest that splitting parameters correlate strongly with tectonic features of Caledonian and Variscan age [ Helffrich , 1995; Restivo and Helffrich , 1999]. However, tectonic activity associated with the opening of the Atlantic and emplacement of the British Tertiary Igneous Province (BTIP) at ∼50–60 Ma, may still be affecting deeper structures beneath the Atlantic's passive margins [e.g., Arrowsmith et al , 2005; Ucisik et al , 2005].…”

Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

“…Restivo and Helffrich () found that SC anisotropy measurements with SNR > 8 were valid for any back azimuth polarization, whereas measurements with 4 < SNR < 8 were only reliable for waves with back azimuths >20° from the fast splitting direction. Over 94% of all measurements had SNR > 4 for both methods.…”

Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

“…However, other methods of obtaining average splitting parameters for a given station that rely on stacking procedures have also been developed. For example, Wolfe and Silver (1998) proposed a method of stacking error surfaces obtained from splitting measurements for phases measured at a variety of backazimuths; a similar stacking technique has been proposed by Restivo and Helffrich (1999). Such stacking methods have the advantage of compensating for noisy data or poor waveform clarity, and making explicit use of null observations.…”