The Use of Direct Shear Waves in Quantifying Seismic Anisotropy: Exploiting Regional Arrays

Abstract

To overcome the potential contamination of the direct S waves by sourceside anisotropy in shear-wave-splitting analysis, we describe a new approach that we call the reference station technique. The technique utilizes direct shear waves recorded at a station pair and depends on maximizing the correlation between the seismic traces at reference and target stations after correcting the reference station for known receiverside anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. The algorithm also provides a delay time between both stations caused, for example, by isotropic heterogeneities. Synthetic tests demonstrate the stability of the estimated parameters, even where variability in near-surface properties (thickness and velocity of sediment layer) exists. We applied the reference station technique to data from seismic experiments at the northern margin of Tibet. Average splitting parameters obtained from the analysis of direct S-wave results are consistent with those obtained from previous SKS splitting measurements. Where differences exist, shear-wave fast polarization estimates resolved from direct S indicate a higher degree of internal consistency for closely spaced stations than those derived from SKS. This is probably due to the much larger number of direct S waves available for splitting measurements compared to SKS for the same observational period, resulting in higher quality measurements. We also demonstrate the ability of the technique to provide improved splitting measurements for temporary stations by following a bootstrap approach in which only a few stations with well-constrained SKS splitting parameters are used as seeds to determine the splitting parameters of a large array in an iterative manner. In addition, the S measurements sample the anisotropic layer with different angles of incidence and back azimuths, thus potentially providing additional constraints on more complicated anisotropic structures, and the interstation delay times could be used for tomographic studies to reduce the bias from anisotropic structure.Online Material: Multisplit software package (C++) with instructions.

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

“…Any potential difference between the thickness of the crust and sedimentary layers will also cause the timing and amplitude of converted phase, but Eken and Tilmann (2014) showed, numerically, its influence on expected splitting parameters would be negligible. During the application of the technique, we let φ and δt vary from 0 to 180 • with an increment of 1 • and from 0 to 3 s with an increment of 0.05 s, respectively.…”

“…We form the station pairs by selecting the same earthquake event recorded at both (reference and target) stations. Eken and Tilmann (2014) and Singh et al (2016) successfully applied the RST to regional arrays with an interstation distance less than 300 km. By taking 300 km interstation spacing as the limit in a similar fashion, we have formed 22 816 station pairs with four horizontal components available at reference and target stations out of 35 649 possible station pairs; these are based on 161 teleseismic events prior to the application of the technique.…”

“…At the first stage, an inverse splitting operator depending on a backward angular rotation with two horizontal components, a time shift, and the reversal of the back angular rotation are employed to correct the reference station for known receiver-side anisotropy (generally inferred from SKS splitting analyses) when estimating the direct-S-derived individual splitting parameter (Eken and Tilmann, 2014). Following the correction of the reference station, S signals are corrected for splitting parameters in a grid search manner at the target stations.…”

“…However, the major problem in including direct S waves in splitting measurements is the contamination of the S wave signals due to the influence of anisotropic structures existing within the sourceside region. Eken and Tilmann (2014) have recently shown that this problem can be overcome using an array-based ap- ITSZ BNSZ ES01 ES02 ES03 ES04 ES05 ES06 ES07 ES08 ES09 ES10 ES11 ES12 ES13 ES14 ES15 ES16 ES17 ES18 ES19 ES20 ES21 ES22 ES23 ES24 ES25 ES26 ES27 ES28 ES29 ES30 ES31 ES32 ES33 ES34 ES35 ES36 ES37 ES38 Es39 ES40 ES41 ES42 ES43 ES44 ES45 ES46 proach, known as the reference station technique (RST). The method assumes an identical source-side anisotropy effect at two closely located stations (reference and target stations) with small differences in epicentral distances.…”

“…The RST has been successfully tested through both synthetic and observed data collected along the northeastern and southwestern parts of the Tibetan Plateau and the Hellenic Trench in the eastern Mediterranean (e.g. Eken and Tilmann, 2014;Singh et al, 2016;Confal et al, 2016). The present study focuses on the southeastern part of Tibet near Namcha Barwa (Fig.…”