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

Abstract

Abstract. The present study deals with detecting seismic anisotropy parameters beneath southeastern Tibet near Namcha Barwa Mountain using the splitting of direct S waves. We employ the reference station technique to remove the effects of source-side anisotropy. Seismic anisotropy parameters, splitting time delays, and fast polarization directions are estimated through analyses of a total of 501 splitting measurements obtained from direct S waves from 25 earthquakes ( ≥ 5.5 magnitude) that were recorded at 42 stations of the Namcha Barwa seismic network. We observe a large variation in time delays ranging from 0.64 to 1.68 s, but in most cases, it is more than 1 s, which suggests a highly anisotropic lithospheric mantle in the region. A comparison between direct S-and SKS-derived splitting parameters shows a close similarity, although some discrepancies exist where null or negligible anisotropy has been reported earlier using SKS. The seismic stations with hitherto null or negligible anisotropy are now supplemented with new measurements with clear anisotropic signatures. Our analyses indicate a sharp change in lateral variations of fast polarization directions (FPDs) from consistent SSW-ENE or W-E to NW-SE direction at the southeastern edge of Tibet. Comparison of the FPDs with Global Positioning System (GPS) measurements, absolute plate motion (APM) directions, and surface geological features indicates that the observed anisotropy and hence inferred deformation patterns are not only due to asthenospheric dynamics but are a combination of lithospheric deformation and sub-lithospheric (asthenospheric) mantle dynamics. Direct S-wave-based station-averaged splitting measurements with increased back-azimuths tend to fill the coverage gaps left in SKS measurements.

Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases

“…It was observed in several studies (e.g. Eken & Tilmann 2014;Tiwari et al 2017) that most of the large differences in FPDs and delay times could be attributed to the low numbers of SK(K)S measurements at those stations. If the two datasets both have reliable measurements and they still produce dissimilar results, then the variance could be attributed to lateral changes in anisotropic structure combined with the path differences between the two phases.…”

“…The maximum similarity of the corrected S wave signal beneath the reference station and target station represents the best estimate of splitting parameters for the respective station pair and event; final splitting parameters are derived by averaging the results from many events and station-pairs. The RST has been applied to the regions with various tectonic settings, such as in the Tibetan Plateau (Eken & Tilmann 2014;Singh et al 2016;Tiwari et al 2017) and the Hellenic Trench in the eastern Mediterranean (Confal et al 2016).…”