Seismic anisotropy inferred from direct &amp;lt;i&amp;gt;S&amp;lt;/i&amp;gt;-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

Ashwani Kant Tiwari,Tuna Eken,Chandrani Singh,Arun Singh

doi:10.5194/se-8-435-2017

Seismic anisotropy inferred from direct &lt;i&gt;S&lt;/i&gt;-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

Ashwani Kant Tiwari, Tuna Eken + Show 2 more

Open Access

https://doi.org/10.5194/se-8-435-2017

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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.

Highlights

The Tibetan Plateau has a long history of deformation within the last 50 million years (e.g. Rowley and Currie, 2006; Henderson et al, 2011)
Lattice-preferred orientation (LPO) of olivine mineral in the mantle as a result of plate interactions is controlled by various geodynamic processes and is considered to be the main cause of the shear wave splitting observations on the teleseismic S and SKS waves
We present here 501 splitting measurements observed for 42 seismic stations of the XE network between the years 2003 and 2004

Summary

Introduction

The Tibetan Plateau has a long history of deformation within the last 50 million years (e.g. Rowley and Currie, 2006; Henderson et al, 2011). The dislocation creep process, which is the creeping motion of crystal dislocation, is considered to be the leading cause of mantle anisotropy (Karato, 1987; Nicolas and Christensen, 1987; Karato and Wu, 1993; Mainprice et al, 2000). It can be caused by either high-stress conditions or large grain size or both, but the nonlinear increase in the strain rate is independent of the grain size (Karato and Wu, 1993). This type of deformation is expected to occur at a depth range of less than 400 km (e.g. Karato, 1984, 1987) where olivine is the most common mineral, and LPO development and observed anisotropy mainly represents the upper 400 km of the mantle (Becker and Faccenna, 2011)

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