Seismic anisotropy beneath the Indian continent from splitting of direct S waves

Abstract

[1] In this study, we investigate the shear wave splitting beneath 39 broadband seismic stations located over various tectonic units of the Indian shield using direct S waves from deep to intermediate depth earthquakes. The delay times between fast and slow axes of anisotropy do not reveal anomalous values that reflect contributions from midmantle or the mantle transition zone. The fast polarization azimuths are generally consistent with the results from SKS splitting that indicate a predominance of plate motion related strain in the Indian continent. Splitting results at station HYB, which was hitherto considered isotropic, indicate significant anisotropy, with a NE oriented fast axis. Although a general trend of SH leading SV is indicated by most of the measurements, invoking radial anisotropy to explain this phenomenon appears farfetched since the near vertical incidence of the S waves in the usable distance range of 40°-80°m akes them more sensitive to azimuthal anisotropy underneath the station. Forward modeling of the dependence of the splitting parameters on incoming polarization assuming horizontal axis of symmetry brings out two layers of anisotropy beneath the Indian shield with fast axis azimuths oriented N15°E in the bottom layer and N60°E in the top layer. While stronger anisotropy (dt ≈ 1 s) in the bottom layer is related to asthenospheric flow, a weaker one (dt ≈ 0.4 s) in the upper layer could represent anisotropy frozen in the lithosphere.

Seismic anisotropy beneath the Eastern Dharwar craton

“…They suggested that this relative fl ow would create foliations in the horizontal plane, which get preserved in the lithosphere. Results of seismic anisotropy from 35 broadband stations in the Indian Shield using direct S waves show that the fast polarization azimuths are in agreement with those obtained from SKS splitting that indicate a predominance of strain related to absolute plate motion in the Indian Shield (Saikia et al, 2010).…”

“…Interestingly, although SK(K)S splitting at station HYB reveals null results, the splitting parameters obtained using direct S phases show fast-axis azimuths both in the NE and NNW-SSE directions (Saikia et al, 2010). In this context, it is relevant to investigate the anisotropic nature of the Eastern Dharwar craton in greater www.gsapubs.org | Volume 4 | Number 4 | LITHOSPHERE detail, to address issues like null anisotropy, the discrepancy between S and SK(K)S anisotropy, and possible presence of multiple layers of anisotropy that have an annihilating effect on each other.…”

“…After performing quality analysis, only those waveforms with high signal-to-noise ratio (S/N > 5) as well as clear arrival of the relevant shear-wave phase were retained for further analysis. To enhance the dominant period of SK(K)S phases and to suppress the effects of noise, the data were fi ltered using a Butterworth band-pass fi lter between 4 s and 20 s. In order to avoid contamination of the direct S waves with other phases like PcS, ScPmP, SP, and PL, which arrive in close proximity (for details, see Saikia et al, 2010), the analysis was performed using raw (unfi ltered) data. Whenever fi ltering became imperative, a series of Butterworth fi lters was used by changing one end from 1 to 5 s while keeping the other end fi xed at 50 s. Eventually, only those measurements that produced stable splitting results for different fi lters were considered.…”

“…Based on these measurements, it was speculated that the onset of seismic anisotropy north of the IndusTsangpo suture zone could mark the termination of the Indian lithosphere beneath Asia. However, many splitting measurements from numerous broadband seismic stations installed during the past decade over diverse geological provinces of the Indian subcontinent (Singh et al, 2006(Singh et al, , 2007Kumar and Singh, 2008;Heintz et al, 2009;Kumar et al, 2010;Saikia et al, 2010;Mandal, 2011) testify to the anisotropic character of the Indian lithosphere and sublithospheric mantle and establish a dominance of the plate motion-related strain. Also, evidence for fossilized anisotropy, particularly in regions close to continental shear zones, is brought out in regions like the Southern granulite terrain and Cuddapah Basin.…”

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

“…In summary, our comparative analysis of splitting parameters shows a good accordance between SKS-and direct-S-derived splitting parameters as previously observed in the Himalaya-Tibet collision zone (e.g. McNamara et al, 1994;Singh et al, 2016;Huang et al, 2011;Eken et al, 2013) and in the Indian shield (Saikia et al, 2010).…”