Azimuthal anisotropy in the mantle transition zone beneath the Tibetan Plateau: Evidence from normal mode coupling

We investigate strong coupling between spheroidal mode 0S25 and toroidal mode 0T25 observed at LSA, a permanent seismic station located on the southern Tibetan Plateau, after some large earthquakes, such as the 2001/06/23 Mw = 8.4 Peru earthquake and the 2011/09/18 Mw = 7.2 Sikkim earthquake. Earth's rotation cannot cause coupling of 0S25–0T25 according to coupling selection rules. Our extensive observations indicate that the local azimuthal anisotropy is the dominant cause for strong coupling of 0S25–0T25. It is noted that the coupling is noticeable when the great circle paths connecting epicenters and LSA intersect an East–West line at about 45°, but not observed when paths are parallel or perpendicular to the East–West line. This result suggests that the local azimuthal anisotropy has a horizontal symmetry axis oriented nearly eastward. Our estimation of coupling kernels for the coupled modes 0S25–0T25 shows a peak sensitivity to the depth range of 400–650 km, suggesting the existence of azimuthal anisotropy in the upper mantle transition zone beneath the Tibetan Plateau. Our observations also indicate that local anisotropy, rather than Earth's rotation, plays a leading role in causing strong coupling of 0S17–0T18 at LSA after the 2006/04/20 Mw = 7.7 Siberia earthquake, suggesting the anisotropic coupling between fundamental spheroidal and toroidal modes is more important than previously realized. The work of this paper proposed that observations of anisotropic coupling below 3.5 mHz may provide constraints on the azimuthal anisotropy in the upper mantle transition zone beneath the seismic station.

► We observe significant coupling of 0S25–0T25 and 0S17–0T18 at Tibetan station LSA.
► Observations indicate that local anisotropy near the station causes the coupling.
► Model suggests azimuthal anisotropy exists in the upper mantle transition zone.
► Observations suggest the symmetry axis of anisotropy is oriented in nearly eastward.