Crustal Anisotropy and Lower Crustal Flow beneath the Eastern Margin of the Tibetan Plateau Revealed by P-to-S Conversions from the Moho

Abstract:

Teleseismic data recorded by broadband stations located along the eastern margin of the Tibetan plateau adjacent to the Sichuan Basin are employed to characterize crustal anisotropy based upon systematic back-azimuthal arrival time variations of the P-to-S conversions (Ps) at the Moho. Stacking of radial receiver functions (RFs) along candidate cosine moveout curves of the Ps phase is used to retrieve the splitting parameters (fast orientations and delay times). A total of 68 stations possess high quality RFs with adequate azimuthal coverage for reliable determination of the splitting parameters. The resulting fast orientations are generally parallel to the surface features with delay times ranging from 0.3 to 0.87 s. Previous studies using upper crustal local earthquakes suggested that the delay times resulting from upper crustal anisotropy are dominantly less than 0.2 s. Thus, the observed crustal anisotropy indicates a highly anisotropic lower crust, with a fast orientation that is parallel to the major surface faults. This hypothesis is consistent with P wave and surface wave velocity anomalies observed in the study area. The anisotropic structure beneath the study area is further explored by using shear wave splitting analysis of the XKS phases (including PKS, SKKS and SKS). Many of the stations display a systematic azimuthal variation of the resulting XKS splitting parameters with a 90-degree periodicity, suggesting complex or two-layer anisotropy. We searched for the 2 pairs of splitting parameters and found that the upper layer parameters are mostly consistent with those obtained using Ps phases from the Moho. The optimal depth of the lower layer estimated based on spatial coherency of XKS splitting parameters is centered at about 200-250 km, indicating an asthenospheric origin.