COMPLEX LITHOSPHERIC SEISMIC ANISOTROPY IN THE VICINITY OF THE MAY 12, 2008, WENCHUAN, CHINA, EARTHQUAKE

The May 12, 2008 magnitude 7.9 Wenchuan earthquake occurred along the Longmenshan thrust fault, which separates the Tibetan Plateau and the Sichuan Basin in southwestern China. Analysis of P-to-S converted phases from the core-mantle-boundary, including PKS, SKKS, and SKS, recorded at two permanent broadband seismic stations in the vicinity of the earthquake revealed systematic dependence of the splitting parameters (fast polarization directions and splitting times) on the geographic location of the ray-piercing points. At station CD2 which is located about 60 km east of the epicenter of the Wenchuan mainshock, XKS raypaths from the northwest, which sample the Tibetan Plateau, show mostly NE-SW fast directions which are parallel to the strike of the Longmenshan fault. Events from the north which sample the westernmost edge of the Sichuan Basin, consistently demonstrate fast directions of about 120 degree (clockwise from the north). Numerous events with a back-azimuth of about 120 show excellent XKS energy on the radial component, but negligible energy on the transverse component (i.e., null measurements), suggesting that the observed azimuthal dependence is unlikely the result of multiple layer or dipping axis anisotropy. Instead, it might reflect rapid lateral variations of heterogeneities in lithospheric fabrics. Similarly, at station ENH which is located on the eastern edge of the Sichuan Basin, events from the north show dominantly 135 degree fast directions, and events from the SE show approximately NE (60 degree from the north) fast directions which is parallel to the strike of fold belts in the area. The splitting parameters do not show 90-degree periodicity. Our measurements suggest that the dominant lithospheric fabrics in the Sichuan Basin have an ENE direction. The study demonstrates that at stations located above vertical boundaries of anisotropy, station-averaged splitting parameters are not representative of the anisotropic structure.