TWO-LAYER ANISOTROPY BENEATH HAWAII REVEALED BY TELESEISMIC SHEAR-WAVE SPLITTING ANALYSES

Splitting of P-to-S converted phases (XKS, including PKS, SKKS, and SKS) at the core-mantle boundary on the receiver side is one of the most effective tools to measure the direction and strength of fabrics in the mantle. This work presents new XKS splitting measurements obtained from broadband seismic stations in the vicinity of Hawaii. Limited quality and quantity of previous XKS studies in Hawaii resulted in contradicting conclusions regarding the characteristics of mantle fabrics and the causes that are responsible for generating the observed seismic anisotropy. This study adapted a robust procedure for measuring and objectively ranking XKS splitting parameters, which include the splitting time and the polarization direction of the fast wave. Manual visualization check was applied to the results of the automatic calculations to ensure that no high quality events were neglected and no low quality results were included. The resulting splitting parameters show systematic variation with the arriving azimuth of the XKS rays, suggesting a two-layer model. The lower layer is consistent with the current absolute plate motion (AMP) direction, implying that mantle flow in the asthenosphere is responsible for generating the observed anisotropy. The fast direction in the upper layer is close to the direction of major fracture zones, suggesting a lithospheric origin.