Complex seismic anisotropy at the border of a very low velocity province at the base of the Earth's mantle

We constrain the anisotropy associated with a very low velocity province ( VLVP) at the base of the Earth's mantle using the SKS and SKKS waves sampling the region. Our selected high- quality data sets consist of 415 SKS and 111 SKKS waveforms for 127 deep earthquakes recorded at distances between 90 degrees and 150 degrees by the seismic stations in three temporary broadband PASSCAL seismic arrays: the Kaapvaal seismic array ( 1997 - 1999), the Tanzania seismic array ( 1994 - 1995), and the Ethiopia/ Kenya seismic array ( 1999 - 2001), as well as the permanent stations in the Global Seismographic Network. These seismic data provide good sampling coverage for some portion of the VLVP and its surrounding areas. Our results show, when the SKS or SKKS phases sample the regions away from the border of the VLVP ( inside or outside the VLVP), the apparent splitting parameters inferred from the SKS phases are consistent with those inferred from the SKKS phases, and their variations strongly correlate with seismic stations but not with the exit points at the core- mantle boundary of these seismic phases. However, when the SKS or SKKS phases sample near the border of the VLVP, the apparent splitting parameters inferred from the SKS phases and SKKS phases are different, and their variations no longer correlate with seismic stations. These features indicate that part of the shear wave splitting for the seismic data sampling the border of the VLVP has to originate from deep mantle, most likely near the border of the VLVP. We assume that the anisotropy in the shallow mantle beneath seismic stations has a horizontal hexagonal symmetry axis and infer the splitting parameters associated with the shallow anisotropy beneath the seismic stations using the SKS and SKKS waveforms for the seismic data sampling the regions away from the border of the VLVP. We then obtain the splitting parameters associated with the lowermost mantle anisotropy using the SKS and SKKS waveforms corrected for the inferred shallow anisotropy beneath seismic stations, assuming that the medium in the lowermost mantle also has a horizontal hexagonal symmetry axis. Our results reveal a complex anisotropy pattern near the border of the VLVP. Such a complex anisotropy pattern may be explained by lattice- preferred orientation of anisotropic mantle aggregates aligned by a complex mantle flow near the VLVP margins. The complex flow pattern near the VLVP margins may indicate strong interactions between the VLVP, a compositional anomaly, and the surrounding normal mantle, and may provide an explanation to the concentration of some hot spots geographically near the borders of the VLVPs in the lowermost mantle.