Seismic detection of a thin laterally varying boundary layer at the base of the mantle beneath the central-Pacific
We explore lowermost mantle structure beneath the Pacific with long‐period recordings of the seismic core phases SKS, SP_dKS, and SKKS from 25 deep earthquakes. SP_dKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SP_dKS arrivals are explained by a laterally varying mantle‐side boundary layer at the CMB, having P‐velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large‐scale low velocity feature in the lower mantle beneath the central‐Pacific. SKS, SP_dKS, and SKKS data for the generally faster‐than‐average circum‐Pacific lower mantle are well‐fit by models lacking any such low‐velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions.
Additional Information© 1996 American Geophysical Union. Received July 31, 1995: revised October 11, 1995; accepted November 7, 1995. Thanks to X.-F. Liu for providing SKS12WM13, S. Grand for help with data and comments, T. Lay, H.-C. Nataf, J. Vidale, Q. Williams and two anonymous referees for constructive reviews. E.J.G. was supported by an NSF Postdoctoral fellowship. Contribution #288 of W.M. Keck Seismological Laboratory and Institute of Tectonics, UCSC, and #5590 of the Geological and Planetary Sciences, Caltech.
Published - 95GL03603.pdf