2012 IRIS Workshop
A large ultra-low-velocity zone at the base of the mantle near Hawaii
Sanne Cottaar: UC Berkeley, Barbara Romanowicz: UC Berkeley
a) Data coverage of S(diff) from three events in the southwestern Pacific towards North America. Dots indicate pierce points above the CMB. Background model is SAW24B16 at 2750 km depth. The black circle is the proposed ULVZ. b) Sketch of the lens-like cause of the postcursors. c) Synthetics for SAW24B16 model. 316 traces for one event are shown in azimuth bins for Sdiff phases between 100 and 110°. d) Same traces as in c. but now with the proposed ULVZ. e) Corresponding observed traces.
Full-resolution graphics file in original format: 0127.tiff
The USArray transportable array has greatly improved the coverage of the lowermost lower mantle beneath the central Pacific. Our study investigates several events in the western Pacific for which diffracted phases towards North America are sensitive to the northern boundary of the Pacific LLSVP beneath Hawaii. The S/Sdiff phases of the event display waveform complexities, including strongly delayed (>30 s) postcursors. Beamforming analysis shows that the observed postcursors originate from a localized area in D”. Through 3D forward modeling using the Spectral Element Method (SEM), we show these observations can be explained by the presence of an unusually large ultra-low-velocity-zone (ULVZ). While some trade-offs exist between size, shape, velocity reduction and height, of the ULVZ, our preferred model consists of a cylindrical structure located ~15° west of Hawaii with a large aspect ratio: a width of ~900 km and height of ~20 km above the core-mantle boundary. The shear wave velocity reduction is ~20%. The location of this ULVZ suggests that it may be the root of a long-lived, stable plume responsible for the Hawaiian volcanic chain.
Acknoweldgements: We thank Akiko To for helpful discussions. This work was supported by NSF/CSEDI grant 067513
For further reading: Cottaar and Romanowicz (2012), An unusually large ULVZ at the base of the mantle near Hawaii, submitted to Nature Geoscience
Keywords: core-mantle_boundary, ulvz, sdiff