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Further structural constraints and uncertainties of a thin laterally varying ultralow-velocity layer at the base of the mantle

Garnero, Edward J. and Helmberger, Donald V. (1998) Further structural constraints and uncertainties of a thin laterally varying ultralow-velocity layer at the base of the mantle. Journal of Geophysical Research B, 103 (B6). 12,495-12,509. ISSN 0148-0227. http://resolver.caltech.edu/CaltechAUTHORS:20121129-104248537

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Abstract

Constraints and uncertainties are presented for modeling of an ultralow-velocity zone layer (ULVZ) at the base of Earth's mantle using an SKS wave with small segments of P wave diffraction at the SKS core entry and exit locations, called SP_dKS. Source or receiver effects are ruled out as causes for the SP_dKS anomalies used to map ULVZ structure, since systematic SP_dKS-SKS travel time moveout behavior is present in profiles of recordings of a given earthquake at many seismographic stations and also for many events recorded at one station. The southwest Pacific region produces strong variability in observed SP_dKS/SKS amplitude ratios (compared to synthetic seismograms), which geographically corresponds to an anomalous ULVZ region. Accurate determination of absolute ULVZ thicknesses requires knowledge of, in addition to magnitude of P wave velocity (V_p) reduction in the layer, the magnitude of S wave velocity (V_S) reduction and density (ρ) perturbation (if any). Synthetic seismogram experiments demonstrate several key points regarding uncertainties and constraints in modeling ULVZ structure: (1) thicker layers (up to 300 km thick) with mild reductions (e.g., −2.5 to −5.0%) cannot reproduce the anomalous SP_dKS behavior seen in the data; (2) for ULVZ layers less than 10 km thick, strong trade-offs exist between discontinuous velocity reductions and linear gradient reductions over a thicker zone; (3) uncertainties preclude precise determination of magnitude of δV_P and δV_S reductions, as well as the δV_S:δV_P ratio; (4) large density increases within the ULVZ (e.g., up to 60% and more) can efficiently broaden and delay the peak of the energy that we identify as SP_dKS for models with strong velocity reductions in the layer; (5) models with extreme Q reductions in the ULVZ can affect SP_dKS waveforms, and dampen spurious ringing energy present in Sd waveshapes due to the ULVZ; and (6) the minimum required V_p reduction for the most anomalous data (around −10%) trades off with thinner ULVZ structures containing larger velocity reductions (with possible density increases as well).


Item Type:Article
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http://dx.doi.org/ 10.1029/98JB00700DOIUNSPECIFIED
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Additional Information:© 1998 American Geophysical Union. Received 29 October 1997; revised February 2, 1998; accepted 18 February 1998. We thank Steve Grand, Thome Lay, Barbara Romanowicz. and Jeroen Ritsema for data, preprints, and helpful discussions. and we also thank the makers of GMT software [Wessel and Smith, 1991]. Reviews by Michael Wysession, Annie Souriau, and Peter Shearer improved the manuscript. This research was partially supported by NSF grant EAR-9896047.
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Funding AgencyGrant Number
NSFEAR-9896047
Record Number:CaltechAUTHORS:20121129-104248537
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121129-104248537
Official Citation: Garnero, E. J. and D. V. Helmberger (1998), Further structural constraints and uncertainties of a thin laterally varying ultralow-velocity layer at the base of the mantle, J. Geophys. Res., 103(B6), 12,495–12,509, doi:10.1029/98JB00700.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35728
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:29 Nov 2012 19:05
Last Modified:27 Dec 2012 03:05

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