Open AccessFurther structural constraints and uncertainties of a thin laterally varying ultralow‐velocity layer at the base of the mantle
Author(s) -
Garnero Edward J.,
Helmberger Donald V.
Publication year - 1998
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/98jb00700
Subject(s) - seismogram , geology , amplitude , mantle (geology) , magnitude (astronomy) , seismology , perturbation (astronomy) , earth structure , geodesy , p wave , geophysics , physics , optics , astrophysics , quantum mechanics , medicine , cardiology , atrial fibrillation
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 d KS . Source or receiver effects are ruled out as causes for the SP d KS anomalies used to map ULVZ structure, since systematic SP d KS ‐ 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 d KS / 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 d KS 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 δ V P and δ V S reductions, as well as the δ V S :δ V 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 d KS for models with strong velocity reductions in the layer; (5) models with extreme Q reductions in the ULVZ can affect SP d KS 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).