Amplitudes of long-period PcP and the core-mantle boundary

[1] Seismic phases that reflect off the core-mantle boundary (CMB) are sensitive to the velocity and density contrasts between the base of the mantle and the core. We measure the amplitudes of CMB reflections to seek effects of large velocity reductions and possible density increases in proposed thin basal layers (ultralow-velocity zones, or ULVZs). We construct globally averaged envelope stacks of PcP and ScP in 3°-wide epicentral distance bins, correcting for propagation and source effects. We measure the amplitude ratios PcP/P and ScP/P and compare them with the predicted range dependence for preliminary reference Earth model (PREM) and several proposed ULVZ models. The amplitude ratios are not compatible with ULVZs containing 30% S wave velocity reductions or with a 3-km-wide core-mantle transition zone. The PcP data cannot distinguish between PREM and models with 10% reductions in both P wave velocity (V P ) and S wave velocity (V S ) and up to a 20% increase in density. The ScP data also match PREM closely but contain greater uncertainty from the correction for mantle attenuation. In well-sampled regions, lateral variations of reflection amplitudes mapped to bounce point locations are consistent with previous detections and nondetections of ULVZs. These results suggest that extreme velocity reductions near the CMB are not global features and that regions with large velocity reductions do not result from global phenomena such as phase changes in an isochemical mantle but rather from local thermal or compositional anomalies.

Section: Amplitude Ratiossupporting

confidence: 85%

Reflection properties of the core‐mantle boundary from global stacks of PcP and ScP

Persh

Vidale

2004

“…Such a model has, in principle, already been proposed [e.g., Sacks and Beach, 1974;$noke and Sacks, 1976], and it has been shown to be compatible with some of the seismological data. However, numerous difficulties remain [e.g., Husebye et al, 1976;Miiller et al, 1977], and this model must be considered speculative at present. An attempt to explain the observations from the lowermost mantle, such as the velocity anomalies and apparent scattering, in terms of a simple (single) boundary layer could provide an important siesmological test on the constitution of the lower mantle.…”

Section: Chemical Compositionmentioning

confidence: 99%

Convection, composition, and the thermal state of the lower mantle

Jeanloz¹,

Richter²

1979

212

We present a thermal model for the lower mantle, which is constructed from petrologically derived estimates of the temperature in the transition zone and from an adiabat based on the thermal properties of MgO and SiO2 measured at high pressures. Superadiabatic contributions to the geotherm through the lower mantle, including those from possible phase transformations but excluding those from thermal boundary layers, are negligibly small. A thermal boundary layer is required at the base of the mantle in order to satisfy our estimate of the lowest possible temperature in the core (about 2800 K); its thickness of about 100 km is well constrained. Even so, our model suggests that we require either a significantly larger heat flux from the core than has been considered reasonable (>50 mW m -2) or the presence of a further thermal boundary layer within the lower mantle in order to arrive at more likely core temperatures (about 3200-3500 K). The latter alternative appears the more plausible, and such a boundary layer requires a barrier to convection. Guided by the present seismological evidence, we suggest that a thermal boundary layer is associated with a chemical discontinuity either at the top of the lower mantle or near its base (D" region). If seismological data require the D" layer to be significantly thicker than about 100 km, this is support for a chemically distinct D" region with multiple thermal boundary layers. Otherwise, the results of this study suggest that the upper and lower mantle may be chemically distinct, hence precluding whole mantle convection.

“…Furthermore, we evaluate the possibility of using PKiKP / PcP amplitude ratios to map out heterogeneities in the lowermost mantle. Previous authors have interpreted PcP / P amplitude ratios in terms of CMB structure [ Muller et al , 1977; Neuberg and Wahr , 1991; Kruger et al , 1995; Persh , 2002]; however, for some distance ranges, PKiKP / PcP ratios can be finer probes because of greater ray path similarity between PKiKP and PcP than between P and PcP .…”

Section: Introductionmentioning

confidence: 99%

Observations of PKiKP/PcP amplitude ratios and implications for Earth structure at the boundaries of the liquid core

Koper

2004

[1] Mining of the database produced by seismic array stations of the International Monitoring System has resulted in hundreds of new observations of precritical reflections from Earth's inner core. Here we present short-period amplitude ratios between these PKiKP phases and corresponding reflections (PcP) from the core-mantle boundary (CMB). PKiKP/PcP amplitude ratios are one of the few seismic observations that directly constrain the change in S velocity and density across the inner core-outer core boundary. We measure the amplitude ratios from optimally tuned array beams and use a bootstrap technique to estimate observational uncertainties. We also use a model resampling technique to estimate uncertainties associated with the background reference model. The optimal models have S velocities at the top of the inner core less than about 2.5 km/s and density jumps across the inner core-outer core boundary of less than about 0.45 g/cm 3 . These values can be reconciled with the higher estimates from normal mode constraints if strong radial gradients are present in the top of the inner core or if the outermost inner core has material properties distinct from the bulk of the inner core. We also find evidence for an ultralowvelocity zone in the lowermost mantle beneath the eastern coast of Australia. A unique, best fitting model cannot be determined, but an example that is consistent with the data has a reduction in P velocity of 10%, a reduction in S velocity of 35%, and an increase in density of 20%. This type of structure could be explained by the presence of partial melt and increased iron content just above the CMB.