Ray parameters of diffracted long period P and S waves and the velocities at the base of the mantle

Mula, Abdel Hafiz; Müller, Gerhard

doi:10.1007/bf01593067

Cited by 43 publications

(37 citation statements)

References 25 publications

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“…While the slowness is the linear slope of the arrivals on a travel time versus distance plot, picking the arrival times is difficult because of the emergent nature of the waveforms that results from the preferential attenuation of higher frequencies during diffraction. As a result of this change in frequency content, and therefore waveshape, any attempt to determine a slowness through successive arrivals results in an "apparent slowness" p,, instead of the true slowness at the CMB [Mula and Miiller, 1980]. We used a linear least squares regression through the times of the SHdiff pulse peak maxima [Mondt, 1977;Mula and Miiller, 1980], which has the same reliability as a multiphase cross-correlation but is easier to obtain because anomalous arrival times cause multiple peaks within and a broadening of the multiphase cross-correlogram [Wysession and Okal, 1989 Because of the frequency-dependent amplitude decay that is observed to occur in seismic waves during the process of diffraction [Mula and Miiller, 1980], the slowness anomaly is quantified by comparison with reflectivity synthetic seismograms [Kennett, 1980] …”

Section: Data and Proceduresmentioning

confidence: 99%

“…Analysis techniques involving synthetic modeling were established nearly 20 years ago [Mondt, 1977;Doornbos and Mondt, 1979a, 1979b, 1980Mula and Miiller, 1980;Mula, 1981]. Later on, similar azimuth profiles were used to examine regional tracks along the CMB [Okal and Geller, 1979 The ray parameter for a diffracted wave is the derivative of the travel time-distance curve T(zS) beyond the shadow boundary and has a linear behavior as indicated by p = dT/dA = Rc, ua/Vc, ua, (•) where p is the ray parameter, T is the travel time, and A is the epicentral distance [Mula and Miiller, 1980;Wysession et al, 1992]. The ray parameter can also be determined from Snell's law and is expressed as a function of the Earth-normalized radius of the CMB, RcMB, and the average seismic wave velocity at the CMB, VcMB.…”

Section: Introductionmentioning

confidence: 99%

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Lateral variations at the base of the mantle from profiles of digital S_diff data

Valenzuela¹,

Wysession²,

Neustadt³

et al. 2000

J. Geophys. Res.

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Abstract. Average long-wavelength shear velocity structure in several regions at the base of the mantle is determined using the profiles of core-diffracted SH waves (SHrill). There is significant lateral variation in the Sdiff apparent ray parameters, suggesting considerable lateral shear velocity anomalies in D", the region just above the core-mantle boundary (CMB). Apparent ray parameters are determined by least squares fits through the pulse maxima of the instrument-deconvolved ground displacements, and heterogeneities are quantified through comparisons with reflectivity synthetic ray parameters. Corrections are applied to the data to account for the effects of the

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Section: Data and Proceduresmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Lateral variations at the base of the mantle from profiles of digital S_diff data

Valenzuela¹,

Wysession²,

Neustadt³

et al. 2000

J. Geophys. Res.

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“…The obtained apparent velocity from ray parameter depends on the frequency of the waves and the velocity structures at the base of the mantle (Chapman and Phinney 1972;Mula and Müller 1980). Therefore, the apparent velocity in Eq.…”

Section: Methodsmentioning

confidence: 99%

Ultra-low velocity zone heterogeneities at the core–mantle boundary from diffracted PKKPab waves

Sun

2017

Earth Planets Space

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Diffracted waves around Earth's core could provide important information of the lowermost mantle that other seismic waves may not. We examined PKKP ab diffraction waves from 52 earthquakes occurring at the western Pacific region and recorded by USArray to probe the velocity structure along the core-mantle boundary (CMB). These diffracted waves emerge at distances up to 10° past the theoretical cutoff epicentral distance and show comparable amplitudes. We measured the ray parameters of PKKP ab diffraction waves by Radon transform analysis that is suitable for largeaperture arrays. These ray parameters show a wide range of values from 4.250 to 4.840 s/deg, suggesting strong lateral heterogeneities in sampling regions at the base of the mantle. We further estimated the P-wave velocity variations by converting these ray parameters and found the CMB regions beneath the northwestern edge of African Anomaly (Ritsma et al. in Science 286:1925-1928) and southern Sumatra Islands exhibit velocity reductions up to 8.5% relative to PREM. We suggest that these low velocity regions are Ultra-low velocity zones, which may be related to partial melt or iron-enriched solids.

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“…the D" region, are somewhat controversial. The following models with different velocity gradient have been proposed for the D" region; negative velocity gradient (CLEARY, 1969;DOORNBOS and MONDT, 1979;DOORNBOS, 1983), neutral (PREM), positive velocity gradient (MITCHELL and HELMBERGER, 1973;MULA, 1981) and the existence of a discontinuity (LAY and HELMBERGER, 1983;YOUNG and LAY, 1987;REVENOUGH and JORDAN, 1991). Here, we discuss the effects of the structures of the D" region on the velocities in the outer core.…”

Section: Effects Of Velocity Structure In the Lowermost Mantlementioning

confidence: 99%

“…CLEARY's (1969) interpretation based on the ray theory was questioned because it is difficult to apply the ray theory to the diffracted waves. Analysis of amplitude decay and apparent velocity of the diffracted waves was done by MONDT (1977), DOORNBOS and MONDT (1979), using the full wave theory and by MULA (1981), WYSESSION et al (1992) using reflectivity method. These results support a slightly positive or slightly negative gradient as an average structure in the D" region.…”

Section: Effects Of Velocity Structure In the Lowermost Mantlementioning

confidence: 99%

Velocities and Chemical Stratification in the Outermost Core

Tanaka

Hamaguchi

1993

J. geomagn. geoelec

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Travel times of long-period SKS and S2KS phases recorded at the GDSN and WWSSN stations were analyzed to retrieve information about the radial structure of the outer core. We propose a new velocity model for the outermost core named KTH. The velocity at the top of the core is 8.016 km s-1, and the velocity in the outermost 200 km of the core is 0.11 km s-1 faster than that. of HALES and ROBERTS (1971), that in the outer 200400 km of the core is identical within 1% to that of HALES and ROBERTS (1971). We find that the inhomogeneity index rl is slightly larger than unity in the outermost 100,200 km of the core where a preferable value of q just beneath the core-mantle boundary is 1.5 which corresponds to Brunt-Vaisala frequency N of 8.6 x 10-4 s-1. We suggest that the outermost 100200 km of the core is stably stratified. The thickness of the stable stratified layer in the present study is not far from those of 7080 km thickness derived from theoretical considerations. By comparing possible lateral heterogeneity in the outermost core with temperature distribution from a geomagnetic study, we infer that a chemically stratified layer is preferable in the outermost core.

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Ray parameters of diffracted long period P and S waves and the velocities at the base of the mantle

Cited by 43 publications

References 25 publications

Lateral variations at the base of the mantle from profiles of digital S_diff data

Lateral variations at the base of the mantle from profiles of digital S_diff data

Ultra-low velocity zone heterogeneities at the core–mantle boundary from diffracted PKKPab waves

Velocities and Chemical Stratification in the Outermost Core

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Ray parameters of diffracted long period P and S waves and the velocities at the base of the mantle

Cited by 43 publications

References 25 publications

Lateral variations at the base of the mantle from profiles of digital Sdiff data

Lateral variations at the base of the mantle from profiles of digital Sdiff data

Ultra-low velocity zone heterogeneities at the core–mantle boundary from diffracted PKKPab waves

Velocities and Chemical Stratification in the Outermost Core

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Product

Resources

About

Lateral variations at the base of the mantle from profiles of digital S_diff data

Lateral variations at the base of the mantle from profiles of digital S_diff data