Further evidence for radial velocity anomalies in the lower mantle

Wright, C.; Lyons, J. A.

doi:10.1007/bf00878727

Cited by 29 publications

(5 citation statements)

References 13 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This study indicates that the D″ layer may be a material boundary layer whose depth can vary by ∼40 km. For P waves the earliest suggestions of a first‐order discontinuity at the top of the D″ layer were proposed by Wright and Lyons [1980] and Wright et al [1985].…”

Section: Introductionmentioning

confidence: 99%

Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific

Kito

Negishi

2004

J. Geophys. Res.

View full text Add to dashboard Cite

The P and S wave velocity structure of the D″ layer beneath the southwestern Pacific was investigated by using short‐period data from 12 deep events in the Tonga‐Fiji region recorded by the J‐Array and the Hi‐net (two large‐aperture seismic arrays) in Japan. Reflected wave beam forming (RWB) and a migration method were used to extract weak signals originating from heterogeneities in the lowermost mantle. In order to acquire high resolution a double‐array method was applied to the data. The results of the RWB method indicate that seismic energy is reflected at discontinuities near the depths of 2520 and 2650 km, which have a negative P wave velocity contrast of 1% at the most. In addition, there is a positive seismic discontinuity at a depth of 2800 km. In the case of the S wave, reflected energy is produced almost at the same depth (2550 km depth). An apparent depth shift (50 km) of the discontinuity at the depth of 2850 km may indicate that the S wave velocity reduction in the lowermost mantle is ∼2–3 times stronger than that of P. A two‐dimensional cross section, constructed with the RWB method, suggests that the observed discontinuities can be characterized as intermittent lateral heterogeneities whose lateral extent is a few hundred kilometers. The migration shows weak evidence of scattering objects which belong to the seismic discontinuities detected by the RWB method. These anomalous structures may represent a part of hot plume generated beneath the southwestern Pacific in the lowermost mantle.

show abstract

Section: Introductionmentioning

confidence: 99%

Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific

Kito

Negishi

2004

J. Geophys. Res.

View full text Add to dashboard Cite

show abstract

“…Evidence for laterally varying P-velocity structure in the lowermost mantle was given in studies of short-period P waves by Vinnik, Lukk & Nikolaev (1972), Wright & Cleary (1972), Wright (1973), Wright & Lyons (1980), Ruff & Lettvin (1984), Wright, Muirhead & Dixon (1989, Schlittenhardt (1986, Doornbos, Spiliopoulos & Stacey (1986), Young & Lay (1989), Baumgardt (1989), Neuberg & Wahr (1991), Houard & Nataf (1992 and Vidale & Benz (1993), and by the use of long-period P waves by Wysession & Okal (1989), Hock (1992) and Wysession et al (1992). Haddon & Cleary (1974), Doornbos (1978), Haddon (1982), Menke (1986a,b) and Bataille & Flattk (1988) present evidence for scattering that can be produced by CMB topography or velocity variations in D" [see also Bataille, Wu & Flattk (1990) for a review].…”

Section: Introductionmentioning

confidence: 99%

P- and S-wave reflections from anomalies in the lowermost mantle

Weber¹

1993

Geophysical Journal International

117

124

View full text Add to dashboard Cite

S U M M A R Y Seismic P and S waves recorded at the G R F array in Germany are used to study the inhomogeneous structure of the boundary layer D" at the base of the mantle. The use of the seismic array allows the detection of small-scale anomalies in the lowermost mantle. The lateral resolution attainable is about 100 to 400 km, i.e. more than 10 times better than with tomographic methods.The analysis of 13 years of G R F broad-band array data yields 255 events with high signal-to-noise ratio which are used to map the lowermost mantle. 74 of these events show anomalous P waves (PdP) which arrive 3-6 s after the direct P wave and have a slowness 0.7-0.8s/" smaller than the slowness of direct P. Other events close to the events with PdP do not have such an anomalous phase.Using slowness, traveltime, amplitude and waveform information it is demonstrated that PdP is caused by an anomalous lower mantle velocity structure below the turning point of the P wave. If the pP phase (i.e. a P wave first reflected at the free surface near the source) is used together with the P phase, distinct and well-separated P-velocity anomalies can be determined under the Nansen Basin, the Kara Sea and northern Siberia. The areas of the bounce points of PdP in the lower mantle have a lateral extension of about 100 by 200 km, but this is not the size of the anomaly, since the resolution of P waves at 1 Hz in this depth is 130 km by 260 km (Fresnel zone). The accuracy to which the depth of the reflector (2612 km under the Nansen Basin and 2605 km under northern Siberia respectively) can be determined for l-D models is f 1 0 to 20km. The velocity contrast at the lower mantle discontinuity is about 3 per cent f l to 1.5 per cent. Areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas. 2-D models of the anomalies reveal the range of adequate models and possible trade-offs. If the lateral extension of the anomaly is about 7" the reflector has to be 40 km deeper than in the l-D model. If the dip of the reflector in the lowermost mantle is only about 1.5" it is difficult to resolve if the reflector is tilted towards the source or towards the receiver. For the anomaly under the Nansen Basin deviations from the great circle path are observed for PdP, indicating 3-D effects. Below the three anomalies the core-mantle boundary (CMB) can be located by PcP with a depth in agreement with standard earth models.The analysis of the S waves (S, SdS and ScS) reveals two S-velocity anomalies in the lowermost mantle under northern Siberia. The first anomaly coincides with the P velocity anomaly under northern Siberia and can be explained by a 1-D model with a reflector depth of 2610 km f 15 km and a velocity contrast of 2.3 per cent. The second S-velocity anomaly is in an area where no P-velocity anomaly can be detected. The corresponding l-D S-velocity model has a reflector depth of 2575 km f 15 km and an S-velocity contrast of 2.6 per cent. The smallest structures that can be resolved with the S waves in th...

show abstract

“…Detailed travel time, ray parameter, and seismic waveform studies have provided evidence for arrivals that reflect from the increased velocity gradient about 150-300 km above the core. P wave studies have mainly used short-period and broadband array data [Wright and Cleary, 1972;Wright, 1973;Wright and Lyons, 1980;Wright et al, 1985;Baumgardt, 1989;Davis and Weber, 1990;Weber and Davis, 1990;Weber andKOrnig, 1990, 1992;Neuberg and Wahr, 1991;HouardandNataf, 1992HouardandNataf, , 1993 Figure 2 indicates the global extent of D" exhibiting evidence for a P and/or S wave discontinuity. Coverage of the lowermost mantle is restricted by the available source and receiver geometries.…”

mentioning

confidence: 99%

The core‐mantle boundary region

Loper¹,

Lay²

1995

J. Geophys. Res.

157

View full text Add to dashboard Cite

The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of the transition zone between the Earth's core and mantle. The core-mantle boundary (CMB) is the most significant internal boundary within our planet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding of the dynamic Earth system. The thermal and chemical processes operating near the CMB have intimate relationships to fundamental events in Earth history, such as core formation, and continue to play a major role in the planet' s evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the mode of thermal convection of the Earth. The D" region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and small scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge that we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious increases in data, computational power, and experimental methodologies used to investigate deep earth structure and processes. In the past decade in particular there have been unprecedented multidisciplinary advances in understanding the CMB region, and there are excellent prospects for developing a comprehensive understanding of this region in the next few decades.Fundamental issues yet to be resolved include the causes of layering extensively observed in D' ', whether or not downwelling slabs accumulate at the base of the mantle, whether plumes arise from the CMB region to feed hotspots at Earth' s surface, the extent of core-mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to which mantle structure influences the geomagnetic field and its reversals. This overview highlights the progress and future directions in geophysical investigations of the CMB region. 1919-1994 IntroductionOur Earth is a dynamic body in continual evolution. An important event in this evolution was separation of the ironalloy core and silicate-oxide mantle early in the history of i_•111.11, [.JIKJOO. UIJ ULIIIlI• I. ll•, lllO. lll stage ,,, o.,• ,, ......... process [Anderson and Hanks, 1972' Ruff and Anderson, 1980; Stevenson, 1981' Ringwood,' 1984; Jones and Drake, 1986; Ida etak, 1987; Stevenson, 1990; Honda et al., 1993a]. This chemical separation profoundly impacted the subsequent mechanical, thermal, and chemical evolution of Earth, as the large density contrast between the core and mantle prohibits convective motions across the core-mantle boundary (CMB).Consequently, boundary layers have formed in the CMB region, and these control the transfer of heat, angular momentum, and material between the core and mantle. The goal ...

show abstract

Further evidence for radial velocity anomalies in the lower mantle

Cited by 29 publications

References 13 publications

Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific

Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific

P- and S-wave reflections from anomalies in the lowermost mantle

The core‐mantle boundary region

Contact Info

Product

Resources

About