Seismic evidence for small-scale heterogeneity throughout the Earth's mantle

Hedlin, Michael A. H.; Shearer, Peter M.; Earle, Paul S.

doi:10.1038/387145a0

Cited by 172 publications

(188 citation statements)

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“…This relative homogeneity of the lower mantle may be real but may alternatively be due to the lack of resolution in current seismological techniques [Nolet, 1990;Machetel, 1990]. Recent analysis of PKP precursor amplitudes [Hedlin et al, 1997] suggests small-scale lower mantle heterogeneities may be ubiquitous. Localized heterogeneities such as subducted lithospheric slabs penetrating the 670 km discontinuity [Grand, 1994;van der Hilst et al, 1991;Fukao et al, 1992] and regions of the D" layer overlying the core-mantle boundary (CMB) [Loper and Lay, 1995] are the visible exceptions to lower mantle homogeneity.…”

Section: Introductionmentioning

confidence: 99%

Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data

Kaneshima¹,

Helffrich²

1998

J. Geophys. Res.

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Abstract. Short period University of Washington, northern California, and southern California seismic array records of a sequence of deep Mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following P. We measure slowness, arrival time, and arrival azimuth of these phases relative to P using array techniques to find where they emanate in the mantle. By combining slowness and back azimuth deviations and the move-out with depth of the arrivals from different earthquakes, we find their most probable origin is through S-to-P conversion at point scatterers in the mid-to lower mantle at 25.7øN 148.2øE 1590 km and 25.5øN 151.0øE 1850 km respectively, separated by -300 km. These features indicate elastically distinct material in the middle of the lower mantle, which, if fragments of subducted slabs, are not related to contemporary subduction. They may represent upwelling convective material from the deep mantle or D", but they are not purely thermal in nature. As chemically distinct bodies, they may represent potential reservoirs of the isotopic and trace element components recognized geochemically.

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Section: Introductionmentioning

confidence: 99%

Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data

Kaneshima¹,

Helffrich²

1998

J. Geophys. Res.

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“…After PKP, the coda amplitudes decay gradually with time. Scattering simulations (Hedlin et al, 1997) indicate that most of the energy at ranges below 125° is due to near surface effects. To illustrate the amount by which the amplitude of the precursors and coda increases from the low levels seen at ranges below 145° we plotted a copy of the average stack from 120 to 125° at all ranges (see the dashed traces).…”

Section: Preliminary Data Analysismentioning

confidence: 99%

“…The suites of curves in Figure 3 indicate the minimum travel time for single-scattered energy scattered at depths ranging from the CMB to 2200 km above. This figure illustrates that most scattered waves will follow PKP Hedlin et al (1997) considered the factors listed in the previous section and modeled a stack of 1600 PKP precursor recordings using Rayleigh-Born scattering theory (Chernov, 1960) applied to the exponential autocorrelation function. The modeling indicated that the global average small-scale heterogeneity has a scale length of ~8 km and is distributed uniformly throughout the lower-mantle with an RMS velocity contrast of 1%.…”

Section: Preliminary Data Analysismentioning

confidence: 99%

“…Numerous studies have used recordings of these precursors to investigate the physical properties of small-scale heterogeneity in the mantle (e.g. Doornbos 1972;Husebye et al, 1976;Hedlin et al, 1997). Although a recent analysis by Hedlin et al (1997) used GSN recordings of the precursors to study heterogeneity throughout the lower mantle, the geometry of the precursors favors studies of the deepest mantle as the preponderance of early arrivals come from great depth.…”

Section: Introductionmentioning

confidence: 99%

“…Although a recent analysis by Hedlin et al (1997) used GSN recordings of the precursors to study heterogeneity throughout the lower mantle, the geometry of the precursors favors studies of the deepest mantle as the preponderance of early arrivals come from great depth. Hedlin et al (1997) modeled the weak scattering that gives rise to precursors using the Born approximation and found that the gradual growth of the precursors with time and range favored a model that included heterogeneity distributed with equal strength at all depths in the lower mantle. Models that include heterogeneity just at the CMB or within D'' produce early arrivals, and thus sharper onsets than are seen in the data.…”

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confidence: 99%

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Probing mid-mantle heterogeneity using PKP coda waves

Hedlin

Shearer

2002

Physics of the Earth and Planetary Interiors

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We investigate the utility of PKP coda waves for studying weak scattering from small-scale heterogeneity in the mid-mantle. Coda waves are potentially a useful probe of heterogeneity at shallow depths because this energy is not preferentially scattered near the CMB, as PKP precursors are, but is due to scattering at all depths. PKP coda waves have not been used for this purpose historically because of interference with other late-arriving energy due to near-surface resonance and scattering. We have analyzed 3624 recordings of PKP precursors and coda made by stations in the IRIS Global Seismographic Network. To study the range and time dependence of the scattered waves, we binned and stacked envelopes of the recordings. PKP scattered waves increase in amplitude rapidly with range as predicted by scattering theory. At ranges below ~125°, we predict and observe essentially no scattered energy preceding PKP. Coda amplitudes at these ranges are independent of range and provide an estimate of energy due to near-surface effects we can expect at all ranges. We use the average coda amplitude at ranges from 120 to 125° to correct coda amplitudes at other ranges. PKP coda waves show a strong dependence on time and range and are clearly influenced by scattering in the lower mantle. PKP coda waves, however, do not provide a tighter constraint on the vertical distribution of mantle heterogeneity than is provided by precursors. This is due, in part, to relatively large scatter in coda amplitudes as revealed by a resampling analysis. Modeling shows that PKP coda amplitudes are not highly sensitive to the vertical distribution of heterogeneity in the mantle. To illustrate this we consider single-scattering in two extreme models of mantle heterogeneity. One allows heterogeneity just at the CMB; the other includes heterogeneity throughout the mantle. The amplitudes of precursors are tightly constrained by our stack and support our earlier conclusion that small-scale heterogeneity is uniformly distributed throughout the lower mantle. The best-fit model includes 8 km scale length heterogeneity with an RMS velocity contrast throughout the mantle of 1%. INTRODUCTIONSince the pioneering work of Haddon and Cleary in the early 1970's, the high-frequency energy that precedes the inner core phase PKP(df) has been known to result from scattering in the mantle (Haddon, 1972;Haddon and Cleary, 1974; Figure 1). These scattered waves arrive free of interference from latearriving near-surface scattered waves and thus provide a unique window into the small-scale structure of the deep Earth. Numerous studies have used recordings of these precursors to investigate the physical properties of small-scale heterogeneity in the mantle (e.g. Doornbos 1972;Husebye et al., 1976;Hedlin et al., 1997). Although a recent analysis by Hedlin et al. (1997) used GSN recordings of the precursors to study heterogeneity throughout the lower mantle, the geometry of the precursors favors studies of the deepest mantle as the preponderance of early arrivals come ...

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PKP precursors: Implications for global scatterers

Waszek

Thomas²,

Deuss

2015

Geophysical Research Letters

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Precursors to the core phase PKP are generated by scattering of seismic energy from heterogeneities in the mantle. Here we examine a large global data set of PKP precursors in individual seismograms and array data, to better understand scattering locations. The precursor amplitudes from individual seismograms are analyzed with respect to the inner core phase PKIKP and mantle phase PP. We find and correct for a hemispherical asymmetry in the precursor/PKIKP amplitudes, resulting from inner core structure. Using ray tracing, we locate scatterers in our array data and use these to infer scattering locations in the individual data. The scattering strength displays regional variation; however, we find no relationship with long‐scale core‐mantle boundary velocity structure. Scattering is observed in all regions of data coverage, as are paths with no precursors. This indicates that scattering occurs from various small‐scale heterogeneities, including but not limited to ultralow velocity zones or partial melt, and slabs.

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Seismic evidence for small-scale heterogeneity throughout the Earth's mantle

Cited by 172 publications

References 27 publications

Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data

Detection of lower mantle scatterers northeast of the Marianna subduction zone using short‐period array data

Probing mid-mantle heterogeneity using PKP coda waves

PKP precursors: Implications for global scatterers

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