Detection of an ultralow velocity zone at the core‐mantle boundary using diffracted PKKPab waves

Rost, Sebastian; Garnero, Edward J.

doi:10.1029/2005jb003850

Cited by 30 publications

(22 citation statements)

References 56 publications

(70 reference statements)

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“…An onset separation greater than 6.4 s between BC and AB branches of PKKP was expected from global models at epicentral distances in the range 105-117°(along minor arc). At shorter distances, PKKP AB was expected to be diffracted at the CMB, the corresponding phase being PKKP diff AB : That phase was clearly observed at distances down to 97°, in agreement to other observations (ISC Bulletin; BORMANN and WENDT, 2002;ROST and GARNERO, 2006). Spectral computation for PKKP BC and PKKP AB was done by using the FFT subroutine described by STEARNS (1975).…”

Section: Observations and Data Processingsupporting

confidence: 55%

High Frequency PKKPBC Around 2.5 Hz Recorded Globally

Ivan

Cormier

2010

Pure Appl. Geophys.

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Anomalous high frequency PKKP BC signals (displaying a large amount of energy around 2.5 Hz), recorded globally for deep and intermediate depth earthquakes, are compared to PKKP AB signals. The attenuation difference t Ã AB À t Ã BC is evaluated from spectral amplitudes in the range 96-111°, being approximately twice the results provided by full-wave theory and PREM (with no low Q l zone in the lowermost mantle and a nearly infinite Q K in the outer core). Most ray paths for such recordings are piercing the D 00 region in the proximity of regions where ultra-low velocity zones (ULVZ) have been previously reported beneath the North Atlantic Ocean, the Southwest Pacific and the southwestern part of South America. If BC amplitudes around 2.5 Hz and at low frequencies (0.5-1.5 Hz) are comparable, the observed attenuation difference (in the frequency range 0.2-2.5 Hz) is small (around 0.25 s) and close to the PREM value. The particle motion of the high-frequency PKKP BC at 2.5 Hz is quite similar to that of the raw recording, suggesting a deep source. An explanation for this might be scattering of the BC branch in some very restricted areas of the lowermost mantle. Alternately, the presence of a thin layer with high attenuation in the D 00 region would most likely be associated with either the ultra-low velocity zone (ULVZ) or light sediments on the underside of the core-mantle boundary (CMB). Correlated to other methods to investigate the lowermost mantle, the high-frequency PKKP BC can be used to map lateral variations of attenuation above the CMB, possibly associated with the boundary of the superplumes, especially when PKKP AB is observed.

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Section: Observations and Data Processingsupporting

confidence: 55%

High Frequency PKKPBC Around 2.5 Hz Recorded Globally

Ivan

Cormier

2010

Pure Appl. Geophys.

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“…Unlike previous studies (Rost and Garnero 2006), in this study, we did not carry out the analysis of absolute travel times of PKKP ab diffraction or relative travel times with respect to PKKP bc . Travel time analysis, though, could verify our results inferred from ray parameters.…”

Section: Comparison With Previous Studiesmentioning

confidence: 99%

“…Our region in the northwestern side of African Anomaly in general agrees with the region of high ULVZ likelihood in their map, but extends further west. Using PKKP data recorded by Yellowknife array, Rost and Garnero (2006) also mapped ULVZs in a region located further north of our study region, with a similar lateral extension. Their results did not show systematic velocity changes, but they did notice that PKKP ab diffraction waves sampling the west of the region show larger departures from theoretical ray paths than those to the east.…”

Section: Ulvzs At the Cmbmentioning

confidence: 99%

“…1). In general, PKKP cd and PKKP df are difficult to see due to very small amplitudes, while PKKP ab and PKKP bc are more easily identified in short-period seismograms (Earle 2002;Rost and Garnero 2006;Ivan and Cormier 2011;Niu et al 2012). …”

Section: Introductionmentioning

confidence: 99%

“…The inserted box shows one-sided diffraction of PKKP ab waves along the CMB seismograms, thus providing unique solutions to smallscale velocity structures at the CMB. In spite of these merits, there are only a few studies utilizing PKKP ab diffraction waves (Rost and Garnero 2006;Ivan and Cormier 2011), which is mainly because of lack of high-quality data. With the increase in temporary/permanent stations all around the world, PKKP ab diffraction phase now may provide more opportunities to resolve the CMB structures in regions that have not been studied previously.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

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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Intermittent and lateral varying ULVZ structure at the northeastern margin of the Pacific LLSVP

Zhao

Garnero

et al. 2017

JGR Solid Earth

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Thin patches with ultralow velocities have been proposed to exist at the core‐mantle boundary (CMB). The detection and mapping of ultralow velocity zones (ULVZs) are difficult, in part, because of limited source‐receiver geometries of seismic phases used in ULVZ modeling. Here we develop a new approach that simultaneously utilizes ScS precursor and postcursor energies to investigate the CMB region for ULVZ structure. We stacked source‐deconvolved ScS waveforms within 1.5° geographic bins to extract ScS precursor and postcursor energies, if present, with ScS effectively removed from waveforms. We investigate the CMB beneath the central Pacific Ocean, and evidence for ULVZs is clearly apparent. Geographic bin stacks possessing similar ScS precursor‐plus‐postcursor behavior are grouped by using cluster analysis to produce more robust waveforms by enhancing the signal‐to‐noise ratios. Synthetic seismograms that demonstrate the amplitude and timing of the ULVZ arrivals are sensitive to ULVZ thickness and internal velocities. To pursue local ULVZ properties we processed 13,850 1‐D synthetic models with various ULVZ thicknesses and internal properties, using the identical ScS‐stripping method as with the data. A best fitting model was found for each geographical bin cluster by using an amplitude‐sensitive cross‐correlation algorithm. While limitations exist due to 1‐D modeling, strong lateral variations are clearly apparent in ULVZ thickness and properties across the large low shear velocity province (LLSVP) margin in our study area. Inside hypothesized LLSVP edges, ULVZs appear to distribute unevenly, suggesting 3‐D variations of convection currents.

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Detection of an ultralow velocity zone at the core‐mantle boundary using diffracted PKKP_ab waves

Cited by 30 publications

References 56 publications

High Frequency PKKPBC Around 2.5 Hz Recorded Globally

High Frequency PKKPBC Around 2.5 Hz Recorded Globally

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

Intermittent and lateral varying ULVZ structure at the northeastern margin of the Pacific LLSVP

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