High Frequency PKKPBC Around 2.5 Hz Recorded Globally

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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Observations of PKKPab Diffraction Waves Well Beyond Cutoff Distance

Chen

Zhang

2021

Seismological Research Letters

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The core mantle boundary (CMB) features the most dramatic contrast in the physical properties within the Earth and plays a fundamental role in the understanding of the dynamic evolution of the Earth’s interior. Seismic core phases such as PKKP sample large area of the lowermost mantle and the uppermost core, thus providing valuable information of the velocity structures on both sides of the CMB. Diffraction Waves Well Beyond Cutoff Distance (PKKPab) is one branch of the triplicated PKKP that can be observed beyond its ray theoretical cutoff distance as a result of diffraction along the CMB. The travel time and slowness of the diffracted PKKPab (denoted as PKKPabdiff) can be used to constrain the P-wave velocities at the lowermost mantle, thus have been investigated in numerous studies. Previous results (Rost and Garnero, 2006) suggest that most of the observations of the PKKPabdiff waves are in the epicentral distance range of 95°–105° (minor arc convention) (PKKPabdiff diffraction length less than 10°). However, high-frequency (∼1 Hz) synthetic seismograms show that the PKKPabdiff waveforms could be observable at distance down to 65°, which indicates that the PKKPabdiff signals could be detected at distances less than 95° in observations. To explore the distance ranges in which PKKPabdiff is observable, we collected global three-component broadband waveforms from 246 events with source depth deeper than 100 km and magnitude above M 6 from 2007 to 2017 available at the Incorporated Research Institutions for Seismology Data Management Center. We analyzed the slowness, polarization, and amplitude of the candidate PKKPabdiff signals, and found 95 events with clear PKKPabdiffsignals, with nearly 60% of the events show PKKPabdiff diffraction lengths greater than 10°, and the longest diffraction distance is beyond 20°. These newly identified PKKPabdiff waves would substantially augment the dataset of core phases for improvements of the CMB velocity models.

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High Frequency PKKPBC Around 2.5 Hz Recorded Globally

Cited by 3 publications

References 42 publications

Geophysical Constraints on Mantle Composition

Geophysical Constraints on Mantle Composition

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

Observations of PKKPab Diffraction Waves Well Beyond Cutoff Distance

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