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High Frequency PKKPBC Around 2.5 Hz Recorded Globally

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Abstract

Anomalous high frequency PKKPBC signals (displaying a large amount of energy around 2.5 Hz), recorded globally for deep and intermediate depth earthquakes, are compared to PKKPAB signals. The attenuation difference \( t_{\text{AB}}^{*} - t_{\text{BC}}^{*} \) 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 Qμ zone in the lowermost mantle and a nearly infinite QK in the outer core). Most ray paths for such recordings are piercing the D″ 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 PKKPBC 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″ 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 PKKPBC can be used to map lateral variations of attenuation above the CMB, possibly associated with the boundary of the superplumes, especially when PKKPAB is observed.

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Acknowledgements

The authors express deep gratitude to the team maintaining the Event Waveform Archive of the Geological Survey of Canada (http://earthquakescanada.nrcan.gc.ca/); also, to the people from IRIS DMC and PIDC, for support in acquiring the high-quality input data for this study. Some figures were produced by using GMT files (Wessel and Smith, 1996). Most computations in time domain have been performed by TauP ToolKit (Crotwell et al., 1999). Comments and suggestions of Sebastian Rost and an anonymous reviewer substantially improved the paper. V.F. Cormier’s participation in this study was funded by the National Science Foundation under grant EAR 07-38492. M. Ivan received a partial financial support from ACSICS PNCDI 2 project D-11-025.

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  1. University of Bucharest, Traian Vuia Str. 6, P.O. Box 37, 70138, Bucharest, Romania

    Marian Ivan

  2. National Institute of Earth Physics, P.O. Box MG-21, Bucuresti, Magurele, Romania

    Marian Ivan

  3. Physics Department, University of Connecticut, 2152 Hillside Road, Storrs, CT, 06269-3046, USA

    Vernon F. Cormier

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Ivan, M., Cormier, V.F. High Frequency PKKPBC Around 2.5 Hz Recorded Globally. Pure Appl. Geophys. 168, 1759–1768 (2011). https://doi.org/10.1007/s00024-010-0192-z

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