Probing two low-velocity regions with PKP b-caustic amplitudes and scattering

Details Probing two low-velocity regions with PKP b-caustic amplitudes and scattering Probing two low-velocity regions with PKP b-caustic amplitudes and scattering

Jul 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.178..503t&link_type=abstract

Geophysical Journal International, Volume 178, Issue 52, pp. 503-512.

Astronomy and Astrophysics

Astronomy

6

Mantle Processes, Body Waves, Wave Scattering And Diffraction

Scientific paper

We probe low-velocity regions of the lower mantle with PKP waves using South Pacific earthquakes recorded at stations from the temporary Ethiopia Afar Geoscientific Lithospheric Experiment array in Ethiopia and the German Regional Seismic Network (GRSN; Germany). The Ethiopian stations show strong scattering of PKPab waves, which arrive as precursors to PKPdf. They also show anomalously large amplitude diffractions from the PKP b-caustic. In contrast, the events recorded at the GRSN show scattering of PKPab as precursors to PKPdf but more normal amplitudes in the PKP b-caustic diffractions. To understand the cause for the observed amplitudes, we test a large set of 1-D models with anomalies in P- and S-wave velocities and density in the lowermost mantle and outer core. Structures that can cause the observed large PKP b-caustic amplitudes are low-velocity regions just above the core-mantle boundary, of the order of 50 km thick for the South Pacific to Ethiopia path, with velocity reductions of around 5 per cent for P- and 25 per cent for the S-wave velocity. Alternatively a 30-km-thick lamella, starting at 50 km above the core-mantle boundary with 10 per cent P-wave velocity reduction and 25 per cent S-wave velocity reduction, also produces a good fit to the data. We find that the low-velocity regions act as waveguides, enhancing the amplitude of the PKP b-caustic diffracted waves and causing superposition of PKP b-diffractions with PKPdf. The low-velocity regions cannot be smooth layers but must show a certain degree of small-scale heterogeneity to produce the high-frequency scattering observed in the data. The path to Germany needs small scatterers in the lowermost mantle, but the low-velocity region is less strong compared with the South Pacific-Ethiopia path. A plausible explanation for these low-velocity regions is the presence of melt inclusions, perhaps entrained in small-scale convective systems that act as high-frequency wavefield scatterers. In regions where these inclusions or convective systems are abundant, they appear as a large-scale low-velocity zone to lower-frequency waves. These observations support ideas that ultra-low-velocity zones at the base of the mantle may be heterogeneous regions of melt-rich material.

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