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The upper mantle degree two pattern: Constraints from geoscope fundamental spheroidal mode eigenfrequency and attenuation measurements
Author(s) -
Romanowicz Barbara,
Roult Genevieve,
Kohl Thomas
Publication year - 1987
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl014i012p01219
Subject(s) - attenuation , geology , spectral line , signature (topology) , mode (computer interface) , degree (music) , hotspot (geology) , mantle (geology) , tectonics , range (aeronautics) , geodesy , geophysics , physics , geometry , seismology , mathematics , optics , materials science , computer science , composite material , astronomy , acoustics , operating system
We have analyzed fundamental spheroidal mode spectra obtained from very long period Geoscope records, in terms of global distribution of eigenfrequencies and attenuation. With a new data set and a different data processing approach, involving time variable filtering and the removal of higher order effects in the spectra, we confirm the strong upper mantle degree two signature in the eigenfrequency data. The higher frequency content of the Geoscope spectra allows us to put better constraints on the depth range at which this pattern originates, which appears to be somewhat shallower than was proposed before. While some effect may persist down to the bottom of the transition zone, the frequency signature of the strongest degree two pattern requires an S velocity heterogeneity concentrated in the depth range 300‐500 km and this signature is also apparent in the distribution of Q, which is indicative of a probable thermal origin. The data also resolve a shallow degree 2 pattern, well explained by heterogeneity spanning from the surface to depths of about 300 km, and associated with the distribution of tectonic provinces. It is shifted by about 45° with respect to the deep degree 2 pattern, and its expression in the Q data is not clear.