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The influence of shear‐velocity heterogeneity on ScS 2 /ScS amplitude ratios and estimates of Q in the mantle
Author(s) -
Chaves Carlos A. M.,
Ritsema Jeroen
Publication year - 2016
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 - 10.1002/2016gl069891
Subject(s) - amplitude , geology , attenuation , seismology , waveform , mantle (geology) , shear (geology) , shear waves , geodesy , seismic wave , geophysics , physics , optics , petrology , quantum mechanics , voltage
Regional waveforms of deep‐focus Tonga‐Fiji earthquakes indicate anomalous traveltime differences (ScS 2 ‐ScS) and amplitude ratios (ScS 2 /ScS) of the phases ScS and ScS 2 . The correlation between the ScS 2 ‐ScS delay time and the ScS 2 /ScS amplitude ratio suggests that shear wave apparent Q in the mantle below the Tonga‐Fiji region is highest when shear wave velocities are lowest. This observation is unexpected if temperature variations were responsible for the seismic anomalies. Using spectral element method waveform simulations for four tomographic models, we demonstrate that focusing and scattering of shear waves by long‐wavelength 3‐D heterogeneity in the mantle may overwhelm the signal from intrinsic attenuation in long‐period ScS 2 /ScS amplitude ratios. The tomographic models reproduce the trends in recorded ScS 2 ‐ScS difference times and ScS 2 /ScS amplitude ratios. Although they cannot be ruled out, variations in shear wave attenuation (i.e., the quality factor Q ) are not necessary to explain the data.