Teleseismic Body Wave Attenuation and Diffraction.

Abstract

Using available digital seismic stations deployed since the 1980’s, the largest data set based on broadband waveforms among studies on body-wave attenuation (t*) and quality factor (Q) are used in this thesis. The use of nearly 300,000 measurements of body-wave spectral ratio from globally distributed stations renders better constraints of t* and Q variations with higher spatial and depth resolutions in the mantle than have been previously available. The maps of body-wave t* correlate well with the variations of t* computed from the most recent surface-wave Q model QRFSI12 indicating that body-wave and surface-wave t* reflect the same intrinsic attenuation even though these waves sample the upper mantle entirely differently. The high correlation between body-wave t* maps and the t* inferred from a thermal interpretation of shear-wave velocity tomography S20RTS suggests that temperature controls both variations in attenuation and velocity in the upper mantle. The distance variations of P- and S-wave t* (t*P and t*S) are inverted for a radial profile of the quality factor Qμ in the lower mantle. On average, t*P and t*S increase by about 0.2 s and 0.7 s, respectively, between epicentral distances of 30 degree and 97 degree. The body-wave spectra are explained best if Qμ increases in the lower mantle with the rate of 0.1/km. The relatively strong increase of t*S compare to t*P (t*S ~ 4 t*P ) suggests that intrinsic attenuation is the cause of the overall trend in our data. The ratio of P- and S-wave quality factor determined in this thesis (QP/Qμ = 2.27) confirms that intrinsic attenuation occurs mostly in shear and that bulk attenuation is negligible in the mantle. Finally, the delay of seismic waves which traversed numerical mantle plumes are calculated in this thesis for the first time. High-resolution numerical simulations of mantle plume are used to investigate the effects of numerical plumes on waveforms. The measurements of wave front delay demonstrate that the delay of shear-waves by plume tails at depths larger than 1000 km are immeasurably small (< 0.2 s) at seismic periods commonly used in waveform analysis.