Teleseismic travel time residuals in North America and anelasticity of the asthenosphere

Details Teleseismic travel time residuals in North America and anelasticity of the asthenosphere Teleseismic travel time residuals in North America and anelasticity of the asthenosphere

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999pepi..116...93v&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 116, Issue 1-4, p. 93-103.

Physics

12

Scientific paper

The slope a of the relation δtS=aδtP+b for the teleseismic S and P travel time residuals δts and δtp is practically the only robust observational value that is equally sensitive to the lateral S and P velocity variations in the upper mantle. Published estimates of a for North America are about twice higher than those at ultrasonic frequencies in olivine at comparable pressures and temperatures. In our study, lateral variations of the S residuals in North America are found not from the S wave readings, which can be easily biased, but from accurately determined delays relative to P of the Ps phases converted from `410 km' and `660 km' discontinuities. Nevertheless, the estimates of a thus derived are close to the values obtained with the standard technique. The discrepancy between the ultrasonic and seismic estimates of a can be explained by physical velocity dispersion. On the assumption of a single absorption band, our value of a is consistent with the published values of Q in the seismic frequency band and velocities in the ultrasonic range, if the high-frequency cutoff of the absorption band in the upper mantle beneath western North America is on the order of several hundred Hertz. However, in the literature there are indications of a cutoff at around 1 Hz. Then our data imply the presence of another absorption band at high frequencies outside the seismic frequency band. This band may correspond to the viscous dissipation in the Walsh [Walsh, J.B., 1969. A new analysis of attenuation in partially melted rock. J. Geophys. Res., 74, 4333-4337] model of penny-shaped inclusions of melt.

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