A seismic Q-profile for the lower mantle from short-period P waves

Abstract

A few studies of the quality factor Q (the ratio of peak-to-dissipated energies of seismic waves) in the Earth's mantle have used a limited data set of spectral ratios for body waves^1–3. Most estimates of Q have come from the analysis of surface waves and free oscillations, but use of such data to estimate Q in the deep mantle suffers from a significant lack of resolution. To improve this resolution in the lower mantle, we have compiled a large data base of digital, short-period P-wave recordings from underground nuclear explosions. We have analysed P and PP waves to develop a set of t^* (seismic travel time divided by effective path Q) station corrections corresponding to the upper mantle beneath each station and a t^*–distance profile. The t^*–distance profile is inverted to determine a Q–depth profile for the lower mantle. A thermal activation model used to interpret the Q–depth profile and to estimate a lower mantle geotherm, enables a thermal boundary layer to be identified in the lowermost 200 km of the mantle. This boundary layer is large enough to be indicative of mantle convection driven by heat flow from the Earth's core.