A large set of teleseismic body wave data covering a broad range in frequency was analysed t o determine the frequency arid depth dependence of Q for P-and S-waves under the northern shield areas of Eurasia. Based o n numerous f * estimates for P-and S-waves covering the seismic band between 0.02 and 8 Hz, a Q model of the Eurasian shield was constructed. The data require a model in which Q increases with frequency and which is characterized by Q values in the upper mantle that are generally higher than those of global average models. The model with the best fit includes a minimum in Q between about 100 and 200 k m depth and high Q values on the order of thousands throughout most of the deeper mantle. These results are generally consistent with proposed models of attenuation as a thermally activated process which is influenced by the temperature and pressure in the earth. Preliminary results suggest that t* under tectonic regions is higher than t* under shield regions for all frequencies over the 0.02 to 8 Hz frequency range suggesting that Q varies regionally as well as with frequency and depth. lntroduction Average Q values for the Earth are well defined from free oscillation measurements (Anderson & Hart 1978) in the long-period band. Translating these measurements into tF and I ; estimates for an average spherically symmetric Earth gives values close to I and 4 s respectively for mantle P-and S-waves in the 30 to 90" distance range. However, with the advent o f digital recording of seismic signals over a broad frequency range, it became apparent that the t* values derived from long period, free oscillation observations cannot be applied to short-period signals. It was found that teleseismic short-period P-waves often

In this paper we discuss some aspects of estimating t* from shortperiod body waves and present some limits on t* ( f ) models for the central and south-western United States (CUS and SWUS). We find that for shortperiod data, with frequencies above 1 or 2Hz, while the average spectral shape is stable, the smaller details of the spectra are not; thus, only an average t*, and not a frequency-dependent t*, can be derived from such information. Also, amplitudes are extremely variable for short-period data, and thus a great deal of data from many stations and azimuths must be used when amplitudes are included in attenuation studies.The predictions of three pairs of models for t * ( f ) in the central and south-western United States are compared with time domain observations of amplitudes and waveforms and frequency domain observations of spectral slopes to put bounds on the attenuation under the different parts of the country. A model with the t* values of the CUS and SWUS converging at low frequencies and differing slightly at high frequencies matches the spectral domain characteristics, but not the time domain amplitudes and waveforms of short-period body waves. A model with t* curves converging at low frequencies, but diverging strongly at high frequencies, matches the time domain observations, but not the spectral shapes. A model with nearly-parallel t* (f) curves for the central and south-western United States satisfies both the time and frequency domain observations.We conclude that use of both time and frequency domain information is essential in determining t * ( f ) models. For the central and south-western United States, a model with nearly-parallel t * ( f ) curves, where At* -0.2 s, satisfies both kinds of data in the 0.3-2 Hz frequency range.

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Seismic observations of the amplitude ratio amplitude P′670P′/amplitude P′P′ suggest a reflection coefficient R ∼ 7–13% for reflections from the underside of the 670‐km discontinuity. This can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region. Velocity and density profiles through the transition region were calculated for olivine, pyroxene, garnet, and olivine + garnet model mineral assemblages. For models with identical upper and lower mantle compositions, the computed reflection coefficients from the phase transitions pertinent to the 670‐km discontinuity are less than 0.5%. These phase transitions produce their maximum R if they are effectively discontinuous, which gives R ∼ 2–3%, except for the garnet to perovskite transformation which gives R ∼ 8%. In general, given realistic impedance contrasts across a phase transition, the change in material properties must occur over a region less than 3 km thick for R to be of the order of 5%. A discontinuous change in chemical composition at 670‐km depth, coupled with a change in phase, can also produce a reflection coefficient of the observed magnitude: R ∼ 2–3% for a first‐order change in phase at 670 km with different iron concentrations on each side of the discontinuity, and R ∼ 4–6% for cases with a discontinuous increase in silica content. Based on our analysis, the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient. Thus although limited by considerable uncertainties, the mineralogical and seismic data suggest a two‐layered model of the mantle with a discontinuous change in both phase and composition at 670 km.

The results ot' short and intermediate period data analyses for the determination of ;i frequency dependent (L model of the niantlc under the shield areas of Eurasia are presented. The spectra of short period P-waves from nuclear explosions in the 1-8 Hz frequency range give $-0.15-0.2 s.Using recordings of Soviet nuclear explosions at NORSAR, P-wave profiles were assembled t o study Q in the upper mantle under the Russian-Fennoscandian shield. Analyses of the relative amplitudes and frequency contents of the various upper mantle arrivals support the existence of an upper mantle low Q layer under this region, but the Q p in this layer is high, around 700, compared t o well studied tectonic regions of the western United States. A variety of measurements were used in the 0.3-1 Hz intermediate frequency band including S-wave periods, P and S rise times, ScS,-ScP periods, and S-SS periods, giving -2 t o 3 s, or $ -0.5 to 0.75 s.