Studies of teleseismic Pand S-wave amplitudes and spectra in the 0.5-4 Hz band show large variations in the attenuative properties of the upper mantle under the United States. The data indicate that attenuation is greatest under the south-western United States including, but not confined to, the Basin and Range province. The lowest attenuation prevails under the north central shield regions. The north-eastern part of the country, consisting of New England and possibly including a larger area along the eastern seaboard, is characterized by moderate attenuation in the mantle.The level of the high-frequency energy in short-period seismic waves and the differences between Q values derived from short-and long-period data indicate that Q is frequency dependent. The form of frequency dependence of t* compatible with the data in the 0.5-4 Hz range does not allow a rapid decrease of t * with increasing frequency. Rather it supports a gradual decrease covering the broader 0.1-4Hz range. The curves of t* versus frequency, for shield-to-shield and mixed shield-to-western United States type paths are parallel with an average difference of 0.2 s in t* in the short-period band, but may diverge towards the long-period band. For both curves t: is below 1 s. For shield-to-shield paths t: must be below 0.5 s at 1 Hz.

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.

In this paper a simple procedure is shown that permits the estimation of resolution for a given set of data having specified measurement errors. For estimation of the resolution obtainable with a given set of data, it is not sufficient to combine the partial derivatives of the observed quantities with respect to the parameters at depth; since the resolution is severely limited by the errors of measurement, these errors must also be considered. The determination of shear velocity for the crust and upper mantle from surface‐wave observations is considered as an example. The relative effectiveness of various combinations of modes for the determination of shear velocity is discussed.

Observations have been made of the local atmospheric pressure field and the long-period seismic noise fields both on the surface of the Earth and in a mine at a depth of 183 metres. The observations show that during windy intervals and in the period range 20-100 s there is a strong correlation between local atmospheric pressure changes and the noise recorded by a vertical seismograph located on the surface. In contrast, over the same range of periods there is no correlation between the seismic noise recorded in the mine and local atmospheric pressure changes except during the passage of acoustic waves. It is shown that the noise in this pass band is not due to the buoyant response of the seismograph, but is caused by the motion of the Earth responding to atmospheric pressure changes.