![]() The amplitude of the overshoot however, is larger than predicted by the theoretical source model and appears to be an artefact of the processing rather than a property of the source pulse itself. Use of the frequency-dependent t* leads to pulses apparently with overshoot. Such M-M pulses should show a positive initial motion followed by a negative overshoot with amplitude less than about a third of that of the positive motion the leading edge of the main pulse being sharper than the trailing edge. The results of the deconvolution of STS seismograms show that neither the frequency-dependent nor the frequency-independent models produce deconvolved seismograms that can be interpreted as a series of pulses of the M-M type. Evidence from spectra for frequency-dependent t* models is also reassessed. In this paper results are presented derived by deconvolving observed P seismograms using each of the two types of attenuation model, in an attempt to determine which best describes the attenuation. It has also been claimed that the P spectra for STS explosions show evidence of the frequency-dependence of t*. Evidence for the frequency-independent t* comes from the observed rate of fall-off of explosion spectra to high frequencies, whereas a frequency-dependent t* is required to bring the amplitudes at 1 Hz predicted from widely accepted source models, such as the Muellar-Murphy (M-M) model derived from close-in observations at the NTS, into agreement with observed amplitudes. One type assumes that the specific quality factor Q and hence t* (the ratio of traveltime to Q) is independent of frequency and the other that Q increases (and hence t* decreases) with frequency in the SP band. Two types of anelastic attenuation model have been derived for short-period P-waves for teleseismic ray paths out of the Shagan River Test Site (STS), USSR and the Nevada Test Site (NTS) USA. ![]() The regional pattern and intensity of both travel-time anomalies and t* measurements suggest that both share a common origin due to the regional variation of the thermal structure of the upper 200 to 400 km of the mantle. Measures of differential frequency content (6t* and 6 Q) generally correlate better with differential travel time than measures of differential ampli- tude and mb. High attenuation usually correlates with slow travel times, lower Pn velocity, and inefficient Pn and Sn propagation. Continental cratons are underlain by mantle having small attenuation at all depths. Regional variations in mantle attenuation are consistent with radiometric or magnetic age and tectonic activity, regions of higher relative attenuation coin- cident with younger, tectonically active crust. In this model, Q is constant with frequency up to a cutoff frequency 1/(2~-m) Hz, where ~-m = O.1 to 0.2 sec. Assuming that lateral heterogeneity biases the apparent Q of surface waves, the frequency dependence of Q can be explained by a relaxation model of intrinsic Q. Forward scattering can generate time-dependent variations in the fre- quency content and complexity of body waves that affect the measurement of t*. ![]() Scattering Q cannot be generally separated from intrinsic Q in the mantle. Both spectral and time domain studies indicate that the frequency dependence and regional variation of the attenuation of P waves parallels that of $ waves with t~ = 4t*.
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