Several types of seismological data, including surface wavegroup and phase velocities, travel times from large explosions, andteleseismic travel time anomalies, have indicated that there aresignificant regional variations in the upper few hundred kilometersof the mantle beneath continental areas. Body wave travel times andamplitudes from large chemical and nuclear explosions are used inthis study to delineate the details of these variations beneathNorth America.
As a preliminary step in this study, theoretical P wave traveltimes, apparent velocities, and amplitudes have been calculatedfor a number of proposed upper mantle models, those of Gutenberg,Jeffreys, Lehman, and Lukk and Nersesov. These quantities have beencalculated for both P and S waves for model CIT11GB, which is derivedfrom surface wave dispersion data. First arrival times for all themodels except that of Lukk and Nersesov are in close agreement,but the travel time curves for later arrivals are both qualitativelyand quantitatively very different. For model CIT11GB, there are twolarge, overlapping regions of triplication of the travel time curve,produced by regions of rapid velocity increase near depths of 400 and600 km. Throughout the distance range from 10 to 40 degrees, thelater arrivals produced by these discontinuities have largeramplitudes than the first arrivals. The amplitudes of body waves,in fact, are extremely sensitive to small variations in the velocitystructure, and provide a powerful tool for studying structuraldetails.
Most of eastern North America, including the Canadian Shieldhas a Pn velocity of about 8.1 km/sec, with a nearly abrupt increasein compressional velocity by ~ 0.3 km/sec near at a depth varyingregionally between 60 and 90 km. Variations in the structure ofthis part of the mantle are significant even within the CanadianShield. The low-velocity zone is a minor feature in easternNorth America and is subject to pronounced regional variations.It is 30 to 50 km thick, and occurs somewhere in the depth rangefrom 80 to 160 km. The velocity decrease is less than 0.2 km/sec.
Consideration of the absolute amplitudes indicates that theattenuation due to anelasticity is negligible for 2 hz waves in theupper 200 km along the southeastern and southwestern margins ofthe Canadian Shield. For compressional waves the average Q forthis region is > 3000. The amplitudes also indicate that thevelocity gradient is at least 2 x 10-3 both above and below thelow-velocity zone, implying that the temperature gradient is < 4.8°C/kmif the regions are chemically homogeneous.
In western North America, the low-velocity zone is a pronouncedfeature, extending to the base of the crust and having minimumvelocities of 7.7 to 7.8 km/sec. Beneath the Colorado Plateau andSouthern Rocky Mountains provinces, there is a rapid velocity increaseof about 0.3 km/sec, similar to that observed in eastern NorthAmerica, but near a depth of 100 km.
Complicated travel time curves observed on profiles withstations in both eastern and western North America can be explainedin detail by a model taking into account the lateral variations inthe structure of the low-velocity zone. These variations involveprimarily the velocity within the zone and the depth to the topof the zone; the depth to the bottom is, for both regions, between140 and 160 km.
The depth to the transition zone near 400 km also variesregionally, by about 30-40 km. These differences imply variationsof 250 °C in the temperature or 6 % in the iron content of themantle, if the phase transformation of olivine to the spinelstructure is assumed responsible. The structural variations atthis depth are not correlated with those at shallower depths, andfollow no obvious simple pattern.
The computer programs used in this study are described inthe Appendices. The program TTINV (Appendix IV) fits sphericallysymmetric earth models to observed travel time data. The method,described in Appendix III, resembles conventional least-squarefitting, using partial derivatives of the travel time with respectto the model parameters to perturb an initial model. The usualill-conditioned nature of least-squares techniques is avoided bya technique which minimizes both the travel time residuals and themodel perturbations.
Spherically symmetric earth models, however, have been foundinadequate to explain most of the observed travel times in thisstudy. TVT4, a computer program that performs ray theory calculationsfor a laterally inhomogeneous earth model, is described in Appendix II.Appendix I gives a derivation of seismic ray theory for an arbitrarilyinhomogeneous earth model.
