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We present a new set of contour maps of the seismic structure of North America and the surrounding ocean basins. These maps include the crustal thickness, whole-crustal average P-wave and S-wave velocity, and seismic velocity of the uppermost mantle, that is, Pn and Sn. We found the following: (1) The average thickness of the crust under North America is 36.7 km (standard deviation [s.d.] ‫4.8ע‬ km), which is 2.5 km thinner than the world average of 39.2 km (s.d. ‫ע‬ 8.5) for continental crust;(2) Histograms of whole-crustal P-and S-wave velocities for the North American crust are bimodal, with the lower peak occurring for crust without a high-velocity (6.9-7.3 km/sec) lower crustal layer; (3) Regions with anomalously high average crustal P-wave velocities correlate with Precambrian and Paleozoic orogens; low average crustal velocities are correlated with modern extensional regimes; (4) The average Pn velocity beneath North America is 8.03 km/sec (s.d. ‫ע‬ 0.19 km/sec); (5) the well-known thin crust beneath the western United States extends into northwest Canada; (6) the average P-wave velocity of layer 3 of oceanic crust is 6.61 km/ sec (s.d. ‫ע‬ 0.47 km/sec). However, the average crustal P-wave velocity under the eastern Pacific seafloor is higher than the western Atlantic seafloor due to the thicker sediment layer on the older Atlantic seafloor.

The recent successful prediction of the triton binding energy, E"with the static Bonn potential is examined. Modified versions of the potential are introduced to isolate separately the effects of the deuteron D-state admixture and the 'So scattering length on E,. Within this model study we find a monotonic relation between E, and the So scattering length and a strict linear dependence of E, on the D-state admixture, in accordance with general well-known trends and earlier separable-potential model studies. The mechanism through which the weaker tensor force (lower D-state admixture) leads to a stronger binding of the triton is investigated by the introduction and study of effective energy-dependent central potentials.Recently, successful microscopic predictions of the triton binding energy (E, ) based only upon realistic twobody forces have been reported. ' These predictions, obtained with energy-independent parametrizations of the full meson-theoretic Bonn interaction, are important because they mark a departure from previous indications that microscopic two-body forces are fundamentally inadequate to describe the triton binding. The difference between these recent predictions and previous ones reflects an uncertainty in our ability to predict E, due to ambiguities in the character of the two-body nucleonnucleon (NN) input. A key issue, then, is the degree to which crucial features of the NN input are constrained by data and the extent to which unconstrained model dependencies internal to the two-body system can affect predictions of the triton binding energy. In order to address this issue, a precise characterization of the essential mechanisms which govern the triton binding is required.We need to know what aspect of the interaction is responsible for variations in predictions, and whether or not this feature of the NN interaction is a direct consequence of the NN data and of the basic assumptions of the theoretical model. In this paper we report the results of several numerical studies which we have performed to address these questions.As a preliminary, we show in Table I the results of calculations of the triton binding energy with some of the "standard" realistic NN interactions, ' along with our own recent results. ' The "realistic" two-body interactions presented in Table I all provide accurate representations of the NN data. " ' Without this common prerequisite, it would, of course, be meaningless to compare their trinucleon predictions.Nevertheless, there are differences among these models, even on-shell. We summarize the bound-state and some low-energy scattering observables in Table I along with the deuteron S-D admixture, the so-called deuteron percent D state (PD ), predieted by the various models. Although the asymptotic deuteron D/S ratio ri and the deuteron quadrupole moment are experimental observables which do constrain PD, the percent D state is not directly observable and varies for reasonable models of the NN interaction from perhaps 4 to 7%. That this is a principal ambiguity in the descr...

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