Can the Earths crust be realistically described in terms of a finite number of plates?
A fundamental tenet of plate tectonic theory is that most tectonic activity is the result of interaction among rigid and relatively inactive lithospheric plates, 12 in the widely used NUVEL-1 model (Stein, 1993). The NUVEL-1 model has been tested repeatedly by space geodesy, and in general has been found to predict site motions in direction and magnitude reasonably well. Significant exceptions have been found even in classic plate boundaries, such as the South America/Pacific plate area, where the Arequipa site has been found to share in the eastward motion of the Pacific plate (Robaudo and Harrison, 1993). However, these anomalies are explainable in terms of plate theory. Viewing the Earth as a whole, on the other hand, we see clearly that large areas, especially on continents, can not be assigned to discrete plates. The diffuse nature of continental plate boundaries has been noted by many authors, such as Stein (1993). Nevertheless, the broad zone of intense seismic activity in, for
