FIGURE 2.2 Faulting associated with the 1944 Dixie Valley, Nevada, earthquake (from Stewart, 1980).

tion of land and sea (Sloss, 1963), motions that are usually referred to as epeirogenic to distinguish them from the more severe forms of deformation associated with mountain building. Geodetic surveys over the cratons of North America and Eurasia appear to indicate that these regions are still going up and down at remarkable rates, although the significance of some of these observations is obscured by an ongoing debate over their accuracy.

Any consideration of the active deformation of intraplate interiors must, of course, recognize the importance of vertical motions associated with the retreat of the ice sheets since the last major continental glaciation. Studies of the contemporary uplift of Fennoscandia as documented by leveling observations and tilted beach terraces are now classic (e.g., Niskanen, 1939). The effects of postglacial rebound are not restricted to areas in proximity to the former ice sheets but are global in extent and of particular importance in coastal areas.

In the following discussion, we outline some of the evidence for active tectonics in intraplate areas, review some of the obstacles to our fuller understanding of their causes, and assess their potential impact on society.

EARTHQUAKE DEFORMATION

While the underlying causes of intraplate earthquakes remain problematical, some are clearly associated with large and, in some cases, destructive surface movements. Such earthquakes are not limited to continental areas (Wiens and Stein, 1984). Surface faulting is but one of the more obvious expressions of earthquake deformation (Figure 2.2). In some cases, geodetic measurements delineate more subtle patterns of motion. It is reasonable to assume, and the limited evidence available suggests, that intraplate earthquakes are characterized by a deformation cycle with preseismic, coseismic, postseismic, and interseismic phases similar to that inferred for interplate events (Thatcher, Chapter 10, this volume). Coseismic, and to a lesser extent, postseismic deformation have been documented by geodetic measurements for a number of intraplate earthquakes in the United States (Table 2.1). Movements of several centimeters are common, and several meters are possible. Undoubtedly similar movements have accompanied other intraplate events both in the United States and elsewhere but have gone undetected owing to the lack of appropriately timed or spaced observations.

Where sufficiently detailed geodetic observations are available, coseismic movements are roughly consistent with the deformation predicted by simple dislocation theory (e.g., Savage and Hastie, 1966). Intraplate postseismic deformation has been attributed to afterslip on the earthquake fault (e.g., Savage and Church, 1974) or to subsequent viscoelastic relaxation (Koseluk and Bischke, 1981).



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