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It is not fully understood how or why the inner areas of tectonic plates deform, leading to large, although infrequent, earthquakes. Smalley et al. offer a potential breakthrough by suggesting that surface deformation in the central United States accumulates at rates comparable to those across plate boundaries. However, we find no statistically significant deformation in three independent analyses of the data set used by Smalley et al., and conclude therefore that only the upper bounds of magnitude and repeat time for large earthquakes can be inferred at present.

Some low-angle faults in the Humboldt Range of north-central Nevada may be east-verging Mesozoic thrusts that were reactivated as normal faults with westerly displacement. Early deformation in nappes underlain by such faults produced east-verging asymmetric folds of bedding and moderate to steep northwest-dipping penetrative cleavage. These structures, as well as a shallow southeast-plunging Z finite strain axis in nappe rocks, imply west to east compression. Later deformation has younger over older juxtaposition and west-verging asymmetric folds of first foliation. These folds have shallow easterly dipping axial planes whose orientations with respect to adjacent low-angle fault planes and lineations in the fault planes indicate westerly displacement. The later deformatio thus implies reactivation of the low-angle faults as normal faults. Though timing of normal fault motion is uncertain, it appears reasonable to associate it with Cenozoic extension in the Great Basin. Reactivation of thrust faults as low-angle normal faults provides further evidence in support of thin-skinned crustal extension. End_of_Article - Last_Page 938