Chapter 25: Earthquakes, faults, and the seismotectonic framework of the contiguous United States
J. W. Dewey, David P. Hill, W. L. Ellsworth, E. R. Engdahl, 1989. "Chapter 25: Earthquakes, faults, and the seismotectonic framework of the contiguous United States", Geophysical Framework of the Continental United States, L. C. Pakiser, Walter D. Mooney
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Recent decades have seen dramatic improvement in the ability of earth scientists to resolve the geometries of earthquake sources and to relate these geometries to local and global tectonic processes. Well-studied seismogenic faults consist of individual segments that are offset from each other along strike or that have different strikes. The boundaries between segments help control or limit rupture propagation during large earthquakes. On many segments of the plate-bounding San Andreas fault system, fault orientation and structure at depth are well represented by the surface fault trace. In the Cordillera away from the San Andreas system, faults that have produced large earthquakes seem commonly to be expressed at the surface by traces that are distorted representations of the faults at depth. In the United States east of the Cordillera, it has not been possible to associate earthquakes with geologically mapped faults as directly as in the western United States. The following observations, however, suggest that many eastern earthquakes occur on reactivated pre-Cenozoic faults: (1) epicenters in some sources coincide with mapped pre-Cenozoic faults; (2) in other sources, planar zones of hypocenters and instrumentally inferred fault planes parallel regional structural trends; and (3) some pre-Cenozoic faults show Cenozoic displacement. Global tectonic models have been extended to account for the distribution and focal depths of earthquakes that occur in California away from the principal plate boundary and to account for the orientations of regional stress tensors east of the plate boundary. Plate-tectonic models have also led to the hypothesis that a large thrust-interface earthquake might someday occur in the Pacific Northwest subduction zone, notwithstanding the absence of such earthquakes in the historic record.