Delineation and interpretation of seismotectonic domains in western North America

Indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western North America. The San Andreas, East-Central California (ECC), Mendocino Triple Junction (MTJ), and Pacific Northwest (PNW) domains are found to have similar stress fields, although the style and rate of deformation in each is different. We propose that all of these domains are generated by shear between the Pacific and North American plates. In the MTJ and PNW domains this shear may be evidence for the initiation of a transform fault connection between the San Andreas and the Queen Charlotte Island fault systems. The ECC, a region including much of the Sierra Nevada, appears to result from the relatively stiff nature of the San Andreas fault system. The Big Bend and region of relatively new faulting in northern California act to make the San Andreas fault system appear stiff on a regional scale. Two other domains, the Northern Basin and Range (NBR) and the Intermountain Seismic Belt (ISB), show evidence of being controlled by more local forces. The NBR has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the Juan de Fuca-North America plate boundary. The ISB can be explained by readjustments between the Colorado Plateau and the Basin and Range Province, which may be caused by the lowering of the Basin and Range due to extension.