Why are there so many earthquakes and faults in the Western United States?
This region of the United States has been tectonically active since the supercontinent Pangea broke up roughly 200 millon years ago, and in large part because it is close to the western boundary of the North American plate. Since the formation of the San Andreas Fault system 25-30 million years ago, the juxtaposition of the Pacific and North American plates has formed many faults in California that accommodate lateral motion between the plates. North and east of California, the Basin and Range province between the Wasatch Mountains in Utah and the Sierra Nevada Mountains in eastern California is actively spreading and stretching westward.
In New Mexico and west Texas, similar spreading has opened a north-south rift that starts in central Colorado and extends into northern Mexico. The geologic conditions and plate tectonic setting in much of the Western U.S. has resulted in the region being underlain by relatively thin crust and having high heat flow, both of which can favor relatively high deformation rates and active faulting.
In contrast, in the Central and Eastern U.S. (CEUS) the crust is thicker, colder, older, and more stable. Furthermore, the CEUS is thousands of miles from active plate boundaries, so the rates of deformation are low in this region. Nevertheless, the CEUS has had some rather large earthquakes in historical times, including a series of major earthquakes near New Madrid, Missouri in 1811-1812, a large earthquake near Charleston, S.C. in 1886, and the Cape Ann earthquake northeast of Boston in 1755.