This slide shows more about the plate boundary zone in western North America. West of the San Andreas fault, the Pacific plate is moving northwest at about 50 mm/yr. However, the plate boundary is not narrow. Compared to the fixed central part of North America, the Sierra Nevada mountains are moving northwest at about 11 mm/yr. So that means that about 20% of the deformation takes place in the Basin and Range. The Basin and Range province is the region with all the narrow, north to northeast trending mountain ranges between Reno and the Sierra Nevada mountains on the west, and Salt Lake City and the Colorado Plateau on the east. One might predict from this that 20% of the large plate boundary earthquakes would be in the Basin and Range province, but the actual percentage is a little larger than 20% because the faults in the Basin and Range province are not organized to accommodate the slip as efficiently as the San Andreas fault. The topography between Reno and Salt Lake city is caused by this deformation. The Basin and Range topography is caused by the spreading, with the ranges approximately perpendicular to the direction of spreading. The separation of the mountains is believed to be controlled by the thickness of the brittle crust of the Earth in this region.
