Subcontinental-scale crustal velocity changes along the Pacific-North America plate boundary

Transient tectonic deformation has long been noted within similar to 100 km of plate boundary fault zones and within active volcanic regions, but it is unknown whether transient motions also occur at larger scales within plates. Relatively localized transients are known to occur as both seismic and episodic aseismic events(1), and are generally ascribed to motions of magma bodies, aseismic creep on faults, or elastic or viscoelastic effects associated with earthquakes. However, triggering phenomena(2,3) and systematic patterns of seismic strain release at subcontinental (similar to 1,000 km) scale along diffuse plate boundaries(4,5) have long suggested that energy transfer occurs at larger scale. Such transfer appears to occur by the interaction of stresses induced by surface wave propagation and magma or groundwater in the crust(6), or from large-scale stress diffusion within the oceanic mantle in the decades following clusters of great earthquakes(7). Here we report geodetic evidence for a coherent, subcontinental-scale change in tectonic velocity along a diffuse similar to 1,000-km-wide deformation zone. Our observations are derived from continuous GPS (Global Positioning System) data collected over the past decade across the Basin and Range province, which absorbs approximately 25 per cent of Pacific-North America relative plate motion. The observed changes in site velocity define a sharp boundary near the centre of the province oriented roughly parallel to the north-northwest relative plate motion vector. We show that sites to the west of this boundary slowed relative to sites east of it by similar to 1mm yr(-1) starting in late 1999.