PLATE BOUNDARY ZONES
Although plate boundaries were initially viewed as narrow, it is now recognized that continental plate boundaries are often broad zones of deformation and faulting (up to 1000 km wide), with plate motion spread far beyond the nominal boundary.
Such zones, usually first identified from seismicity, can also occur in oceanic lithosphere.
Gordon & Stein 
Diffuse plate boundary zones cover about 15% of earth's surface.