Stick-slip advance of the Kohat Plateau in Pakistan

Throughout most of the Himalaya, slip of the Indian Plate is restrained by friction on the interface between the plate and the overlying wedge of Himalayan rocks. Every few hundred years, this interface-or decollement-ruptures in one or more M-w >= 8 earthquakes. In contrast, in the westernmost Himalaya, the Indian Plate slips aseismically beneath wide plateaux fronting the Kohistan Mountains. The plateaux are underlain by viscous docollements that are unable to sustain large earthquakes(1). Potwar, the widest of these plateaux is underlain by viscous salt(2,3), which currently permits it to slide at rates of about 3 mm yr(-1) (refs 4,5), much slower than its 2 Myr average(6,7). This deceleration has been attributed to recently increased friction through the loss of salt from its decollement. Here we use interferometric synthetic aperture radar and seismic data to assess movement of the Kohat Plateau-the narrowest and thickest plateau(8,9). We find that in 1992 an 80 km(2) patch of the decollement ruptured in a rare M(w)6.0 earthquake, suggesting that parts of the decollement are locally grounded. We conclude that this hybrid seismic and aseismic behaviour represents an evolution of the mode of slip of the plateaux from steady creep towards increasingly widespread seismic rupture.