Stick–slip advance of the Kohat Plateau in Pakistan

Abstract

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 décollement—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 décollements that are unable to sustain large earthquakes¹. 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 décollement. 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 km² patch of the décollement ruptured in a rare M_w 6.0 earthquake, suggesting that parts of the décollement 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.