A seismic gap is a segment of an active fault known to produce significant earthquakes, that has not slipped in an unusually long time when compared with other segments along the same structure. Seismic gap hypothesis/theory states that, over long periods of time, the displacement on any segment must be equal to that experienced by all the other parts of the fault.Any large and longstanding gap is therefore considered to be the fault segment most likely to suffer future earthquakes.
One such is the central seismic gap, which runs northeast of Delhi along a region woven with unstable faults and including over 10 million people. Until April 25, observers had been concerned by the paucity of earthquakes in the gap: the longer there were no quakes, the more the pent up stresses, and the stronger a future quake will be. A research team’s conclusion describes an active thrust fault below Uttarakhand pregnant with enough tension to unleash a quake measuring at least 8 on the Richter scale. This, in a state already prone to crippling landslides and floods, and with 70% of its population (of about 10 million) residing in rural areas. They attribute the tremendous tension to a geometry of rock that has partially separated from a layer beneath and caused folds and deformations. The technical term for this geometry is a décollement:the landscape and erosion rate patterns suggest that the décollement beneath the state of Uttarakhand provides a sufficiently large and coherent fault segment capable of hosting a great earthquake.
Décollement (detachment) folds develop during folding, secondary to separation of a (more competent) layer from an underlying (less competent) layer as deformation proceeds.