Plateau Pop-up during the 1897 Assam earthquake

The great Assam earthquake of 12 June 1897 reduced to rubble all masonry buildings within a region of NE India roughly the size of England, and its felt area exceeded that of the 1755 Lisbon Earthquake. Hitherto it was believed that rupture occurred on a north-dipping Himalayan thrust propagating south of Bhutan . We show here that this view is incorrect. The northern edge of the Shillong Plateau rose violently more than 11 m during rupture of a buried, 110km-long, reverse fault, dipping steeply away from the Himalaya. The stress drop implied by the rupture geometry and the prodigious fault slip of 18±7 m, explains observed epicentral accelerations exceeding 1 g vertically, and surface velocities exceeding 3 m/s. Our finding represents the first quantitative observation of active deformation of a "pop-up" structure, and confirms that faults bounding such structures can penetrate the whole crust. Plateau uplift in the past 2-5 million years has caused the Indian plate to contract locally by 4±2 mm/year, reducing seismic risk in Bhutan, but increasing it to the large populations of northern Bangladesh.