................................................................. Plateau `pop-up' in the great 1897 Assam earthquake Roger Bilham*² & Philip England² * CIRES & Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA ² Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK .............................................................................................................................................. The great Assam earthquake of 12 June 1897 reduced to rubble all masonry buildings within a region of northeastern India roughly the size of England, and was felt over an area exceeding that of the great 1755 Lisbon earthquake 1 . Hitherto it was believed that rupture occurred on a north-dipping Himalayan thrust fault propagating south of Bhutan 2±5 . But here we show that the northern edge of the Shillong plateau rose violently by at least 11 m during the Assam earthquake, and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the Himalaya. The stress drop implied by the rupture geometry and the prodigious fault slip of 18 6 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s -1 (ref. 1). This quantitative observation of active deformation of a `pop-up' structure con®rms 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 6 2 mm yr 2 1 , reducing seismic risk in Bhutan but increasing the risk in northern Bangladesh. Central to our analysis are the long-neglected survey reports of Bond 6 who located and remeasured the original points of the 1862 trigonometrical survey across the Shillong plateau. Despite the discovery, under appalling conditions, of 8 m of uplift and 4 m of displacement of parts of this plateau, his results were dismissed 7 , because they failed to meet the triangle closure standards of the Survey of India. Bond, and later Oldham 1 , who attempted to interpret the data, suspected that closure errors were caused by continuing movements following the mainshock, an idea that was many years ahead of its time. Oldham, noting the existence of a northward increasing strain gradient in Bond's data, recommended remeasurement of a survey along the northern edge of the plateau, but this was not to occur until 1936 8 (Fig. 1). These northern measurements had a mean error typical of Survey of India accura- cies (3.3 mrad), but did not overlap the earlier re-survey and, as in Bond's survey, included neither scale nor azimuth constraints. The results of the surveys are available as the locations of points 7,9 with their apparent post-seismic displacements, calculated holding ®xed two points within each network 1,6,8,10 . We seek the values of these parameters that best ®t the observed angle changes. The absence of scale and orientation information permits only the analysis of angular changes 11,10 , which we derive from these pub- lished data. We treat the angular changes as though they re¯ect deformation of an elastic half-space, distorted by slip on a single rectangular plane representing the 1897 earthquake. In doing this, we neglect deformation caused by other earthquakes between 1860±69 and 1897 and by post-seismic deformation before the re-surveys. The surface distortion caused by slip on a buried dislocation can be calculated from nine parameters that describe its geometry and slip 12 . The problem is suf®ciently nonlinear that many local minima exist; we therefore sought a global minimum by systematically searching parameter space. Results of this search are contoured in Fig. 2b. The best-®tting solution, in the sense of minimizing the mis®ts to the angular changes normalized by their uncertainties, corresponds to a slip of 16 m on a fault plane striking ESE for 110 km and dipping SSW at 578 beneath the northern edge of the plateau (Fig. 2); slip on the plane extends from 9 to 45 km beneath the surface, with a rake of 768. Several features of the solution are unexpected. First, all previous studies favoured slip on a plane dipping northwards from the south of the plateau. Furthermore, the extension of the rupture plane to letters to nature 806 NATURE | VOL 410 | 12 APRIL 2001 | www.nature.com 0 500 km Bay of Bengal Shillong Dhaka Bhutan Assam Indo- Burman Ranges India Tibet 1885 1905 1803 1833 1934 1950 II II II IX X Figure 1 Area shaken by the great 1897 Assam earthquake, and location of major Himalayan ruptures in the past 200 years 30 . Masonry structures were damaged within Oldham's 1 intensity IX contour (orange), and destroyed within the intensity X ellipse (violet). The earthquake was felt by persons within the intensity II region (green). The orientation of the curious `Mexican-hat' shape of the epicentral region mapped by Oldham corresponds to the strike of the causal subsurface rupture derived from geodetic data (Fig. 2). © 2001 Macmillan Magazines Ltd