Plateau Pop-up during the 1897 Assam earthquake
Roger Bilham* and Philip England .
Bilham, R. and P. England, Plateau pop-up during the great 1897 Assam earthquake. Nature(Lond), 410, 806-809, 2001
*CIRES & Geological Sciences, University of Colorado, Boulder, CO 80309-0399, USA
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 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, 110-km-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/s1. 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.
Central to our analysis are the long-neglected survey reports of Captain J. Bond who was assigned to locate and remeasure the original points of the 1862 trigonometrical survey across the Shillong Plateau. Many of Bond's team were stricken with illness, and one died, yet despite the discovery of 8 m of uplift and 4 m of displacement of parts of the Plateau, his results were dismissed by his superiors, , because they failed to meet the triangle closure standards of the Survey of India. Bond, and later Oldham1, 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 19369 (Fig. 1). These northern measurements had a mean error typical of Survey of India accuracies (3.3 µrad), but did not overlap the earlier re-survey and, as in Bond's survey, included neither scale nor azimuth constraints.
Figure 1. Area shaken by the 1897 Assam Earthquake and location of major Himalayan ruptures in the past 200 years30. Masonry structures were damaged within OldhamÕs1 intensity IX contour, and destroyed within the Intensity X ellipse. The earthquake was felt by persons within the Intensity II region. The orientation of the curious Ņmexican-hatÓ shape of the epicentral region mapped by Oldham corresponds to the strike of the causal subsurface rupture derived from geodetic data (Fig. 2).
The results of the surveys are available as the locations of points7, with their apparent post-seismic displacements, calculated holding fixed two points within each network1,6,,. We seek the values of these parameters that best fit the observed angle changes. The absence of scale and orientation information permits only the analysis of angular changes,10, which we derive from these published data. We treat the angular changes as though they reflect 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. The problem is sufficiently non-linear 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 fitting solution, in the sense of minimising the misfits to the angular changes normalised by their uncertainties, corresponds to a slip of 16 m on a fault plane striking ESE for 110 km and dipping SSW at 57o 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 76o.
Several features of the solution are unexpected. First, all previous studies favored slip on a plane dipping northwards from the south of the plateau. Furthermore, the extension of the rupture plane to the surface corresponds to no mapped fault. In the absence of a named fault we refer to rupture occurring on the "Oldham fault". The easternmost edge of the Oldham fault, and the latitude of its western edge, are well constrained by the triangulation data. (Fig. 2B). The longitude of the westward termination is consistent with 10 m of up-to-the east vertical slip on the Chedrang fault, (Fig. 2). Boundary element calculations treating the Chedrang fault as a frictionless surface driven by coseismic stress changes caused by slip on the Oldham fault emulate the observed distribution of slip only when the ends of the two faults approach within a few km. Finally, the projection of the preferred rupture plane to the surface follows slope breaks on an ESE topographic escarpment along the northern edge of the Plateau (Fig. 2C), and follows the strike and location of Oldham's "Mexican-hat" outline of the rupture zone1 corresponding to the zone of highest shaking intensity and aftershock productivity1. (Fig. 1).
Figure 2a. Trigonometrical stations remeasured on6 and north9 of the Shillong Plateau following the 1897 earthquake. Shear strains for each triangle are displayed as bars whose orientations are parallel to the smallest (most compressional) principal strain and whose lengths are proportional to the shear strain, G = Ć(g12 + g22) where g1= du/dx Š dv/dy, g2= du/dy + dv/dx, and u,v are components of displacement in the easterly (x) and northerly (y) directions11. White rectangles are calculated from the triangulation observations, and black bars show the strains calculated for the best-fitting planar dislocation. Red rectangle indicates subsurface location of this SW dipping dislocation; thick black line with teeth shows the surface intersection of the continuation of this plane to the land surface (slip terminated 9 km below the surface). Short black line at western edge of fault plane indicates location of Chedrang fault, where Oldham1 observed 11 m of co-seismic slip. Line with open teeth to south of the Plateau shows location of the Dauki fault. 2b Trade-offs between parameters of the model dislocation. Each panel displays the variation in root-mean-square misfit to the observed angular changes, with all parameters except those illustrated held fixed to their best-fitting values. y1 and y2 refer to latitudinal coordinate of, respectively, the western and eastern ends of the fault. 2c Topographic section through the central Shillong Plateau showing projected surface location of the Oldham fault. A peneplanation surface is evident at Å1.6 km, with isolated peaks rising to Å2 km.
Our best-fitting value of 16 m for the slip is one of the greatest for any measured earthquake. A lower bound on the magnitude of this slip comes from the magnitude of the shear strains near the top of the rupture plane (Fig. 2) and, independently, from Oldham's observations on the Chedrang fault, which imply a minimum of 11 m of displacement on the main fault plane. There is a trade-off between the depth of the top of rupture and the magnitude of the slip (Fig. 2A), with greater slip (up to 25 m) allowed if the fault is buried more deeply. Slip of less than 10 m, however, produces significantly worse fits to the observations. The dip of the fault and the rake of the slip are constrained to within 15o by the distribution of shear strains (Fig. 2A); specifically, a dip of less than 45o would produce strains in the northern network that are excluded by the data. The NNE-SSW compressional strain implied by our rupture is consistent with the P-axes of smaller earthquakes in the region during this century.,, The maximum depth of the lower edge of the rupture is not well constrained by the observations (Fig. 2A). The geodetic data suggest that rupture extended to a depth of at least 35 km, and may well have cut through to the base of the crust (Fig. 3), here estimated to lie at 43-46 km depth. This suggestion is consistent with the evidence that earthquakes occur to depths of 30Ń50 km beneath the Shillong plateau15. The unusually high ratio of slip to fault length implies a static stress drop at the high end of the observed range, consistent with the violence of the event. It was in this earthquake that, for the first time, accelerations exceeding 1g were identified as responsible for propelling objects into the air. From European seismograms Richter calculated a magnitude of Ms=8.7 although retrospective calibration of these same records yields Ms=8.0±0.1. The parameters of the rupture shown in Fig. 2 correspond to Mw=8.1.
The triangulation data exclude significant slip on the Dauki fault, at the southern margin of the plateau, during the 1897 event. The question remains as to whether this undoubtedly major fault, is a Himalayan thrust fault, as previous interpretations of the 1897 event have concluded. In Miocene time this fault accommodated westward translation of the Indo-Burman ranges, but, increasingly since Pliocene time, the fault has permitted reverse slip. If the Dauki fault is, indeed, a major fault bounding the plateau, then it is reasonable to assume that, like the Oldham fault, it cuts to the base of the crust. If the Dauki fault were a gently dipping thrust fault (6”-15”N) it would intersect the Oldham fault in the mid-crust (6-14 km) (Fig. 3). For the two faults not to intersect within the crust the Dauki fault would need to dip at more than 40”.
We therefore interpret the Shillong Plateau as a pop-up structure bounded by (at least) two reverse faults (Fig. 3). This interpretation is consistent with, but not required by, gravity data that suggest the plateau is uncompensated15,18. It is unclear why the Shillong Plateau should be the only major pop-up structure in the northern Indian shield. One possible explanation may lie in the proximity of two major loads on the lithosphere that are not present elsewhere along the Himalayan chain: the sediments of the Bengal fan are up to 23 km thick to the south and east of the Plateau (Fig. 3), and the Indo-Burman ranges represent an overthrust load that is moving westwards over the Indian lithosphere22. Either, or both, of these loads may amplify the stresses already acting on the Indian plate due to the load of the Himalaya. Alternatively, the explanation may lie in a pre-existing heterogeneity; the Miocene strike-slip Dauki fault may have nucleated dip-slip faulting as the region approached the Himalaya in Pliocene time.
We now proceed to estimate the rate of uplift and shortening of the Plateau. The time of uplift of the Plateau has been inferred from a coarsening of sediment lithology that starts in the Pliocene (2-5 MyBP)21. The 2-km-high plateau is surfaced by Arch¾an rocks, and equivalent rocks lie 4Ń5 km below sea level to the north and south of the plateau22 (Fig. 3). The mean surface level of the Shillong Plateau is approximately horizontal (Fig. 2C), indicating that the cumulative vertical contributions of slip on the Dauki and Oldham faults have been similar since the Plateau began to emerge. If we assume that the relative vertical displacement between the top of the Arch¾an rocks on the plateau, and their equivalents to north and south has been caused by slip on those faults over the past 2Ń5 Myr, then a rate of vertical displacement of 2.5±1 mm/yr is implied on each of the faults. Assuming they each dip at 50o, these rates translate into fault slip rates of 3.3±1.3 mm/yr and to horizontal shortening rates of 4±2 mm/yr on the two faults combined. These estimates are consistent with 1997-9 GPS measurements that indicate the central Shillong Plateau moves south at a rate of 6.3±3.8 mm/year relative to points in central and southern India.
Figure 3. N/S section from Tibet to the Bay of Bengal showing schematic geometry of Plateau pop-up. Although the dip of the Dauki fault is conjectural, it would intersect the 1987 rupture were its dip less than Å 40”, resulting in a geometry that is incompatible with continued slip on both faults. Slip is inhibited on reverse faults with dip greater than 50”
From these rates we estimate a recurrence interval for earthquakes resembling the 1897 event (slipÅ15 m) to be 3-8 kyr on each fault. Pal¾oseismic investigations along the northern edge of the Plateau indicate a 500 year interval between the past four high-intensity shaking events, but these may record accelerations from moderate local earthquakes and large events in the Bhutan Himalaya, in addition to 1897-type events.
Our finding of reverse slip on the Oldham fault ends a century of speculation as to the mechanism of the 1897 earthquake. The Shillong plateau is not being built as part of a system of thin-skinned thrusting, but is bounded by a high-angle reverse fault to its north, and probably also to its south. The Shillong plateau thus resembles the 'pop-up' structures that border thrust belts elsewhere. The Shillong faults, and the smaller high-angle compressional faulting in the foreland of the entire Himalayan arc, are located in the region flexed by the load of the Himalaya. This association implies that the mechanical cause of the deformation is a superposition of elastic stresses due to bending of the Indian plate and in-plane compressional stress from IndiaÕs collision with Tibet. As pointed out by Rogers25, most if not all of such structures are, or were when active, proximal to large mountain chains that could have provided the same state of stress that drives the uplift of the Shillong Plateau. There is thus no need to invoke basal tractions due to subduction to explain pop-up structures, as has often been done for the western Americas.,,
The absence of slip beneath the Bhutan Himalaya in 1897 suggests that the 400 km region between the great Himalayan ruptures of 1934 and 1950 (Fig. 1) has remained a seismic gap for at least the past two centuries. At the higher end of our estimated slip rates, the faults bounding the Shillong Plateau could absorb one third of the inferred Himalayan contraction rate of 18 mm/yr, correspondingly increasing the interval between great earthquakes in the Bhutan Himalaya.
Our conclusions also raise important issues concerning the seismic hazard potential of the Shillong Plateau. The >300-km length of the Dauki fault has not slipped recently, but were it to slip in a single earthquake its potential maximum magnitude (M³8) would constitute a significant seismic threat to nearby densely populated regions of Bangladesh, and to the megacity of Dhaka less than 150 km to the south (Fig. 1). The interval between these giant plateau-building earthquakes fortunately exceeds 3000 years.
The investigation was funded by the National Science Foundation and the Natural Environment Research Council. RB received a John Simon Guggenheim Memorial Foundation fellowship while at Oxford University.
Correspondence and requests for materials should be addressed to R. B.
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