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of the Geological Survey of India inferred that the changes in survey angles and elevations following the great Assam earthquake of June 12, 1897, were due to co-seismic tectonic movements. C.S. Middlemiss reached the same conclusion for the Kangra earthquake of April 4, 1905, also in the well-surveyed foothills of the Himalaya (9).
Mechanical Theories of Faulting
The notion that earthquakes result from fault movements linked the geophysical disciplines of seismology and geodesy directly to structural geology and tectonics, whose practitioners sought to explain the form, arrangement, and interrelationships among the rock structures in the upper part of the Earth’s crust. Although Hutton, Lyell, and the other founders of the discipline of geology had investigated the great vertical deformations required by the rise of mountain belts, the association of these deformations with large horizontal movements was not established until the latter part of the nineteenth century (10). Geological mapping showed that some horizontal movements could be accommodated by the ductile folding of sedimentary strata and plastic distortion of igneous rocks, but that much of the deformation takes place as cataclastic flow (i.e., as slippage in thin zones of failure in the brittle materials that make up the outer layers of the crust). Planes of failure on the larger geological scales are referred to as faults, classified as normal, reverse, or strike-slip according to their orientation and the direction of slip (Figure 2.5).
In 1905, E.M. Anderson (11) developed a successful theory of these faulting types, based on the premises that one of the principal compressive stresses is oriented vertically and that failure is initiated according to a rule published in 1781 by the French engineer and mathematician Charles Augustin de Coulomb. The Coulomb criterion states that slippage occurs when the shear stress on a plane reaches a critical value tc that depends linearly on the effective normal stress sneff acting across that plane:
tc = t0 + µsneff, (2.1)
where t0 is the (zero-pressure) cohesive strength of the rock and µ is a dimensionless number called the coefficient of internal friction, which usually lies between 0.5 and 1.0. Anderson’s theory made quantitative predictions about the angles of shallow faulting that fit the observations rather well (except in regions where fault planes were controlled by strength anisotropy like sedimentary layering). However, it could not explain the existence of large, nearly horizontal thrust sheets that formed at deeper structural levels in many mountain belts. Owing to the large lithostatic load, the total normal stress sn acting on such fault planes was