Complete determination of local crustal deformation from geodetic observations

Abstract

Local crustal deformation can be specified by a set of thirteen geodetic deformation rate parameters, referred to a local moving reference frame to obviate datum problems. These parameters comprise six independent components of an intrinsic strain rate tensor; three independent components of a rigid-body rotation rate tensor; the rate of change of the absolute intensity of gravity; and three components of a gravity gradient rate vector. Determination of these parameters requires an integrated programme of observations of changes in the length, azimuth and zenith distance of observation lines, and of levelling and gravity changes between neighbouring points. Regular geodetic observations on nearhorizontal lines are inadequate to resolve the vertical components of the shear strain and rotation rate tensors, and to determine the vertical extensional strain; these can be found by adding measurements with borehole tiltmeters and vertical borehole strain meters. Adoption of a coherent deformation model emphasises that a wide variety of geodetic techniques must be combined to describe completely the deformation of the crust, and that the perpetuation of a traditional subdivision of techniques into “horizontal”, “vertical”, etc., is inimical to a rational and coherent approach to the general problem of strain in the earth.