Engineering topics

Ockendon, J. R.

Publications and source records attributed to Ockendon, J. R..

3 records

NASA NTRS1978

Geoid Anomalies and the Near-Surface Dipole Distribution of Mass

Although geoid or surface gravity anomalies cannot be uniquely related to an interior distribution of mass, they can be related to a surface mass distribution. However, over horizontal distances greater than about 100 km, the condition of isostatic equilibrium above the asthenosphere is a good approximation and the total mass per unit column is zero. Thus the surface distribution of mass is also zero. For this case we show that the surface gravitational potential anomaly can be uniquely related to a surface dipole distribution of mass. Variations in the thickness of the crust and lithosphere can be expected to produce undulations in the geoid.

Turcotte, D. L.↗

NASA NTRS1977

On the gravitational potential and field anomalies due to thin mass layers

The gravitational potential and field anomalies for thin mass layers are derived using the technique of matched asymptotic expansions. An inner solution is obtained using an expansion in powers of the thickness and it is shown that the outer solution is given by a surface distribution of mass sources and dipoles. Coefficients are evaluated by matching the inner expansion of the outer solution with the outer expansion of the inner solution. The leading term in the inner expansion for the normal gravitational field gives the Bouguer formula. The leading term in the expansion for the gravitational potential gives an expression for the perturbation to the geoid. The predictions given by this term are compared with measurements by satellite altimetry. The second-order terms in the expansion for the gravitational field are required to predict the gravity anomaly at a continental margin. The results are compared with observations.

Ockendon, J. R.↗

NASA NTRS1977

Lithospheric instabilities

In this paper we define a mantle geoid. This is the height that hot solid mantle rock from the asthenosphere would attain if it were not confined by the lithosphere. The mantle geoid lies 3.25 km below the hydrogeoid (sea level). Hot mantle rock cannot entirely penetrate the continental lithosphere. One consequence of this partial penetration is rifting; as a result of rifting an accreting plate margin may be created. Hot mantle rock from the asthenosphere can penetrate through the oceanic lithosphere if the sea floor lies below the mantle geoid. Penetration of the oceanic lithosphere by this solid mantle rock is a necessary condition for the initiation of subduction. We argue that the same processes that are associated with rifting in continental lithosphere will be associated with behind arc spreading and the initiation of subduction in the oceanic lithosphere.

Turcotte, D. L.↗