The state of stress in the upper part of the earth's crust

Abstract

The object of the present article is to present new results of absolute rock-stress measurements performed in the upper part of the earth's crust as a contribution to the discussions of the problem of 1. (1) whether the earth is expanding or contracting and 2. (2) whether continental drift and convection currents exist. A description of the prevalent uniform stress patterns found by extensive absolute rock-stress measurements in the Fennoscandian bedrock area is presented. Further measurements have been performed outside Fennoscandia, from Spitzbergen in the north to Zambia in the south. These measurements indicate that the horizontal stress field found in Fennoscandia may belong to a general stress system according to which large parts of the earth's crust are stressed. Measurements performed in the Atlantic area in coastal zones, on Iceland and other islands seem to show that the Atlantic ocean floor behaves as a rigid plate stressed by horizontal compressive and shear forces, in all probability, emanating from the zone of contact between ocean floor and continental crust. The Mid-Atlantic Ridge and the numerous east—west fracture zones in the ocean floor seem to represent an orthogonal fracture system probably deriving from horizontal shear stresses in the floor. The method of measurement used enables a complete and highly accurate determination of the three-dimensional state of stress in the earth's crust, thus including the magnitude and direction of the shear stresses. The magnitude and orientation of the stress ellipsoids at depths of 160 and 400 m in a granite massif in Zambia are shown in tables and in ellipsoid models. The horizontal stress field in the earth's crust probably has changed its direction and magnitude in geologic times as may be inferred from the present measurements. A study of the directions of shear-fracture lines occurring in old and resistent granite bedrock will tell about these changes. At many places there would seem to exist a close relationship between the state of stress in the upper part of the earth's crust (today as well as in past geologic times) and the direction of the fracture systems in the bedrock as manifested, for instance, by the river courses. An example is provided by the reported measurements in Zambia. If this were a general situation throughout the world it would show that the shear stress in the crust constitutes an important factor for the formation of the landscape. Horizontal compressive stresses in the bedrock were found at all points of measurement on Iceland, in specific areas of the same order of magnitude as in Fennoscandia. Where the Mid-Atlantic Ridge is assumed to encounter Iceland, the orientation of the maximum horizontal shear in the Icelandic bedrock agrees with the orientation of the ridge; this suggests that the ridge may have its origin in shear stresses in the crust. The maximum horizontal pressure acts in a direction at 45° to the ridge instead of the 90° direction to be expected, where new material extruded to form the Atlantic Ocean floor. In the southwestern corner of Iceland stress measurements were made in a large bedrock area of granophyre. Age determinations of the rocks were made in additional bore-holes drilled down to 130 and 115 m below sea level. Disturbances of dynamic stress equilibrium now acting in the earth's crust seem to be due to changes in the horizontal stress field. Since earthquakes are most probably due to sudden changes in the stress field of the earth's crust, a continuous recording of the absolute values of such changes seems to provide a possibility to predict earthquakes. In the last part of the article comments are presented on some objections raised to the interpretation of certain observations regarding the earth's crust adduced as evidence for theories of expansion and continental drift.