Microearthquake Survey of the Mid-Atlantic Ridge

Abstract

THE association of earthquakes with the axis of the Mid-Atlantic ridge is well known^1–4, and it has been realized that where a median valley existed, the pattern of epicentres stratified it^5,6. The first motions and distribution of earthquakes along fracture zones confirm the hypothesis of transform faults and hence give support to ideas of sea floor spreading and plate tectonics^7,8. Seismological observations on Iceland have demonstrated the anomalous nature of the upper mantle beneath the ridge axis^9–12, but the anomalous character of Iceland itself limits the value of this work. Further progress in the seismological study of the ridge axis requires a greater precision of observation than the present network of seismographic stations can provide. Epicentral location on the Mid-Atlantic ridge is still too crude to allow precise correlations with physiographic features, and depth determination has not progressed beyond a qualitative “shallow”. Only by direct seismographic observations on the ridge axis can we resolve these problems and understand more about the tectonic and magmatic processes taking place in the zone where new oceanic lithosphere is being created. Ocean bottom seismographs have therefore been under development over the past few years under the joint sponsorships of the Institute of Geological Sciences and the United Kingdom Atomic Energy Authority.