I. Seismological study of the Ninetyeast and Chagos-Laccadive Ridges, Indian Ocean. II. Models for asymmetric and oblique spreading at midocean ridges. III. Attenuation studies using split normal modes.

Abstract

Part I of this work is a study of the seismicity of the Ninetyeast and Chagos-Laccadive Ridges in the Indian Ocean. These two features, in the interior of the oceanic plate, both show unusual seismicity. The mechanisms of these earthquakes were studied using body and surface Waves. This analysis shows that the Ninetyeast Ridge is still an active zone of deformation within the plate, along which substantial relative motion is taking place. The Chagos-Laccadive Ridge, though far less active, shows unusual seismicity in its southern portion. The seismicity on both ridges differs substantially from any previously discussed in the ocean basins. Both features should still be regarded as active today, though they do not fit into the classic ridge-transform-trench classifications. Part II of this work is a study of the mechanics of oblique and asymmetric seafloor spreading. It proposes that asymmetric seafloor spreading occurs as a consequence of the relative motion between ridges and slow moving mantle material below. A mechanical model of asymmetric spreading predicts that the trailing flank of a ridge migrating with respect to the mantle spreads fastest. These predictions are tested against published data and found to be in good agreement in most places. Oblique spreading is said to occur at midocean ridges which spread slowly (half rate less than 3 cm/yr), while the spreading is perpendicular at faster spreading ridges. This relation is explored using the ratio of the power dissipated at ridges to that on transform faults to determine the most energetically favorable ridge-transform geometry. The angle of oblique spreading (θ) is approximately related to the spreading rate by sin θ ˜ V^(−1), in good agreement with observations. Part III of this Work is a study of the attenuation of the longest period normal modes of the earth. The rotationally and elliptically split normal modes of the earth are observed for the 1960 Chilean and the 1964 Alaskan earthquakes by analysis in the time domain. Synthetic seismograms are computed using theoretical results which show the dependence of the amplitude and phase of the singlets on source location, depth, mechanism and the position of the receiver. By comparing these synthetics to the filtered record, the Qs of the longest period spheroidal (_0^S_2-_0^S_5) and torsional (_0^T_3, _0^T_4) modes can be estimated. In addition, the Q of the fundamental radial mode _O^S_O is measured.