Seismicity of the Indian Ocean and a possible nascent island arc between Ceylon and Australia

Abstract

The epicenters of about 900 earthquakes in the Indian Ocean, Africa, and adjacent areas that occurred from 1950 to 1966 were relocated by computer. These epicenters delineate many transform faults, ridge crests, and triple junctions much more precisely than was done previously. A series of great NNW-striking transform faults south of Tasmania successively offset the mid-ocean ridge a total of about 1500 km. The distribution of epicenters and of two focal mechanism solutions for the southwest brance of the mid-Indian ridge are indicative of sea-floor spreading with ridge crests oriented approximately WNW and transform faults NNE. Previous reports of sediment thicknesses near this branch of the ridge are in good accord with the inferred pattern of spreading ridge crests and transform faults. Likewise, failures to detect measurable sea-floor spreading along this branch can be ascribed to the unfortunate orientation of the few published profiles nearly parallel to ridge crests. An incipient tectonic feature, possibly a nascent island arc, may be associated with a seismic zone in the northeast Indian Ocean between Ceylon and Australia. The large width of the zone of shallow shocks, the relative abundance of earthquakes larger than magnitude 7, and the absence of observed topographic features associated with the seismic zone are indicative of a nascent tectonic feature that may be related to a decrease or change in the relative motion between the Indian-Australian plate and-the Eurasian plate, possibly as a result of continental collision. Other interpretations of the tectonics of this unusual zone are also discussed. Regardless of its tectonic significance, this zone deserves further study since it is the most seismically active region in the oceans that has not been identified as either a ridge, an arc, or a transform fault. The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W., Washington, D.C. 20037. Document J 70-003; $1.00. Payment must accompany order.