Compressive Stress between Two Mid-Ocean Ridges

Abstract

ONE of the possible driving mechanisms of plate motions^1–3 is a push from mid-ocean ridges. This could arise from the injection of magma at the ridge crest⁴, or to hydrostatic overpressure resulting from the elevation of the ridge above the ocean basins⁵. If the plates are pushed apart at ridges, an area between two mid-ocean ridges should be under compression. On May 9, 1971, an earthquake of magnitude 6.2 (m_b reported by NOAA) occurred within the Antarctic plate between the Chile Ridge and the Pacific–Antarctic Ridge, followed within 24 h by several aftershocks of magnitude 5.0 or greater (Fig. 1). An analysis of the focal mechanism of the principal shock shows that the plate is at least locally under horizontal compressive stress, with the maximum stress axis approximately perpendicular to the Pacific–Antarctic Ridge.