A large normal-fault earthquake in the overriding wedge of the Lesser Antilles subduction zone: The earthquake of 8 October 1974

Abstract

A large earthquake ( Ms = 7.6) occurred in the overriding wedge of the Caribbean plate on 8 October 1974. Hypocenters of locally recorded aftershocks suggest that the main shock rupture extended from about 40 km, immediately above the inferred thrust interface between the Caribbean and American plates, to shallow crustal depths of about 10 km. The focal mechanism solution and aftershock hypocenters taken together imply that the earthquake occurred on a southeast-dipping normal fault that strikes NNE to NE, transverse to the regional strike of the northern Lesser Antilles arc. The location and focal mechanism of the 1974 shock are thus similar to those of small earthquakes occurring in the Adak Canyon region of the central Aleutians and studied by LaForge and Engdahl (1979). Both regions, the northern Lesser Antilles and the central Aleutians, are zones of oblique plate convergence, and it is possible that wedge normal faulting, transverse to the island arc, is a consequence of this oblique convergence. Alternatively, the large normal-fault earthquake in the northern Lesser Antilles may have been a consequence of the resistance of the aseismic Barracuda Ridge, on the American plate, to subduction beneath the Caribbean plate. The focal mechanism of the 1974 earthquake indicates that the shock should not have released a significant amount of the elastic strain that is thought to have been accumulating along the thrust boundary of this segment of the Lesser Antilles arc since the great earthquake of 1843.