Seismicity and earthquake focal mechanisms along the Mid-Atlantic plate boundary between Iceland and the Azores

Abstract

The seismicity along the mid-Atlantic plate boundary in the North Atlantic is in a general way consistent with the concept of sea floor spreading. Some detailed studies of the seismicity of Iceland show, however, that the simple picture of an orthogonal system of spreading ridges and transform faults is an oversimplification. On the Reykjanes Peninsula, for example, the plate boundary has both transform fault and spreading ridge characteristics. The Tjörnes fracture zone may be described as a multiple transform fault with at least two parallel strike-slip faults taking up the transform motion. The largest earthquakes within the volcanic rift zones of Iceland appear to be associated with central volcanoes and are not directly related to the rifting process. The Charlie-Gibbs fracture zone and the southern part of the Reykjanes Ridge are examples of a typical transform fault and of a spreading ridge, respectively. The northern part of the Reykjanes Ridge, on the other hand, appears to be spreading obliquely. South of the Charlie-Gibbs fracture zone focal mechanism solutions of two earthquakes on or near the plate boundary have a substantial component of thrust faulting. Crustal compression in this area may be related to mantle plume activity under Iceland and the Azores.