Source parameters of earthquakes along the coastal margin of West Africa and comparisons with earthquakes in other coastal margin settings

Abstract

The coastal margins of Western Africa and most of the eastern Americas are considered to be stable continental interiors where large earthquakes occur infrequently. Since these regions have similar geologic and tectonic histories, it might seem reasonable to expect similarities between their large earthquakes. On the other hand, current plate forces acting on these regions differ and may lead to regional differences in earthquakes. To examine the similarities of coastal margin earthquakes we first determined the source parameters for six earthquakes of M ⩾ 5.5 occurring along the coastal margin of West Africa, a region that has not previously been studied in detail. Results of this analysis indicate that the earthquakes are shallow (8–15 km) strike-slip events along high-angle faults (dips generally > 80°), with nodal planes having orientations similar to those of faults mapped in the exposed Precambrian basement. Five out of seven events show complexities in their source-time functions. East of the Cameroon line, most P axes trend northeast-southwest to east-west. Earthquakes west of the Cameroon line show less coherency of stress field, with P axes for onshore events trending northwest-southeast and for offshore events north-south to northeast-southwest. In contrast, M ⩾ 5.5 earthquakes of the eastern coastal margin of North and South America have reverse mechanisms, with more moderate dips on nodal planes (45–70°) and over 50% have east-west oriented P axes, which reflect stresses caused by the motion of North and South America relative to Europe and Africa. In North America, earthquakes appear to occur on faults that cannot be associated with surficial geology; occur shallower or deeper than West African events and have shorter rupture lengths for the equivalent moment. These differences suggest that West African events may not serve as useful analogs for events expected along the American coastal margin.