SOURCE PARAMETERS OF EARTHQUAKES ALONG THE COASTAL MARGIN OF WEST-AFRICA AND COMPARISONS WITH EARTHQUAKES IN OTHER COASTAL MARGIN SETTINGS

The coastal margins of Western Africa and most of the eastern Americas are considered to be stable continental interiors where large earthqu akes occur infrequently. Since these regions have similar geologic and tectonic histories, it might seem reasonable to expect similarities b etween their large earthquakes. On the other hand, current plate force s acting on these regions differ and may lead to regional differences in earthquakes. To examine the similarities of coastal margin earthqua kes we first determined the source parameters for six earthquakes of M greater-than-or-equal-to 5.5 occurring along the coastal margin of We st Africa, a region that has not previously been studied in detail. Re sults of this analysis indicate that the earthquakes are shallow (8-15 km) strike-slip events along high-angle faults (dips generally > 80-d egrees), with nodal planes having orientations similar to those of fau lts mapped in the exposed Precambrian basement. Five out of seven even ts show complexities in their source-time functions. East of the Camer oon line, most P axes trend northeast-southwest to east-west. Earthqua kes west of the Cameroon line show less coherency of stress field, wit h P axes for onshore events trending northwest-southeast and for offsh ore events north-south to northeast-southwest. In contrast, M greater- than-or-equal-to 5.5 earthquakes of the eastern coastal margin of Nort h and South America have reverse mechanisms, with more moderate dips o n nodal planes (45-70-degrees) and over 50% have east-west oriented P axes, which reflect stresses caused by the motion of North and South A merica relative to Europe and Africa. In North America, earthquakes ap pear to occur on faults that cannot be associated with surficial geolo gy; occur shallower or deeper than West African events and have shorte r rupture lengths for the equivalent moment. These differences suggest that West African events may not serve as useful analogs for events e xpected along the American coastal margin.