SEISMOTECTONICS OF MADAGASCAR

Instrumental seismic activity in Madagascar over a 16-years period, as well as the largest earthquakes experienced over the last century, ar e studied using seismic signal recordings and the archives of the Anta nanarivo Geophysical Observatory (Institut et Observatoire de Geophysi que d'Antananarivo). Earthquakes occur in most of the island. The know n focal mechanisms indicate a general E-W extension, consistent with t he stretching currently observed in the Mozambique channel and on the eastern edge of the African continent. Seismic areas cover the main Pl iocene-Quaternary tectonic formations. Other events are located near P an-African tectonics structures, preferentially reactivated within the Precambrian basement complex. A regional origin of stresses cannot ex plain completely the spatial distribution of all the earthquakes in th e central highlands of Madagascar. An other possible stress origin can be a thermal bulge on which the main Pliocene Quaternary tectonic and volcanic structures of this region are situated. The current tectonic phase of extension seems to correspond to an early stage of reactivat ion. Madagascar could be part of the E-W extension zone associated wit h diffuse seismicity affecting East Africa south of the Equator (C) 19 98 Elsevier Science B.V. All rights reserved.