Seismotectonics of Madagascar

Instrumental seismic activity in Madagascar over a 16-years period, as well as the largest earthquakes experienced over the last century, are studied using seismic signal recordings and the archives of the Antananarivo Geophysical Observatory (Institut et Observatoire de Geophysique d'Antananarivo). Earthquakes occur in most of the island. The known focal mechanisms indicate a general E-W extension, consistent with the stretching currently observed in the Mozambique channel and on the eastern edge of the African continent. Seismic areas cover the main Pliocene-Quaternary tectonic formations. Other events are located near Pan-African tectonics structures, preferentially reactivated within the Precambrian basement complex. A regional origin of stresses cannot explain completely the spatial distribution of all the earthquakes in the 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 reactivation. Madagascar could be part of the E-W extension zone associated with diffuse seismicity affecting East Africa south of the Equator (C) 1998 Elsevier Science B.V. All rights reserved.