Marine Geophysical Researches 20: 171–182, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 171 Tectonic setting of the Azores Plateau deduced from a OBS survey J. M. Miranda 1 , L. A. Mendes Victor 1 , J. Z. Simões 1 , J. F. Luis 1 , L. Matias 1 , H. Shimamura 2 , H. Shiobara 2 , H Nemoto 2 , H. Mochizuki 2 , A. Hirn 3 & J. C. L´ epine 3 1 CGUL - Universidade de Lisboa, Rua da Esc Politecnica, 58, 1200 Lisboa, Portugal 2 University of Hokkaido, Hokkaido, Sapporo, Japan 3 Institut de Physique du Globe de Paris, 4 Place Jussieu, 75004 Paris, France Received 15 July 1998, accepted 7 May 1999 Key words: Azores triple junction, seismicity, tutonics, OBS Abstract The studies of Azores seismicity generally show shocks with either normal faulting or right-lateral strike-slip along the ESE direction, compatible with a eastward relative motion of the Eurasian (EU) relative to the African (AF) plate. However, the 1 January 1980 earthquake was interpreted as a clear left-lateral strike-slip shock along the N150E direction. This pattern is difficult to explain in terms of the relative motion between the EU, AF and North American (NA) plates: all available models for the present day movement of this triple junction fail to explain the regional variability in the stress conditions of the area. Here we present data from a 34-day long Ocean Bottom Seismograph array deployment. We show that the seismicity is distributed along a band aligned with the island chain itself, and is concentrated along several faults with an approximate N150E strike, cutting the Azores plateau in all the area covered by the OBS network. The combination of these new results with other geophysical data permits us to conclude that the tectonic setting of the Azores plateau is characterised by the existence of two sets of faults, in the N120E and N150E directions, defining several crustal blocks, whose relative motion accommodates the interaction of the three megaplates. The deformation of these tectonic blocks is probably driven by the shear between the EU and AF plates. This model explains well the spatial variability of the stress conditions in the Azores domain, the combination of dextral and sinistral strike slip mechanisms and the observed seismotectonics of the Azores islands. Introduction The Azores islands are located near the triple junction between the EU, AF and NA plates (Figure 1). Its seismicity, as recorded by the WWSSN has been the subject of many studies (Udias et al., 1976, Grimison and Chen, 1986, 1988; Buforn et al., 1988). In spite of some small discrepancies in the location of the events, there appears to be a significant concentration of epi- centers along a narrow band aligned with the islands, interpreted as the seismic signature of either a sec- ondary oblique spreading center (Krause and Watkins, 1970), a leaky transform zone (McKenzie, 1972), or a series of en echelon basins, with volcanic massifs separating and straddling them (Searle, 1980). Interpretation of the low-energy seismic activity recorded by the local seismological network has al- ways been difficult (Nunes, 1991), mainly due to the event location uncertainties as a consequence of the geometry of the local seismological network and its low technical standards. The available data reinforced the above conclusions but were not accurate enough to discriminate between the above models. The compila- tion of the seismic events for the period 1980–1990 is presented in Figure 2. Our knowledge of the present day movement in the Azores area is mainly constrained by the focal mechanisms of the main shocks. They show essen- tially a combination of WNW–ESE normal faulting, along the direction of the island chain itself, with a near E–W right lateral strike slip (McKenzie, 1972;