Tectonic and seismological data collected in the field following the September 13, 1986, Kalamata earthquake (south Peloponnesus) are presented and analyzed to discuss the earthquake rupture process and the regional tectonics. The event occurred on the Kalamata normal fault whose trace was mapped with SPOT images and topographic and field observations. This fault is part of an approximately NNW-SSE en ¿chelon system cutting through the Hellenic nappes. The fault striking N15¿E on the average, with a dip of about 50¿, has a minimum cumulated Quaternary throw of the order of 1 km. The measured coseismic slip is 6--18 cm over a length of 6 km. The main shock focal mechanism obtained from long-period waveform modeling (strike=201¿ (+10¿, -20¿), dip=45¿¿5¿, rake=283¿ (+10¿, -25¿)) represents almost pure east-west extension and is in good agreement with tectonic observations. The centroid depth is constrained to 5¿3 km and the seismic moment to 7.0¿2.5¿1017 N m. Over 700 aftershocks, located by a 16-station network installed after the earthquake for a period of 2 weeks, define two clusters separated by a ''gap'' of aftershock activity, from the surface to a depth of about 10 km.

The main cluster, to the south, defines a 45¿ west dipping plane which lies on the downward extension of the fault mapped at the surface. Focal mechanisms of aftershocks on this fault plane are homogeneous and represent E-W extension as the main shock. In contrast, the majority of focal mechanisms in the uppermost part of the footwall show more or less E-W compression, probably corresponding to postseismic stress release. The northern cluster of aftershocks is very dense and located away from the surface rupture, within a relay zone between the Kalamata and the next en ¿chelon faults to the NW, the Thouria faults. There focal mechanisms represent extension from about N115¿ to N70¿ and N20¿, corresponding mostly to fault reactivation in an area where nonrigid deformations prevail. The main shock probably initiated in this rely zone 3--4 s before the rupture front reached the main fault plane and released most of the energy there, the rupture presumably propagating southward. The focal mechanism of the Kalamata earthquake and that of the April 27, 1965, earthquake located to the northwest of Crete, as well as the regional active normal fault pattern, imply that E-W extension oblique to the Hellenic arc is presently the dominant tectonic regime. E-W oblique to the Hellenic arc is presently the dominant tectonic regime. faults parallel to the Hellenic structures but mostly on newly formed N-S normal faults across those structures. The latter faults are responsible for the apparent segmentation of the Hellenic belt from southern Peloponnesus to Crete. The existence of active E-W extension in this region implies a recent change in the tectonic regime and consequently a change in boundary conditions at the subduction zone, probably in response to the incoming margin of Africa. ¿ American Geophysical Union 1988