We present the results of a dense seismological experiment in the western part of the Gulf of Corinth (Psathopyrgos-Aigion area), one of the most active rifts in the Aegean region for which we have precise tectonic information. The network included 51 digital stations that operated during July and August 1991, covering a surface of 40 x 40 km2.Among the 5000 recorded events with M L ranging between 1.0 and 3.0, we precisely located 774 events. We obtained 148 well-constrained focal mechanisms using P-wave first motions. Of these, 60 also have mechanisms obtained by combining the P-wave first motions with the S-wave polarization directions. The observed seismicity is mainly located between 6 and 11 km depth. Most of the fault-plane solutions correspond to E-W-striking normal faulting, in agreement with the geological evidence. Most of the well-determined mechanisms indicate a nodal plane dipping 10-25" due north and a steep south-dipping plane. A similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the Corinth Rift. We discuss this evidence and the inference of a deep detachment zone, a structure where the major faults seen at the surface appear to root. A large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone. This feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes.

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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 échelon 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 foot wall 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 échelon 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 relay 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 stretching occurs partly on reactivated NW‐SE 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.

The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr −1 in an approximately N10E • direction. On 2013 May 21, an earthquake swarm was initiated with a series of small events 4 km southeast of Aigion city. In the next days, the seismic activity became more intense, with outbursts of several stronger events of magnitude between 3.3 and 3.7. The seismicity migrated towards the east during June, followed by a sudden activation of the western part of the swarm on July 15th. More than 1500 events have been detected and manually analysed during the period between 2013 May 21 and August 31, using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method. Waveform similarity-based analysis was performed, revealing several distinct multiplets within the earthquake swarm. High-resolution relocation was applied using the double-difference algorithm HypoDD, incorporating both catalogue and cross-correlation differential traveltime data, which managed to separate the initial seismic cloud into several smaller, densely concentrated spatial clusters of strongly correlated events. Focal mechanism solutions for over 170 events were determined using P-wave first motion polarities, while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence. Selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm. The solutions are mainly in agreement with the regional NNE-SSW extension, representing typical normal faulting on 30-50 • north-dipping planes, while a few exhibit slip in an NNE-SSW direction, on a roughly subhorizontal plane. Moment magnitudes were calculated by spectral analysis of S waves, yielding b-values between 1.1 and 1.2 in their frequency-magnitude distribution. The seismic moment release history indicates swarm-like activity during the first phase, which could have acted as a preparatory stage for the second phase (after 12 July) that presented a more typical main-shock-aftershock behaviour. The spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the NNEdipping Pirgaki normal fault, outcropping ∼8 km to the south. The slow velocity of eastward migration of the epicentres during June implies triggering by fluids. The situation appears different in the second phase of the sequence, which was probably triggered by a build-up of stress during the first one. The relatively deep hypocentres of the 2013 swarm, compared 2044

The 12 stations Corinth Rift Laboratory Seismological Network (CRLNET) aims at monitoring the seismicity (Ml > 1) in the CRL area and at constraining the geometry of active structures at depth. Two years of microseismicity (2000)(2001) recorded by the CRLNET in the Aigion area shows: (1) background seismicity inside the Corinth rift at depth of 4.5-11 km, deepening towards the north and no activity in the upper 4 km of the crust -this seismicity is not clearly related to major faults observed at the surface -; (2) a swarm, 6 km south of the city of Aigion, associated with the Mw = 4. RésuméPremiers résultats du réseau sismique (CRLN) de la partie ouest du rift de Corinthe : évidence de la réactivation d'une ancienne faille. Le réseau de 12 stations sismologiques installées dans la région du Corinth Rift Laboratory (CRL) permet de localiser la microsismicité (Ml > 1) de cette zone depuis avril 2000, avec une précision de l'ordre du kilomètre afin, en particulier, de contraindre la géométrie des structures actives en profondeur. La microsismicité enregistrée correspond à une sismicité de fond sous le golfe de Corinthe entre 4,5 et 11 km de profondeur, à laquelle se superpose une activité en essaims. Les quatre premiers kilomètres de la croûte sont asismiques. La sismicité s'approfondit vers le nord et n'est pas directement associée aux failles actives observées en surface. Le séisme du 8 avril 2001 (Mw = 4,2) s'est produit à 6 km de profondeur. L'étude du mécanisme au foyer et des répliques indique qu'il s'est produit sur un plan de faille orienté SW-NE, plongeant vers le nord-ouest à 40 • . C'est un séisme en faille normale avec une composante décrochante dextre, qui s'est probablement produit sur une ancienne structure réactivée dans le champ de contrainte actuel (extension ∼N10 •

Abstract. Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present-day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present-day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.

A preliminary study of the aftershocks of three earthquakes that occurred near to Corinth (Greece) in 1981 is combined with observations of the morphology and faulting to understand the evolution of the Eastern Gulf of Corinth. The well located aftershocks form a zone 6 0 k m long and 20km wide. They do not lie on the main fault planes and are mostly located between the north-dipping faulting on which the first two earthquakes occurred and the south-dipping faulting associated with the third event. A cluster of aftershocks also lies in the footwall of the eastern end of the south-dipping fault of the third event.Morphologically, it is observed that in the evolution of the Eastern Gulf of Corinth, antithetic faulting apparently predates the appearance of the main faulting at the surface. This evolution can be explained by motion on a deep seated, shallow angle, aseismic normal fault. A model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes.