A ten years analysis of deformation in the Corinthian Gulf via GPS and SAR Interferometry

The Corinthian Gulf in Greece, is the most active of a series of extending grabens which accomodate the deformation in the highly seismic Aegean region. The geodetic network established in the region has about 200 points: 50 1st order points and ~150 2nd order points. The network covers an area of about 100 x 80 km2, which correspond to an average density of 1 point every 5 km2. This dense network allows to study the main active faults in the region. Eleven field surveys were organized in 1990, 1991, 1992, 1993, 1994, June 1995, October 1995, 1997, and 2001. Two earthquakes occurred in the vicinity during the ten years period: the 1992, 18 November Ms=5.9 Galaxidi earthquake and the 1995, 15 June 1995 Ms=6.2 Aigion one. With respect to the stable Europe, we find for Peloponnessos an average displacement rate of 30 mm/yr in the N215° direction, similar to that found in previous studies. Our results show that most of the deformation in the Corinthian Gulf is localizes off-shore, in a narrow band, in the central part of the Gulf. The extension rate measured over 10 years is 11 mm/yr in the N185° direction in the middle of the Gulf (Xiloxastro) and 16 mm/yr in the N185° direction in its western part (Aigion). The southern block appears un-deformed, except the region of Aigion event. Using CNES DIAPASON software, we derived 85 interferograms of the Corinthian Gulf from 38 raw ERS SAR images acquired between 1992 and 1999. The interferograms sampling the 1995 earthquake show a clear coseismic signal reaching 250 +/- 15 mm at Psaromita cape, a value consistent with the GPS measurements. No post-seismic motion, within the error bars of SAR interferometry (+/- 15 mm), is observed during the 1995-1999 period

Analysis of eleven years of deformation measured by GPS in the Corinth Rift Laboratory area [Analyse de onze années de mesures de deformations collectées par GPS dans la zone du laboratoire du rift de corinthe]

International audienceGPS (Global Positioning System) data collected in the Corinth Rift during eleven campaigns between 1990 and 2001 provide velocities of 57 points with ∼ 1.5 mmyr−1 accuracy. Peloponnesos moves at 30 mmyr−1 to the N215◦ E with respect to a fixed Europe. Extension across the rift is accommodated in a narrow band offshore. Its rate increases from east to west and is 16 mmyr−1 near Aigion. Both sides of the rift behave as clockwise rotating blocks with rates of 7 ± 0.5◦ Myr−1 and 2.8±0.8◦ Myr−1, respectively for the northern and southern blocks. After removing block rotations, the northern block shows a north–south extension rate of 120 ± 50 nstrain yr−1, whereas the southern block indicates the internal deformation is still inside the error bar (< 20 nstrain yr−1). The strain accumulation across the major faults located along the southern coast of the Corinth Gulf is less than 1 mmyr−1. This implies long recurrence periods for large earthquakes on these faults.Analyse de onze années de mesures de deformations collectées par GPS dans la zone du laboratoire du rift de corinthe.Les données GPS collectées dans le rift de Corinthe au cours de onze campagnes de mesure entre 1990 et 2001 ontpermis d’obtenir des vitesses pour 57 points, avec une incertitude de ∼ 1,5 mma−1. Le Péloponnèse se déplace avec unevitesse de 30 mma−1 en direction N215◦ E par rapport au système de référence Europe fixe. L’extension à travers le rift est accommodéedans une bande étroite en mer, dans le golfe. Le taux d’extension augmente d’est en ouest et est de 16 mman−1 prèsd’Aigion. Les deux côtés du golfe de Corinthe se comportent comme des blocs intéressés par des rotations horaires dont les tauxsont de 7 ± 0,5◦ Ma et 2,8 ± 0, 8◦ Ma−1 respectivement pour les blocs nord et sud. Après avoir retiré les rotations des blocs,le bloc nord montre une extension nord–sud de 120 ± 50 nstrain a−1, alors que le bloc sud indique que la déformation interneest encore dans la barre d’erreur (< 20 nstrain a−1). L’accumulation de la déformation à travers les failles majeures localiséesle long de la côte sud du golfe de Corinthe est inférieure à 1 mma−1. Cela implique, pour ces failles, des temps de chargementet récurrence longs (500–1000 ans) pour les séismes de magnitude 6,5 à 7 s’y produisant

Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)

International audienceThis paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes

Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995

International audienceBetween 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S-N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S-N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might be wider in the center (15–20 km), and is more diffuse in the east. The analysis of the displacements observed after the Ms = 6.2, June 15, 1995, and the Ms = S.9, November 18, 1992, earthquakes, both located in the west of the gulf, together with seismological and tectonic observations shows that these two earthquakes occurred on low-angle (≤35°) north dipping normal faults located between 4.5 and 10 km depth in the inner part of the rift. Assuming that the deformation is concentrated in relatively narrow deforming zones, we use a simple model of a dislocation in an elastic half-space to study the implication of the localization. Using the geometry of the known seismogenic faults, our observations imply continuous aseismic deformation in the uppermost crust of the inner rift. This model predicts geodetic strain rates close to seismic strain rates in opposition to previous estimates. This is because our model takes into account the activity on low-angle normal faults in the inner rift and an effective seismogenic layer of 6–7 km, about half that usually assumed