Simultaneous estimation of secular and episodic crustal motion via geodetic observations

International audienceRepeated high quality geodetic observations allow the estimation of the free surface velocity field for a region. Usually, a yearly secular rate is estimated, while the possible episodic motion (seismic slip) due to an earthquake is evaluated via inversion of the geodetic data. The episodic motion influence is subtracted from the total field. The actual region and the respective geodetic observations affected by the seismic event are often assessed by rather vague criteria

Crustal deformations from sparce geodetic data

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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

The Ms=6.2, June 15, 1995 Aigion earthquake (Greece): Evidence for low angle normal faulting in the Corinth rift

We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (&amp;lt;4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38° 21.7′ N, 22° 12.0′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277°, dip 33°, rake -77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E-W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip - if any - has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift