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Meghraoui M.,Institute Of Physique Du Globe Of Strasbourg Umr 7516 | Aksoy M.E.,Institute Of Physique Du Globe Of Strasbourg Umr 7516 | Aksoy M.E.,Technical University of Istanbul | Aksoy M.E.,University of Lisbon | And 4 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2012

The Ganos fault is the westernmost segment of the North Anatolian Fault that experienced the Mw = 7.4 earthquake of 9 August 1912. The earthquake revealed 45-km-long of surface ruptures inland, trending N70°E, and 5.5 m of maximum right lateral offset near Güzelköy. The long-term deformation of the fault is clearly expressed by several pull-apart basins and sag ponds, pressure and shutter ridges and offset streams. In parallel with detailed geomorphologic investigations, we measured co-seismic and cumulative displacements along the fault, and selected the Güzelköy site for paleoseismology. A microtopographic survey at the site yields 10.5 ± 0.5 m and 35.4 ± 1.5 m cumulative lateral offsets of stream channels and geomorphologic features. Seven paleoseismic parallel and cross-fault trenches document successive faulting events and provide the timing of past earthquakes on the Ganos fault segment. Radiocarbon dating of successive colluvial wedges in trench T1, and the fresh scarplet above (probably 1912 surface rupture) indicate the occurrence of three faulting events since the 14th century. Parallel trenches (3, 5, 6 and 7) expose paleo-channels and show a cumulative right-lateral offset of 16.5 ± 1.5 m next to the fault, and 21.3 ± 1.5 m total channel deflection. Radiocarbon dating of past channel units and fault scarp-related colluvial deposits imply an average 17 +/- 5 mm/year slip rate and 323 ± 142 years recurrence interval of large earthquakes during the last 1000 years on the Ganos fault. The succession of past faulting events and inferred slip rate west of the Marmara Sea provide more constraint on the long-term faulting behavior in the seismic gap of the North Anatolian Fault and may contribute to a better seismic hazard assessment in the Istanbul region. © 2012 by the American Geophysical Union.

Meghraoui M.,Institute Of Physique Du Globe Of Strasbourg Umr 7516 | Pondrelli S.,Instituto Nazionale Of Geofisica E Volcanologia
Annals of Geophysics | Year: 2012

We present a synthesis of the active tectonics of the northern Atlas Mountains, and suggest a kinematic model of transpression and block rotation that illustrates the mechanics of this section of the Africa-Eurasia plate boundary. Neotectonic structures and significant shallow seismicity (with Mw >5.0) indicate that coeval E-W-trending, right-lateral faulting and NE-SW, thrust-related folding result from oblique convergence at the plate boundary, which forms a transpressional system. The strain distribution obtained from fault-fold structures and P axes of focal mechanism solutions, and the geodetic (NUVEL-1 and GPS) convergence show that the shortening and convergence directions are not coaxial. The transpressional strain is partitioned along the strike and the quantitative description of the displacement field yields a compression-to-transcurrence ratio varying from 33% near Gibraltar, to 50% along the Tunisian Atlas. Shortening directions oriented NNE and NNW for the Pliocene and Quaternary, respectively, and the S shape of the Quaternary anticline axes, are in agreement with the 2.24°/Myr to 3.9°/Myr modeled clockwise rotation of the small tectonic blocks and with the paleomagnetic data. The convergence between Africa and Eurasia is absorbed along the Atlas Mountains at the upper crustal level, by means of thrusting above decollement systems, which are controlled by subdued transcurrent faults. The Tell Atlas of northwest Algeria, which has experienced numerous large earthquakes with respect to the other regions, is interpreted as a restraining bend that localizes the strain distribution along the plate boundary. © 2012 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved.

Candela T.,Joseph Fourier University | Renard F.,Joseph Fourier University | Bouchon M.,Joseph Fourier University | Schmittbuhl J.,Institute Of Physique Du Globe Of Strasbourg Umr 7516 | Brodsky E.E.,University of California at Santa Cruz
Bulletin of the Seismological Society of America | Year: 2011

We propose that a controlling parameter of static stress drop during an earthquake is related to the scaling properties of the fault-surface topography. Using high resolution laser distance meters, we have accurately measured the roughness scaling properties of two fault surfaces in different geological settings (the French Alps and Nevada). The data show that fault-surface topography is scale dependent and may be accurately described by a self-affine geometry with a slight anisotropy characterized by two extreme roughness exponents (HR), H|| = 0:6 in the direction of slip and H⊥ = 0:8 perpendicular to slip. Disregarding plastic processes like rock fragmentation and focusing on elastic deformation of the topography, which is the dominant mode at large scales, the stress drop is proportional to the deformation, which is a spatial derivative of the slip. The evolution of stress-drop fluctuations on the fault plane can be derived directly from the self-affine property of the fault surface, with the length scale (λ) as stdΔσ(λ) ∝ λHR-1. Assuming no characteristic length scale in fault roughness and a rupture cascade model, we show that as the rupture grows, the average stress drop, and its variability should decrease with increasing source dimension. That is for the average stress drop Δσ(r) ∝ rHR-1, where r is the radius of a circular rupture. This result is a direct consequence of the elastic squeeze of fault asperities that induces the largest spatial fluctuations of the shear strength before and after the earthquake at local (small) scales with peculiar spatial correlations.

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